Non-seismic and Complex Source Tsunami: Unseen Hazard

Abstract

Tsunamis, commonly induced by undersea earthquakes, are formidable natural hazards capable of causing widespread devastation. This comprehensive chapter examines the complex dynamics of tsunamis, their generation mechanisms, and their broad-reaching impacts. The multifaceted nature of tsunami triggers, both seismic and non-seismic, is dissected, highlighting the role of undersea earthquakes, landslides, volcanic eruptions, and meteorological events in driving these devastating natural phenomena. The intricate interplay of seismic parameters such as magnitude, depth, and activity type is elaborated, underscored by an insightful case study on the 2011 Tohoku Earthquake and Tsunami. A pivotal part of the discussion lies in the exploration of non-seismic triggers of tsunamis, an area often overshadowed in tsunami studies. The impact of landslide-induced and volcanically triggered tsunamis is considered alongside the contentious topic of meteorologically influenced tsunami events. Delving further into the genesis of tsunamis, the chapter explores the influences of bathymetry and tectonic structures, particularly in the context of non-seismic tsunami generation. The chapter serves as a beacon for continuous research and predictive modeling in the field of tsunami studies, emphasizing the necessity for societal preparedness and strategic risk mitigation against these potent natural disasters.

1. Introduction

Tsunamis, characterized as a series of ocean waves with long wavelengths and periods, have long been a source of fascination and fear. These remarkable natural phenomena, while beautiful in their raw power and scale, can also bring about catastrophic destruction and loss of life. Their impacts have been felt around the globe, in the Pacific Ocean’s Ring of Fire, the Indian Ocean, the Atlantic Ocean, and even in enclosed bodies of water like the Mediterranean Sea. While tsunamis have traditionally been associated with seismic events such as undersea earthquakes, a growing body of research has shed light on a range of non-seismic triggers, including landslides, volcanic eruptions, and certain meteorological events [1, 2, 3]. These non-seismic and complex sources of tsunamis are the primary focus of this chapter.

The study of tsunamis spans several disciplines, including geology, oceanography, and disaster risk management. Understanding these phenomena requires a holistic perspective, encompassing not just the triggers of tsunamis but also their propagation, the coastal impact, and the aftermath of these events. Seismic events such as earthquakes are the most well-known triggers of tsunamis. When tectonic plates shift on the seafloor, they can displace large volumes of water, setting off a series of waves that travel across vast distances at high speeds [4, 5]. The 2004 Indian Ocean Tsunami, triggered by a massive undersea earthquake off the coast of Sumatra, is a notable example of a seismic tsunami. However, not all tsunamis originate from seismic events. Non-seismic sources, although less common, can also trigger tsunamis. For instance, landslides, whether they occur underwater or fall into water from the land, can displace water and generate a tsunami. Similarly, volcanic eruptions can trigger tsunamis in several ways, such as through the collapse of part of the volcanic edifice or the ejection of pyroclastic flows into the sea. Furthermore, certain meteorological phenomena, like atmospheric pressure changes associated with fast-moving weather systems, can generate meteotsunamis [6, 7].

The purpose of this chapter is to delve deeper into the less-studied realm of non-seismic and complex-source tsunamis. This exploration aims to contribute to a more comprehensive understanding of tsunamis, enhancing our collective ability to predict and mitigate the impact of these devastating events. The chapter is organized into several sections. Following the introduction, we will discuss the general characteristics of tsunamis and how they differ from regular ocean waves. The subsequent sections delve into seismic and non-seismic triggers of tsunamis, each accompanied by a relevant case study. The chapter concludes with a discussion of the current methods and technologies for early warning systems and the ongoing research in the field. The forthcoming discussions aim to provide readers with a robust understanding of the different triggers of tsunamis, the mechanics of wave propagation, and the impacts of these powerful events.

2. Understanding tsunamis: from origin to impact

Tsunamis are unique oceanic events with distinctive properties that differentiate them from regular waves. These differences are extensively outlined in Table 1. Refer to Table 1 for a detailed comparison of tsunamis and regular ocean waves based on parameters such as wavelength, wave speed, wave height, and energy. Additionally, Figure 1 provides a visual representation of the mechanisms behind tsunami generation, illustrating the profound impact of seismic activities beneath the ocean’s surface.

	Tsunami	Regular Wave
Wavelength	200 to 500 kilometers	100 to 200 meters
Wave Speed	Dependent on water depth, typically 500 to 600 km/h in deep ocean	Dependent on wind speed, typically 5 to 90 km/h
Wave Height	In open ocean: usually less than 1 meter; near shore: can exceed 30 meters	In open ocean: 1 to 15 meters; near shore: can be higher due to wave shoaling
Energy	Very high, due to long wave periods and vast wavelengths. Energy does not dissipate quickly, allowing tsunamis to travel across entire ocean basins	Less than tsunamis, due to shorter wavelengths and wave periods. Energy dissipates more quickly, limiting the reach of waves

Table 1.

Characteristics of tsunamis vs. regular waves [8].

Figure 1.

The process of tsunami generation [9].

In Figure 1(a), titled “Faulting,” we see a cross-section of the ocean floor during tectonic activity. Here, a subducting plate moves beneath another, causing a displacement along a fault line. This shift results in an upward thrust of the seabed, which, in turn, propels the water above it, leading to the formation of a tsunami wave. The diagram shows a direct correlation between the seabed’s upward movement and the elevation of the water surface, forming a wave that begins to travel across the ocean.

Figure 1(b), labeled “Submarine slump,” depicts an alternative tsunami-generating event. In this scenario, a mass of sediment on the ocean floor, destabilized perhaps by seismic shaking or other factors, slumps down a slope. This submarine landslide displaces the water column, generating a wave in a process analogous to that observed in Figure 1(a). The resulting wave, or tsunami, then propagates away from the site of the slump, carrying the energy of the initial disturbance across the sea.

Through these illustrations, Figure 1 effectively elucidates the initial seismic events that can lead to the powerful and often devastating phenomenon of tsunamis, highlighting the dynamics of fault motion and submarine landslides as primary catalysts.

2.1 General characteristics of tsunamis

Tsunamis are not typical sea waves. They are distinguished by their long wavelengths, often reaching hundreds of kilometers. This means that unlike surface waves, which involve a superficial layer of the sea, tsunamis engage the entire depth of the ocean, from the seabed to the surface. Consequently, the speed of a tsunami wave is dictated by the depth of the ocean. In the deep ocean, these waves can reach astonishing speeds of 500–600 kilometers per hour, comparable to a commercial jet’s cruising speed [5, 10]. Despite their high speeds, tsunamis often go unnoticed in the open ocean. Their amplitude in deep water is usually less than one meter, and the wave crests can be hundreds of kilometers apart. Hence, ships at sea rarely detect a passing tsunami. However, as a tsunami approaches the shore and enters shallower water, its speed decreases while its amplitude grows. This transformation, known as wave shoaling, can result in destructive waves reaching heights of tens of meters when they hit the coastline. The energy a tsunami carries is immense, and it dissipates very slowly. The long wavelengths mean that tsunamis can travel across entire ocean basins with limited energy loss. This explains why a tsunami triggered off the coast of Chile can cause destruction in Japan, more than 10,000 kilometers away [11, 12].

Tsunamis are not single waves but a series of waves known as a wave train. The first wave is not always the largest, and subsequent waves may arrive minutes or even hours apart. This is why residents are often caught unaware by later, larger waves after a smaller initial wave. The devastation a tsunami can bring about is not limited to its colossal waves. Tsunamis can also cause strong ocean currents, rapid sea-level rise, and flooding, often referred to as tsunami run-up. Run-up refers to the maximum vertical height onshore above sea level reached by the tsunami. Tsunamis leave behind a distinctive sedimentary record that can provide critical insights into past events. These deposits can be used to reconstruct the frequency and magnitude of past tsunamis, thus informing risk assessments [2, 13, 14].

2.2 Distinction between tsunamis and regular ocean waves

At a casual glance, one might mistake a tsunami for an ordinary sea wave. However, tsunamis and typical ocean waves are fundamentally different in several ways, from their generation to their physical characteristics and behavior. Regular ocean waves, often referred to as wind waves, are primarily driven by the action of wind on the sea surface. These waves typically have wavelengths in the range of tens to hundreds of meters, and their period (time between successive waves) is usually between 5 and 20 seconds [15]. Wind waves involve only the uppermost layer of the ocean, and their energy rapidly dissipates with depth. In contrast, tsunamis are born from large-scale, violent disturbances in the ocean, often tectonic in origin. The wavelengths of tsunamis span hundreds of kilometers, and their period can range from several minutes to over an hour. Moreover, tsunamis engage the entire water column, meaning their energy extends all the way down to the sea floor. This gives tsunamis their high speed in deep water and their capacity to travel vast distances with little energy loss. The behavior of tsunamis as they reach the shore is also different from regular waves. While wind waves break and dissipate their energy at the shoreline, tsunamis undergo wave shoaling, increasing their amplitude dramatically. A tsunami’s energy can inundate the land, causing flooding and destruction far beyond the immediate coastline [5, 13]. Understanding these differences is crucial for effective tsunami hazard assessment and mitigation. It helps underscore why tsunamis cannot be treated like large regular waves and why they require unique approaches in terms of prediction, warning, and response.

3. Seismic triggers of tsunamis

Undersea earthquakes are the most common triggers of tsunamis. Their generation conditions, along with the influence of the earthquake’s magnitude, depth, and type of seismic activity on the resulting tsunami, are complex and multifaceted. The 2011 Tohoku Earthquake and Tsunami serve as a case study illustrating these effects. Table 2 lists major tsunami events triggered by undersea earthquakes, detailing the year, location, earthquake magnitude, depth, and the resulting tsunami’s impact.

Year	Location	Earthquake Magnitude	Depth	Tsunami Impact
2011	Pacific Ocean, off the east coast of Tohoku, Japan	9.0–9.1	29	Estimated 15,899 deaths, caused by nuclear accidents at Fukushima Daiichi Nuclear Power Plant
2004	Indian Ocean, off the west coast of northern Sumatra	9.1–9.3	30	Estimated 230,000–280,000 deaths across 14 countries, one of the deadliest natural disasters in history
1960	Pacific Ocean, near Valdivia, Chile	9.4–9.6	35	Estimated 1000–6000 deaths, caused damage in Hawaii, Japan, the Philippines, the east coast of New Zealand, southeast Australia, and the Aleutian Islands
1952	Pacific Ocean, off the east coast of Kamchatka Peninsula, Russia	9.0–9.3	30	No deaths were reported, caused damage in Hawaii and Japan
1755	Atlantic Ocean, near Lisbon, Portugal	8.5–9.0	30	Estimated 10,000–100,000 deaths, one of the deadliest earthquakes in history

Table 2.

Major tsunami events triggered by undersea earthquakes [16].

3.1 Undersea earthquakes; conditions leading to tsunami generation

Undersea earthquakes are the most common cause of tsunamis, and understanding their behavior is critical to our grasp of these devastating events. In the heart of tectonic plate interactions lies the driving force behind such earthquakes. These tectonic forces take shape in the form of plate boundaries, with convergent boundaries (also known as subduction zones) being of significant interest due to their tsunami-generating capacity. The conditions necessary for a tsunami to be generated by an undersea earthquake are nuanced. Firstly, earthquakes must occur beneath the ocean or sea floor. This ensures that there is sufficient water above the epicenter to be displaced by the earthquake’s energy [17, 18, 19]. Secondly, the earthquake must be of a sufficient magnitude, generally considered to be a magnitude of 7.0 or higher on the Richter scale. Lower-magnitude earthquakes can still generate tsunamis but are less likely to result in significant wave heights upon reaching shorelines. Thirdly, the earthquake must result in significant vertical displacement of the sea floor. It is this displacement that transfers energy from the earth to the water, generating tsunami waves. Horizontal movements, which are typical of transform boundaries, do not cause significant displacement of water and thus are not typically associated with tsunamis [17, 20].

Subduction zones, where one tectonic plate is forced beneath another, are hotspots for these powerful, tsunami-generating undersea earthquakes. This is due to the intense pressure that builds up over time as the subducting plate is forced into the mantle. Eventually, this pressure is released in the form of an undersea earthquake. If the conditions mentioned above are met, a tsunami can be generated. One example of this is the 2004 Indian Ocean earthquake and tsunami, which was generated by a powerful undersea earthquake off the coast of northern Sumatra. The earthquake occurred in a subduction zone where the Indo-Australian Plate is being subducted beneath the Eurasian Plate. This event resulted in significant vertical displacement of the sea floor, generating a tsunami that killed hundreds of thousands of people around the Indian Ocean [21, 22].

3.2 Impact of magnitude, depth, and type of seismic activity

The impact of undersea earthquakes in generating tsunamis is dictated by several factors. The magnitude of the earthquake is a key factor – the greater the magnitude, the more energy is released and the greater the potential for a tsunami. Depth is also crucial, as shallow earthquakes are more likely to result in significant vertical displacement of the sea floor, which is necessary for tsunami generation. The type of seismic activity is also vital. As stated earlier, subduction zone earthquakes, which typically result in vertical displacement, are the most common cause of tsunamis. Other types of seismic activity, such as volcanic eruptions or landslides, can also generate tsunamis under the right conditions, but these are less frequent. There is a nuanced relationship between the magnitude, depth, and type of seismic activity. For instance, a high-magnitude earthquake at a very deep depth might not generate a tsunami due to the lack of displacement at the sea floor. Conversely, a lower-magnitude, shallow earthquake in a subduction zone could generate a large tsunami due to significant vertical displacement [22, 23].

3.3 Case study: 2011 Tohoku earthquake and tsunami

The Tohoku earthquake and tsunami on March 11, 2011, epitomizes the destructive power of tsunamis and highlights the role of undersea earthquakes in their generation. With a moment magnitude of 9.0–9.1, the Tohoku earthquake is the fourth largest earthquake on record and the largest ever recorded in Japan. The earthquake occurred off the northeast coast of Honshu, Japan’s main island, in a subduction zone where the Pacific Plate is being forced beneath the North American Plate. The earthquake resulted in massive vertical displacement of the sea floor – up to 50 meters in some areas – generating a series of devastating tsunami waves [24, 25].

The tsunami waves reached up to 40.5 meters in height and traveled up to 10 kilometers inland in some areas. The tsunami resulted in over 15,000 deaths, and the subsequent Fukushima Daiichi nuclear disaster made it the costliest natural disaster in history. The Tohoku earthquake and tsunami demonstrated the critical need for improved tsunami forecasting and preparedness. Despite Japan’s advanced tsunami warning system and strict building codes, the magnitude of the event far exceeded expectations, leading to widespread devastation. It underscored the fact that, while we have made significant advancements in our understanding of tsunamis and their seismic triggers, there is still much work to be done [26]. In conclusion, while undersea earthquakes are the primary trigger of tsunamis, the conditions leading to their generation are multifaceted, involving the interplay of earthquake magnitude, depth, and type of seismic activity. Case studies such as the 2011 Tohoku Earthquake and Tsunami provide valuable insights into these processes, contributing to our understanding and potentially mitigating the devastation caused by these natural disasters.

3.4 Seismic triggers and their impact on tsunami generation

Understanding the seismic triggers of tsunamis begins with the realization that undersea earthquakes play a significant role due to their capability to displace substantial volumes of water abruptly. The relationship between an earthquake’s characteristics and their impacts on ocean floor topography is a complex interplay significantly influencing tsunami genesis. The magnitude, depth, and location of an earthquake can directly affect the extent of seafloor deformation and, in turn, the size and energy of the resulting tsunami. Larger and shallower earthquakes typically cause more substantial displacement of the seafloor, potentially triggering more significant tsunamis. However, the tsunami’s propagation and amplification depend on not only the characteristics of the earthquake but also the topographical and tectonic features of the ocean floor. These elements influence the tsunami waves as they traverse the ocean basin, causing them to refract, reflect, or amplify in certain conditions [27, 28].

While seismic tsunamis are common, their unpredictability and potential for causing widespread devastation necessitate continuous efforts in refining prediction methods and devising effective mitigation strategies. Another intriguing aspect of seismic activities associated with tsunamis is the sequence of earthquakes—namely, foreshocks, mainshocks, and aftershocks. Foreshocks precede the mainshock, the most significant earthquake in the sequence, while aftershocks follow it. The ability to distinguish foreshocks from mainshocks and aftershocks, however, is an area of ongoing research, indicating that seismic patterns may not always conform to this sequence and that generalizations should be made cautiously. The process of tsunami wave formation, triggered by undersea earthquakes, entails a series of stages. Starting from the initial seafloor displacement to the eventual propagation of waves across the ocean, each stage is a complex interplay of numerous factors. The energy released during the earthquake initiates seafloor displacement, thereby triggering the creation and propagation of tsunami waves [29, 30]. The understanding of seismic triggers and their role in tsunami generation, albeit complex, plays a pivotal role in enhancing our predictive capabilities and devising effective disaster management strategies. This condensed overview of seismic triggers provides an essential foundation for further exploring the other non-seismic triggers of tsunamis.

3.5 The role of seismic magnitude and depth in tsunami generation

The magnitude of an undersea earthquake plays a crucial role in determining the size of the resulting tsunami. Higher-magnitude earthquakes have the potential to displace larger volumes of water, creating more significant tsunami waves. However, the depth of the earthquake also plays a critical role. Shallow earthquakes can cause more substantial displacement of the seafloor and therefore generate more substantial tsunami waves than deep earthquakes with the same magnitude. The generation of tsunamis is inherently a multifaceted process influenced by a plethora of parameters. Among these, the seismic magnitude and depth of the initiating earthquake play crucial roles. Tsunamis can be instigated by undersea earthquakes of varying magnitudes and depths, affecting the amplitude, energy, and overall destructive potential of the resulting waves. Consequently, understanding the role of seismic magnitude and depth in tsunami genesis is of paramount importance, as it contributes to improved tsunami forecasting and preparedness measures. Influence of Seismic Magnitude Seismic magnitude fundamentally impacts the scale and power of the resulting tsunami. Typically, earthquakes with higher magnitudes displace larger volumes of water, leading to more substantial tsunami waves. For instance, the Great Chilean Earthquake in 1960, the most powerful earthquake ever recorded with a magnitude of 9.5, triggered a catastrophic tsunami that affected coastlines over a vast geographical area. Similarly, the 2011 Tohoku earthquake in Japan, with a magnitude of 9.0–9.1, generated a devastating tsunami that caused extensive damage and loss of life [31].

The Role of Depth The depth of the earthquake’s hypocenter is another critical factor affecting tsunami genesis. Deep-focus earthquakes, despite possibly having high magnitudes, are less likely to generate destructive tsunamis due to the energy dissipating before reaching the ocean surface. On the contrary, shallow-focus earthquakes, particularly those occurring at depths less than 70 km, tend to produce more significant tsunamis, as the seismic energy is more directly transferred to the overlying water column. For example, the 2004 Indian Ocean Tsunami was triggered by a shallow undersea earthquake off the west coast of northern Sumatra, resulting in one of the deadliest natural disasters in recorded history. The Interplay between Magnitude and Depth Both the magnitude and depth of the earthquake must be considered concurrently to evaluate tsunami potential effectively. A high-magnitude, deep-focus earthquake may generate a less destructive tsunami compared to a lower-magnitude, shallow-focus earthquake due to differences in energy transfer to the ocean surface. As such, tsunami warning systems must account for both parameters to provide accurate and timely alerts [32, 33].

Implications for Tsunami Early Warning Systems Comprehending the role of seismic magnitude and depth in tsunami generation is integral to enhancing the effectiveness of tsunami early warning systems. These systems rely on rapid and accurate assessment of these earthquake parameters to estimate the potential tsunami threat and disseminate timely alerts to vulnerable coastal communities. Therefore, continued research and advancements in seismological studies and computational modeling are required to refine further our understanding of the complex interplay between seismic magnitude, depth, and tsunami generation. Future Research Directions Future research should focus on enhancing our understanding of the nuanced relationship between seismic magnitude, depth, and tsunami genesis. This may involve developing more sophisticated models that can accurately simulate the complex dynamics of undersea earthquakes and the resulting tsunami wave propagation. Additionally, further investigations into past tsunami events can provide valuable insights and data that can be used to refine these models, ultimately leading to improved tsunami forecasting and mitigation strategies [34].

4. Non-seismic triggers of tsunamis

In addition to undersea earthquakes, tsunamis can also be triggered by other mechanisms, such as landslides, volcanic eruptions, and meteorological events. These non-seismic triggers each have unique conditions necessary for tsunami generation, which are summarized in Table 3. For a visual comparison of seismic and non-seismic tsunami triggers, refer to Figure 2.

Figure 2.

Seismic tsunami wave propagation [37].

Trigger Type	Conditions for Tsunami Generation	Examples
Landslide	Occur when a large amount of material slips into the water, displacing water and creating a wave.	The 1958 Lituya Bay landslide in Alaska generated a mega-tsunami with a recorded run-up of 525 meters.
Volcanic Eruption	Occur when there is a violent underwater explosion, or when a caldera collapses and displaces water.	The 1883 eruption of Krakatoa generated tsunamis that resulted in over 36,000 deaths.
Meteorological Events	Occur when atmospheric conditions cause sudden changes in sea levels. These tsunamis are also known as meteotsunamis.	In 2008, a meteotsunami hit Boothbay Harbor in Maine, resulting in significant damage to boats and waterfront property.

Table 3.

Non-seismic triggers of tsunamis [7, 35, 36].

4.1 Landslides

Landslides represent one of the significant non-seismic triggers of tsunamis, and understanding their potential to generate tsunamis is crucial for comprehensive tsunami risk assessments. A landslide occurs when a mass of rock, earth, or debris moves down a slope due to gravity. These movements can be slow or sudden, and when they happen underwater or impact the water from the land, they can displace large amounts of water and generate tsunamis. There are two main types of landslides that can cause tsunamis: subaerial and submarine. Subaerial landslides are those that occur on land and then slide into the water, displacing it and creating a tsunami. Submarine landslides occur directly underwater. Both types can cause significant displacement of water, leading to potentially devastating tsunamis [38].

The magnitude of a landslide and the potential tsunami largely depends on the volume and velocity of the displaced material. Larger and faster landslides displace more water, creating bigger waves. The topography of the seafloor also plays a role; slopes that are too steep or too shallow may not be conducive to tsunami generation, as the water displaced by the landslide might not propagate into a wave that travels far from the source. Moreover, landslides can either be triggered by seismic activity, such as an earthquake, or they can occur independently of any seismic event. It is also worth noting that while most landslides that cause tsunamis are natural, human activity like construction or mining can also contribute to landslide events and potentially induce a tsunami. Monitoring and modeling landslide events are essential in predicting their tsunami potential. The use of submarine mapping techniques, landslide-tsunami models, and sediment analysis can provide valuable insight into past events and improve our predictions for future landslide-induced tsunamis [6, 35, 39].

4.2 Volcanic eruptions as triggers for tsunamis

Volcanic eruptions are another prominent non-seismic source of tsunamis. Volcanic tsunamis can occur because of several different mechanisms, each dependent on the specific circumstances of the eruption. The most straightforward mechanism is through the pyroclastic flow or material from the eruption entering the sea, displacing water, and initiating a tsunami. Pyroclastic flows are rapid currents of hot gas and volcanic matter that flow down the side of a volcano during an eruption. When such a flow reaches the sea, it can displace large volumes of water, causing a tsunami. Another mechanism through which volcanic eruptions can cause tsunamis is the collapse of the volcanic edifice itself, either during or after the eruption. If a significant portion of the volcano collapses into the sea, it can displace enough water to create a tsunami. This is what occurred in the famous 1883 eruption of Krakatoa in Indonesia, which led to a devastating tsunami. Volcanic eruptions can also generate tsunamis through phreatomagmatic explosions – these occur when magma encounters water, creating steam, and causing an explosive reaction. If this reaction is significant enough, it can displace large volumes of water, leading to a tsunami [3, 40, 41]. It is worth noting that the characteristics of a volcanic tsunami – such as its initial wave height, wavelength, and speed – can be very different from those of a seismic tsunami. Therefore, different models and mitigation strategies may be required to effectively manage these types of events.

4.3 Meteorological events and tsunamis

Meteorological events such as storms and hurricanes do not typically generate tsunamis as they are generally understood. However, they can produce phenomena known as meteorological tsunamis or meteotsunamis, which are sea wave disturbances that are similar in their impact to seismic tsunamis, although they are generated by very different mechanisms. Meteotsunamis are most often caused by fast-moving weather systems, such as squall lines, that generate pressure disturbances over the sea. These pressure changes can set up long-wavelength waves in the body of water. If the speed of the weather system is similar to the speed of the wave in the water, then resonance can occur, amplifying the wave and potentially leading to a meteotsunami. Meteotsunamis can cause significant damage and loss of life, particularly if they coincide with high tide or if they strike areas with a large population. Because the mechanisms of generation are so different, meteotsunamis require different prediction and mitigation strategies than seismic tsunamis. It is also noteworthy that some climate scientists are suggesting that climate change may increase the frequency and intensity of these meteorological events and hence meteotsunamis [7, 42]. Therefore, as our understanding of the potential impacts of climate change grows, it is becoming increasingly important to include the study of meteotsunamis in our assessments of tsunami hazards.

4.4 Case study: 1958 Lituya Bay landslide and tsunami

The 1958 Lituya Bay Landslide and Tsunami in Alaska provides a compelling example of the destructive potential of non-seismically generated tsunamis. This event remained the largest recorded mega-tsunami and was triggered by a landslide following an earthquake with a magnitude of 7.8 on the Richter scale. The earthquake itself did not generate a significant tsunami, but the secondary landslide did, dramatically illustrating the potential for non-seismic sources to generate tsunamis. The landslide caused 30 million cubic meters of rock to fall into the narrow inlet of Lituya Bay, displacing a massive amount of water. The subsequent wave was of unimaginable scale, reaching a height of 1720 feet (524 meters) and wiping out all vegetation and trees up to this elevation on the opposite shoreline [35]. While this event did not result in a high death toll (only two fatalities were reported), it nonetheless provides an excellent illustration of the incredible destructive potential of landslide-generated tsunamis. This case also emphasizes the importance of considering these types of non-seismic events in tsunami hazard assessments, particularly in areas prone to landslides. The Lituya Bay event remains a benchmark for tsunami studies, and its effects continue to be felt in the area. It stands as a powerful testament to the need for increased awareness and preparedness for all types of tsunami-generating mechanisms, whether seismic or non-seismic in origin.

4.5 Deepening the examination of non-seismic tsunami triggers

Non-seismic triggers of tsunamis are an increasingly recognized source of potential hazards. While they may not occur as frequently as seismic events, their impacts can be as devastating, if not more so, given the limited warning time and the unpredictability of such events. In the realm of landslide-generated tsunamis, there is a considerable amount of variation depending on the specific conditions and location of the landslide. For instance, the volume of displaced material, the velocity at which the landslide occurs, and the depth at which the landslide mass enters the water all play significant roles in determining the scale and impact of the tsunami [43]. Landslides can occur both above and below the water surface, and both types have the potential to generate tsunamis. Underwater landslides, however, may be particularly effective in generating tsunamis due to the proximity of the energy release to the water column and the potential for significant displacement of water.

Volcanic eruptions as tsunami triggers are multifaceted in nature. While the most straightforward cause is the collapse of the volcanic edifice or caldera into the sea, producing a landslide and, subsequently, a tsunami, other mechanisms are also at play. Pyroclastic flows and related phenomena can displace large volumes of water, as can the ejection of volcanic material into the sea. Even the seismic activity associated with the eruption can generate tsunamis, making this type of trigger particularly complex. Finally, meteorological events such as hurricanes and typhoons are another type of non-seismic trigger. These events can induce storm surges that have similar impacts as tsunamis, especially when combined with astronomical tides [44]. Furthermore, atmospheric pressure changes associated with severe weather events can generate meteotsunamis, a type of tsunami that is not caused by seismic activity or landslides but is nonetheless capable of causing significant damage. While there is a wealth of existing literature that discusses these non-seismic triggers, there is still much to be learned. As we increase our understanding of these processes, our ability to predict and mitigate the impacts of these types of tsunamis will undoubtedly improve.

4.6 Further expanding the scope of non-seismic tsunami triggers

Continuing from the above discussion, it is essential to mention that non-seismic triggers are not merely academic curiosities but have been the source of some of the most destructive tsunamis in recorded history. For example, the 1958 Lituya Bay tsunami, caused by a landslide in a fjord in Alaska, generated a wave that reached a record height of 524 meters when it surged up the opposite slope of the fjord. This event illustrates the potential for non-seismic tsunamis to exceed the scale of seismic tsunamis under specific local conditions. In the volcanic realm, the 1883 eruption of Krakatoa is an infamous example. The explosion and subsequent collapse of the volcano generated a tsunami that killed more than 36,000 people [45]. More recently, the 2018 Anak Krakatoa collapse in the same area also generated a significant tsunami, demonstrating that volcanic tsunamis are not only a historical concern but an ongoing hazard [46]. Regarding meteorological tsunamis, the 2011 Tohoku tsunami, though primarily seismic in origin, was accompanied by a meteotsunami that traveled across the Pacific, demonstrating the wide-ranging impacts of these events. Given these historical examples, it is clear that non-seismic triggers represent a significant and somewhat under-appreciated risk. Enhancing our understanding of these triggers and their effects will improve our predictive models and allow for better mitigation and preparedness efforts.

4.7 Tsunami generation through landslide and volcanic mechanisms

While seismic activities indeed constitute the majority of tsunami-triggering events, it is essential not to overlook non-seismic triggers like landslides and volcanic eruptions, which can also initiate these formidable natural phenomena. The potential of landslides to generate tsunamis arises when a considerable volume of material, like a portion of a mountain or hill, collapses into a water body. This abrupt displacement of water can set off waves that, depending on the volume and speed of the landslide, can escalate into devastating tsunamis. Landslide-triggered tsunamis have been responsible for some historically significant events. The Lituya Bay tsunami in Alaska in 1958 is a prime example, where a massive landslide triggered a mega-tsunami with wave run-up reported as high as 520 meters. Such examples provide a stark reminder of the enormous destructive potential of landslides as tsunami triggers [47]. Just like landslides, volcanic eruptions are a potent non-seismic trigger for tsunamis. The mechanism of tsunami generation through volcanic eruptions can encompass multiple processes, such as pyroclastic flows entering water bodies, flank collapse, and caldera collapse. A striking instance of a tsunami generated by a volcanic eruption was observed during the infamous Krakatoa eruption in 1883. This catastrophic event generated a tsunami that led to widespread destruction and significant loss of life across the Indonesian archipelago. Though they may not be as frequent as their seismic counterparts, tsunamis induced by landslides and volcanic eruptions are far from negligible. Understanding these non-seismic sources and their associated complexities will undoubtedly enhance our predictive capabilities and preparedness, paving the way for effective tsunami hazard mitigation [48].

4.8 Meteorological events: a controversial non-seismic tsunami trigger

Meteorological tsunamis, also referred to as ‘meteotsunamis,’ are an intriguing category of non-seismic events known to trigger tsunamis. These are long ocean waves induced by swift changes in barometric pressure associated with fast-moving weather systems like squalls, frontal passages, and thunderstorms. Though not as formidable as seismic tsunamis, meteotsunamis can induce significant coastal inundation and damage, particularly in areas with a resonance-enhancing coastline configuration. They are primarily observed in enclosed or semi-enclosed basins like the Mediterranean Sea, the Adriatic Sea, and the Great Lakes, where the geographical configuration encourages the amplification of these waves. However, they have been observed along open coasts globally, including the East Coast of the United States and Japan. The generation of meteotsunamis is a complex interplay between the atmosphere, ocean, and coastline. Initial sea-level perturbations are caused by atmospheric pressure changes or wind stress. The wave thus generated travels across the ocean, undergoing modifications due to variations in water depth, seafloor topography, and current. Upon reaching the coast, it can be amplified or attenuated depending on the shape and orientation of the coastline and the water depth [49].

When a storm system travels over the ocean at a speed mirroring the speed of long ocean waves in that region, a phenomenon known as the “Proudman resonance,” it can induce a meteotsunami. The wave can then cross the ocean, undergoing modifications due to variations in water depth, seafloor topography, and current. Upon reaching the coast, it can experience amplification due to local resonances, causing potentially significant coastal flooding and damage. Numerical modeling and observational studies have proven instrumental in understanding meteotsunami genesis, propagation, and impact. Advancements in atmospheric and oceanic modeling now enable us to simulate meteotsunami events with considerable accuracy. These models typically employ equations of fluid dynamics to simulate the interaction between the atmosphere and ocean, and wave propagation [50].

Despite these advancements, there remain hurdles in accurately predicting meteotsunamis, particularly in real-time. The rapid temporal and spatial changes in barometric pressure and wind speed associated with storm systems, which are challenging to predict, are primarily responsible. Furthermore, the resonance conditions that can substantially amplify these waves at the coast are complex and site-specific, necessitating high-resolution bathymetric and topographic data. In conclusion, continuous research and technological advancements are progressively enhancing our understanding of meteotsunamis, their impact on coastlines, and our ability to predict them. This understanding is crucial for devising appropriate mitigation strategies and early warning systems to minimize these events’ impact. With meteotsunamis emerging as a recognized hazard, increased research is required to comprehend their occurrence patterns, the conditions leading to significant events, and their potential impact under future climate change scenarios [51].

4.9 Case study: 1958 Lituya Bay landslide and tsunami

The 1958 Lituya Bay tsunami, triggered by a landslide in Alaska, serves as a noteworthy case study for understanding the mechanisms and impacts of non-seismically induced tsunamis. Occurring on July 9, 1958, in Lituya Bay, located on the Fairweather Fault in the Alaska Panhandle, the event remains one of the most significant landslide-induced tsunamis in recorded history. The event was triggered by an earthquake with a magnitude of 7.8 on the Richter scale. This earthquake induced a massive landslide on the steep slopes of the Gilbert Inlet at the head of the bay. An estimated 30 million cubic meters of rock fell from a height of several hundred meters into the bay, displacing an equivalent volume of water and triggering a massive wave. The wave surged up the opposite slope, reaching an unprecedented run-up height of 524 meters (approximately 1722 feet) and removing all vegetation and soil down to bedrock. This run-up height remains the highest ever recorded for a tsunami and offers a stark reminder of the potential for substantial wave amplification in narrow, steep-sided inlets. Observations of the aftermath and analysis of the event provided valuable insights into the generation and propagation of landslide-induced tsunamis. It highlighted the role of the initial impulse – both the magnitude and direction – given by the landslide to the water body in determining the wave characteristics. The steepness of the slope, the velocity of the landslide, and the volume of the displaced material were key factors influencing the magnitude and direction of the tsunami [35].

The incident emphasized the potential for significant local amplification of the tsunami height, particularly in enclosed or semi-enclosed water bodies with specific topographic features. In the case of Lituya Bay, the narrow, steep-sided inlet served to focus the wave energy, leading to the extraordinary run-up height observed. The event also highlighted the importance of local geological and geomorphological conditions in tsunami generation and propagation. The presence of the Fairweather Fault, one of the fastest-moving transform faults globally, likely contributed to the instability of the slopes around the bay and the triggering of the massive landslide [35]. Despite being a rare event, the Lituya Bay landslide and tsunami underscore the importance of considering non-seismic tsunami triggers in hazard assessment and mitigation strategies. It demonstrates that non-seismic events can induce tsunamis, with local effects far surpassing those of seismic tsunamis.

Since the 1958 event, continued research has improved our understanding of landslide-induced tsunamis. Advances in numerical modeling techniques now allow for more accurate simulations of such events, contributing to enhanced tsunami hazard assessments. While significant progress has been made, challenges remain. Predicting the occurrence of landslides, particularly submarine landslides, is difficult due to the multitude of contributing factors and their inherent unpredictability. More research is required to better understand these triggers and enhance our predictive capabilities, ultimately reducing the risk posed by these devastating events [52]. In conclusion, the 1958 Lituya Bay event serves as a potent reminder of the hazards posed by non-seismic tsunamis. It emphasizes the need for continued research and investment in monitoring and early warning systems to mitigate these events’ impacts.

5. Modeling tsunami generation from non-seismic sources

Tsunami modeling is a critical tool for understanding the dynamics of tsunami generation and propagation, particularly from non-seismic sources. The non-seismic triggers, such as landslides, volcanic eruptions, and meteorological phenomena require different computational models and methodologies as compared to seismic tsunamis. Non-seismic tsunamis have specific characteristics that pose unique challenges for modeling. For instance, landslides and volcanic eruptions often involve a complex interplay of solid and fluid dynamics, making their modeling a multi-physics problem. Meteorological tsunamis, on the other hand, require detailed knowledge of atmospheric processes and their interaction with the ocean [52].

One of the pioneering efforts in the modeling of non-seismic tsunamis was the work of Harbitz [53], who developed a two-phase flow model to simulate submarine landslide-induced tsunamis. This model considers the momentum exchange between the solid landslide material and the water, allowing for a more realistic representation of the tsunami generation process [53]. Such multiphase flow models have since been extended and refined by many researchers, incorporating features like complex landslide kinematics, granular flow physics, and varying bathymetry. Today, these models form the basis for our understanding and prediction of landslide-induced tsunamis. The modeling of volcanic tsunamis is a similarly complex task. It involves understanding and simulating processes such as the collapse of volcanic edifices, the interaction of hot pyroclastic flows with water, and the explosive entry of magma into a water body. Recent advancements in computational fluid dynamics (CFD) have made it possible to simulate such complex processes with reasonable accuracy. Researchers have used CFD to study various volcanic processes and their potential to generate tsunamis, contributing significantly to our understanding of these phenomena [54].

Despite these challenges, considerable progress has been made in the modeling of non-seismic tsunamis over the past few decades. Numerical models have become increasingly sophisticated, incorporating more realistic physics and higher levels of detail. These models have provided invaluable insights into the generation and propagation mechanisms of non-seismic tsunamis, contributing to improved hazard assessments and mitigation strategies. However, there is still much work to be done. The inherent unpredictability and complexity of non-seismic tsunami triggers require the continual refinement of models and methodologies. Ongoing research and investment in computational resources are needed to advance our understanding of these events and to mitigate their risks effectively. In summary, modeling tsunami generation from non-seismic sources is a complex but essential task. Continued research in this area is critical for improving our understanding of these phenomena, enhancing tsunami hazard assessments, and developing effective mitigation strategies.

6. Role of bathymetry in non-seismic tsunami generation

Bathymetry, the underwater topography of ocean floors, plays an intricate role in the generation and propagation of non-seismic tsunamis. This role is amplified considering the complex interactions involved in the non-seismic triggers of tsunamis, such as landslides, volcanic eruptions, or meteorological phenomena. Bathymetry directly affects the initiation and development of landslides, both submarine and coastal, which are crucial triggers of non-seismic tsunamis. The stability of slopes underwater and their likelihood of failure, leading to a landslide, depend heavily on their geometry, which is determined by bathymetry. Steeper slopes are more likely to fail, while underwater canyons can guide the path of a sliding mass of sediments, directing the energy of the resulting tsunami in specific directions. The propagation of a tsunami wave, once generated, is significantly affected by bathymetric features. The wave’s speed, direction, and amplitude can be altered by the underwater topography as the wave travels across the ocean floor. Shoaling effects, where the wave height increases as the water depth decreases, can significantly amplify a tsunami’s destructive power in coastal regions. Volcanic eruptions occurring underwater or on islands can also generate tsunamis, and bathymetry plays a significant role here as well. Submarine volcanic eruptions can directly displace water, triggering a tsunami. The bathymetry around the volcano determines how this initial wave evolves and propagates [48, 55].

An interesting aspect of bathymetry’s role in non-seismic tsunamis comes from the study of meteorological tsunamis, which are generated by atmospheric disturbances. Bathymetry can modulate these tsunamis, with the resonance between the wave frequency and the natural frequencies of semi-enclosed basins, such as the Mediterranean and Adriatic Seas, increasing the wave’s amplitude. Bathymetry’s influence on non-seismic tsunamis cannot be overstated, making accurate bathymetric data essential for assessing the potential impact of these events. Advanced technologies like multibeam sonar have allowed for detailed mapping of the ocean floor, providing the necessary data for more accurate modeling and prediction of non-seismic tsunamis [8, 56].

Despite the progress made, challenges remain in understanding and predicting the interactions between bathymetry and non-seismic tsunamis. For instance, the complexities of landslide dynamics, the influence of various bathymetric features on wave propagation, and the resonance effects in semi-enclosed basins all require further study. In summary, bathymetry plays an essential role in the generation and propagation of non-seismic tsunamis. Ongoing research and data collection are crucial to deepen our understanding of these interactions and improve our ability to predict and mitigate the impacts of these potentially devastating events.

7. Role of tectonic structures in non-seismic tsunami generation

Tectonic structures, whether undersea or terrestrial, play an integral role in the generation and propagation of non-seismic tsunamis. Non-seismic tsunamis occur due to triggers that do not involve tectonic earthquakes but are often closely tied to tectonic features. Understanding these features and how they interact with the generation and propagation of tsunamis is crucial for predicting the impact and formulating mitigation strategies. Undersea landslides, an important cause of non-seismic tsunamis, can be associated with tectonic structures. Submarine landslides occur where sediment deposited on a sloping sea floor becomes unstable and moves downslope. This instability can be caused by a variety of factors, but a key one is the geometry of the sea floor, which is largely determined by tectonic processes [57]. Fault lines and the presence of weak, fractured, or deformed sedimentary layers due to tectonic activity can act as zones of weakness that make a slope susceptible to failure.

Similarly, volcanic eruptions and associated processes can cause non-seismic tsunamis, and these volcanoes are typically located near tectonic plate boundaries. The violent release of magma and gases can cause deformation of the sea floor, leading to displacement of water and the generation of a tsunami. In some cases, a flank or side of the volcano might collapse, triggering a massive landslide and subsequent tsunami, as happened in the case of the 1883 Krakatoa eruption. A significant source of non-seismic tsunamis is meteor impacts, which, while not directly influenced by tectonic structures, interact with the bathymetry and undersea topography that are tectonically formed. The impact crater and its size and shape, determined by the impactor’s size, speed, and angle of impact, also influence the initial tsunami generation [58].

The propagation and impact of non-seismic tsunamis, once generated, can also be influenced by tectonic structures. Submarine ridges and trenches can focus or defocus tsunami energy, leading to larger or smaller wave amplitudes at certain locations. Tsunamis also experience refraction due to changes in water depth, which is primarily controlled by tectonic processes. In summary, tectonic structures play a significant role in the generation and propagation of non-seismic tsunamis, and understanding these roles is crucial for assessing the potential impact and risk associated with these events.

8. Modeling non-seismic tsunami generation

Modeling tsunami generation, especially those resulting from non-seismic triggers such as landslides, volcanic eruptions, and meteorological phenomena, is a challenging task due to the intricate dynamics and unique features associated with these events. Recent advancements in computational techniques and the growing availability of high-resolution bathymetric and topographic data have started to enable more accurate and detailed modeling of these tsunamis.

Modeling non-seismic tsunamis involves many complex processes, including the initial trigger (like a landslide or a volcanic eruption), the propagation of the tsunami waves across the ocean, and their interaction with the coastline. These processes occur over different temporal and spatial scales, demanding highly flexible and efficient computational models. For example, simulating a landslide-triggered tsunami may require modeling the landslide motion, the wave generation, and wave propagation, each of which occurs on a different time scale and may require different model parameters. Tsunamis when modeling landslide-triggered tsunamis, one has to consider the dynamics of the landslide itself, which include its initial position, volume, shape, and motion. This is often done using geotechnical models, which are then coupled with hydrodynamic models to simulate wave generation and propagation. Several numerical methods have been employed for these purposes, including finite element, finite difference, and boundary integral equation methods, each with its unique strengths and limitations. One of the challenges in modeling landslide-triggered tsunamis is accurately representing the underwater topography and the landslide geometry. Advances in seafloor mapping technologies and satellite imagery have provided more detailed bathymetric and topographic data, enabling more precise landslide and tsunami modeling. Generation Modeling tsunamis triggered by volcanic activities requires a different approach. It demands a comprehensive understanding of the various processes involved in volcanic eruptions, such as the explosion, the collapse of the volcanic edifice, the ejection of pyroclastic material, and the resultant displacement of water. Each of these processes can contribute to the generation of a tsunami and should be accurately represented in the model [59, 60].

Another challenge in modeling volcanic tsunamis is the unpredictability of volcanic behavior. The timing, size, and type of eruption can vary greatly, making it difficult to develop a generic model. Nevertheless, progress has been made in modeling specific types of volcanic events, such as flank collapses and pyroclastic flows, which are known to generate tsunamis. Tsunamis induced by meteorological phenomena, often referred to as meteotsunamis, present their unique modeling challenges. These tsunamis are generated by atmospheric disturbances, such as pressure jumps, squalls, or frontal passages, which create wave trains in the ocean. The wave trains can amplify under certain conditions, such as resonance with the natural frequencies of the basin or coastline [8, 54, 61].

Modeling meteotsunamis require coupling atmospheric and oceanic models, which is a complex task. The atmospheric model should be able to represent the spatial and temporal variations in atmospheric pressure, while the oceanic model should be able to simulate the wave response to these pressure variations. One of the key challenges in this process is capturing the resonance effects, which can greatly amplify the wave heights. While significant progress has been made in modeling non-seismic tsunamis, there is still room for improvement. The current models can often provide valuable insights into the potential tsunami hazards associated with different events and guide the development of mitigation strategies. However, they are typically based on simplified assumptions, such as linear wave theory or idealized bathymetry, which may not accurately represent the complex reality. Future advancements in tsunami modeling will likely come from several directions. On the one hand, further developments in computational techniques and technologies, such as high-performance computing and machine learning, could allow more accurate and efficient simulations. On the other hand, more detailed and comprehensive data collection, through advanced seafloor mapping, satellite imagery, and oceanographic observations, could provide the necessary inputs for these models. In addition, there is a growing interest in developing probabilistic tsunami hazard assessments, which incorporate the uncertainties in the source parameters and provide estimates of the likelihood of different tsunami scenarios. This approach, already used in seismic hazard assessment, could offer a more comprehensive understanding of the tsunami risks and inform the planning and design of coastal infrastructure [62, 63, 64, 65].

Table 4 provides a comparison of different tsunami modeling techniques, outlining their underlying assumptions, computational requirements, and typical use cases. The table juxtaposes linear wave theory-based models, which are primarily used for initial simulations and long-distance propagation studies, with more complex Nonlinear models that provide greater accuracy for near-field simulations and inundation studies. It also introduces the coupled atmospheric-oceanic models, which consider both atmospheric and oceanic dynamics to study phenomena like meteotsunamis. The computational requirements increase with the complexity and sophistication of the model, underscoring the trade-off between model accuracy and computational efficiency. The choice of a suitable modeling approach depends on the specific research question and the resources at hand. The Linear Wave Theory is governed by the wave equation, which measures the wave elevation, i.e., the vertical displacement of the water surface, as a function of gravity and the spatial part of the wave. This equation is especially useful for predicting the behavior of smaller, less complex waves but may not adequately capture the behavior of larger or more complex tsunamis. The governing equation for linear wave theory can be expressed as:

Modeling Technique	Underlying Assumptions	Computational Requirements	Typical Use Cases	References
Linear wave theory-based models (Shallow Water Equations)	Neglects nonlinear wave interactions, assumes small wave amplitudes and wave speeds are determined solely by water depth	Lower computational cost, less computationally intensive	Ideal for initial simulations, studying long-distance tsunami propagation	[66]
Nonlinear models (Boussinesq Equations)	Considers nonlinear wave interactions, allows larger wave amplitudes and wave speeds are determined by both water depth and wave amplitude	Higher computational cost, more computationally intensive	More accurate for near-field simulations, studying wave run-up and inundation	[67]
Coupled atmospheric-oceanic models (WRF-HyCOM)	Represents atmospheric pressure variations and oceanic wave response, integrates both atmospheric and oceanic dynamics	Complex, requires both atmospheric and oceanic data	Needed for meteotsunami modeling, studying the impact of atmospheric disturbances on the ocean	[68]

Table 4.

Comparison of tsunami modeling techniques.

$${\partial }^2\mathrm{\eta }/{\mathrm{\partial t}}^2=\mathrm{g}{\text{∇}}^2\mathrm{\eta }$$E1

where η is the wave elevation (the vertical displacement of the water surface), g is the acceleration due to gravity, t is time, and ∇² is the Laplace operator representing the spatial part of the wave (it gives us the divergence of the gradient of η).

For more complex modeling, particularly in shallow water environments where most tsunami impact occurs, the Nonlinear Shallow Water Equations, or Saint–Venant equations, are often employed. These equations are derived from the Navier–Stokes equations and represent a system of hyperbolic partial differential equations. The Saint–Venant equations model the total water depth and velocities in two directions, considering gravity, bathymetry, and spatial coordinates. Although these equations simplify real-world conditions, they provide a solid foundation for tsunami simulation models, helping predict wave propagation and run-up with greater accuracy. As with all models, the accuracy of predictions based on these equations depends on the quality of the input data and how well the assumptions of the model match real-world conditions. The Saint-Venant equations are a set of hyperbolic partial differential equations derived from the Navier-Stokes equations. They are used to model the shallow water approximations and are particularly useful for tsunami wave propagation and run-up simulations. The Saint-Venant equations can be written as:

$$\mathrm{\partial h}/\mathrm{\partial t}+\partial \left(\mathrm{hu}\right)/\mathrm{\partial x}+\partial \left(\mathrm{hv}\right)/\mathrm{\partial y}=0.$$E2

$$\partial \left(\mathrm{hu}\right)/\mathrm{\partial t}+\partial \left({\mathrm{hu}}^2+\mathrm{½}{\mathrm{gh}}^2\right)/\mathrm{\partial x}+\partial \left(\mathrm{huv}\right)/\mathrm{\partial y}=\mathrm{gh}\mathrm{\partial b}/\mathrm{\partial x}$$E3

$$\partial \left(\mathrm{hv}\right)/\mathrm{\partial t}+\partial \left(\mathrm{huv}\right)/\mathrm{\partial x}+\partial \left({\mathrm{hv}}^2+\mathrm{½}{\mathrm{gh}}^2\right)/\mathrm{\partial y}=\mathrm{gh}\mathrm{\partial b}/\mathrm{\partial y}$$E4

where h is the total water depth, u and v are the velocities in the x and y directions, respectively, g is the acceleration due to gravity, t is time, b is the bathymetry (depth of water at rest), and x and y are the spatial coordinates.

In the ever-evolving field of tsunami research, various modeling techniques have been developed to understand and predict the behavior of these massive wave events. Table 4 presents a comparative analysis of the most prominent tsunami modeling techniques. The table contrasts these techniques based on factors like computational efficiency, accuracy, application range, and underlying principles. By understanding the strengths and limitations of each method, researchers and policymakers can make informed decisions in both academic and real-world tsunami mitigation efforts. As tsunamis continue to pose significant threats to coastal communities worldwide, such comprehensive comparisons are crucial for the advancement of preventative strategies and response mechanisms.

9. Characteristics of tsunamis: wavelength, speed, energy, and destructive power

Tsunamis, often incorrectly referred to as tidal waves, are a series of waves with long wavelengths and periods that are typically caused by large-volume displacements of water due to undersea earthquakes, landslides, volcanic eruptions, or meteor impacts. Understanding the key characteristics of tsunamis – namely their wavelength, speed, energy, and destructive power – is crucial for predicting their behavior and mitigating their impact. This knowledge is particularly relevant for communities living in coastal areas, where the effects of tsunamis are most devastating [69].

9.1 Explanation and importance of these characteristics

• Wavelength: The wavelength of a tsunami refers to the distance between successive wave crests. Tsunamis have exceptionally long wavelengths, often exceeding hundreds of kilometers, which distinguish them from typical wind-generated waves with lengths of just tens of meters [70]. The long wavelength means tsunamis are classified as shallow-water waves, regardless of the depth at which they travel. This property impacts the way tsunamis behave as they approach shorelines, with implications for the resultant flooding and destruction.

• Speed: Tsunami speed depends on the depth of the water in which it’s traveling. In the open ocean, tsunamis can reach incredible speeds of over 700 km/h, roughly the speed of a jet airplane. This high speed allows tsunamis to cover vast distances across ocean basins in a relatively short time [71].

• Energy: Tsunamis carry enormous energy, sourced from the initial geophysical event that triggered them. This energy is initially spread across the wave front as it propagates through the ocean. As a tsunami nears the shore and the water depth decreases, wave speed drops, and wave height increases as the energy is compressed into a smaller volume of water. This compression often leads to the devastating impact of tsunamis when they hit land [72, 73].

• Destructive Power: The destructive power of a tsunami is determined by a combination of its size, speed, and the local topography of the land it impacts. The long wavelength and high energy lead to extensive inundation of coastlines. Moreover, tsunamis do not arrive as a single destructive wave but as a series, with successive waves causing additional damage [72, 73].

9.2 How these properties change as tsunamis approach shorelines

As tsunamis travel from the deep ocean toward the shore, their characteristics transform dramatically, largely due to decreasing water depth. Understanding these transformations can help predict the likely impact of a tsunami on coastal regions. As the depth of water decreases, the speed of the tsunami reduces due to drag along the seafloor. This slowing down of the tsunami wave causes the energy of the wave to be compressed into a smaller volume, causing an increase in the wave’s height – a process known as wave shoaling. The wave can grow to be many meters high, far exceeding typical ocean waves [69, 74]. The long wavelength of tsunamis also results in wave run-up on the shore, which is the vertical height above sea level that a wave reaches up a slope or structure. The run-up can exceed the height of the tsunami in deep water by many times, resulting in further inundation and potential damage. The interaction of a tsunami wave with the coastal topography can also create local effects that influence its impact. For example, bays, inlets, and the shape of the seafloor can focus or defocus the tsunami energy, significantly influencing the height and power of the tsunami at the point of impact [70, 71]. In summary, understanding the characteristics of tsunamis and how they change as tsunamis approach shorelines is crucial for the development of accurate tsunami warning systems and the implementation of effective mitigation strategies. It allows for a more precise prediction of the areas at risk, the potential extent of inundation, and the likely destructive power of an incoming tsunami.

10. Coastal impact and inland penetration

Tsunamis pose a significant threat to coastal communities around the world. Once these waves reach the shoreline, they can cause widespread devastation, loss of life, and extensive economic damage. Understanding how tsunamis interact with coastal features and penetrate inland is key to effective disaster management planning and risk mitigation. This includes understanding the run-up process, the extent of inland penetration and inundation, and learning from past events such as the 2004 Indian Ocean Tsunami.

• The Run-up Process: What Happens When a Tsunami Reaches the Shore? The run-up process begins when a tsunami wave reaches shallow water near the coast. At this point, the wave slows down and starts to grow in height due to the wave shoaling effect. This height increase can be amplified by local topographic features, such as the shape of the seafloor, coastal embayment’s, and offshore reefs. As the wave reaches the shoreline, it continues to rise above the normal sea level, a phenomenon known as run-up. The vertical height of the wave above the mean sea level at its maximum inundation point is referred to as the run-up height. This is typically the most destructive part of a tsunami, as the water rushes onto land, causing extensive damage to infrastructure and loss of life [70, 75]. The run-up process is influenced by a range of factors, including the characteristics of the tsunami (e.g., wave height, period, and wavelength), the bathymetry (shape of the seafloor), and the local coastal topography. Given the complexity of these interactions, numerical models are often used to predict the run-up height and extent of inundation in specific coastal settings [76].

• Inland Penetration and Inundation Extent: The extent of inland penetration and inundation of a tsunami wave is largely determined by the local topography, the tsunami’s energy and momentum, and the coastal land use and vegetative cover. Flat, low-lying areas are particularly vulnerable to extensive inland penetration and inundation. Coastal vegetation, such as mangroves, can provide some level of natural defense by absorbing energy and reducing the speed of the incoming water, although their effectiveness depends on the tsunami’s size and the vegetation’s density [77]. The process of inundation extends beyond the initial impact of the tsunami wave. As the wave retreats toward the sea, it carries with it debris and sediment, causing further damage and reshaping the coastal landscape. Additionally, tsunamis often consist of multiple waves, each with the potential for run-up and inundation. Therefore, the damage from a tsunami event can continue for several hours after the first wave impact. Predicting the extent of inland penetration and inundation is crucial for coastal risk management. Numerical models are commonly used for these predictions, incorporating data on tsunami characteristics, bathymetry, and coastal topography. However, these models require validation and calibration based on field observations from past tsunami events [75, 76].

• Case Study: 2004 Indian Ocean Tsunami: The 2004 Indian Ocean Tsunami, triggered by one of the most powerful earthquakes in recorded history off the west coast of northern Sumatra, offers a stark illustration of the devastating impact of tsunamis on coastal communities. The earthquake, with a magnitude of 9.1–9.3, generated a tsunami that radiated across the entire Indian Ocean, causing widespread damage and loss of life in 14 countries. In terms of the run-up process and inland penetration, the impacts varied greatly across the affected regions, reflecting differences in local topography, coastal features, and the magnitude of the tsunami waves. The highest recorded run-up height was over 50 meters on the west coast of Sumatra, near the earthquake epicenter. On the coasts of Thailand, Sri Lanka, India, and the Maldives, run-up heights of 2 to 15 meters were commonly reported [78]. The extent of inundation was also highly variable, reaching up to 5 kilometers inland in Sumatra’s flat coastal plains. In other areas, the inundation was less extensive but still caused significant damage due to the high population density and extensive infrastructure along the coasts [79]. The 2004 Indian Ocean Tsunami underscored the need for comprehensive tsunami risk management, including early warning systems, public education, and land-use planning in coastal areas. It also highlighted the importance of understanding the complex processes of tsunami run-up, coastal impact, and inland penetration for predicting the potential impacts of future events and developing effective mitigation strategies.

11. Mitigation measures and warning systems

Tsunamis, due to their destructive nature and potential for significant loss of life and property, necessitate robust mitigation measures and early warning systems. Effective warning systems for tsunamis involve a combination of seismic activity monitoring, deep-ocean assessments, and coastal sea-level monitoring. Figure 3 encapsulates the intricate system designed for tsunami monitoring, early warning, and hazard assessment, detailing the sequence from seismic detection to community response.

Figure 3.

Tsunami monitoring, early warning, and hazard assessment [80].

Panel a of Figure 3 depicts the initial phase where an earthquake occurs, causing a shift in the seafloor and potentially generating a tsunami. This seismic activity is detected by ocean-bottom sensors, which relay information to seismic sensors on land. The data collected is critical for determining the magnitude of the earthquake and the likelihood of a tsunami.

In Panel b, the focus shifts to the underwater topography and fault dynamics that contribute to tsunami generation. The diagram illustrates an earthquake occurring along a shallow dip fault, with parameters such as the rupture patch length and width, and the sediment layers that might influence the nature of the seismic event. The accretionary wedge, splay faults, and plate roughness are labeled, showcasing the complexity of the ocean floor where tsunamis originate.

Panel c is divided into two key components of tsunami risk management: the forecast and warning system, and the prediction and long-term assessment. In the forecast and warning section, a flowchart illustrates the progression from a monitoring network, which could include instruments like DART buoys, to the broadcast of warnings through various communication channels, culminating in the evacuation of people to safer areas. The lower part of Panel c highlights the importance of historical data for predicting future events, inundation mapping for understanding potential impact areas, and the development of protection and relocation strategies to minimize risk.

Together, these visuals in Figure 3 present a comprehensive overview of the sophisticated approach to managing the threat of tsunamis, from undersea detection to strategic community planning. This multi-tiered strategy is vital for safeguarding coastal regions from the often catastrophic consequences of these formidable natural events.

11.1 Traditional and modern tsunami warning systems

Historically, people living in tsunami-prone regions have relied on natural signs, such as the sudden recession of the sea, ground shaking, or specific animal behaviors, to warn of an incoming tsunami. However, these traditional warning signs provide little advance warning and may not be recognized by all community members, especially visitors or those who are asleep or indoors. Modern tsunami warning systems, in contrast, leverage scientific and technological advancements to rapidly detect seismic events, determine their tsunami generation potential, and disseminate warnings to potentially affected communities. These systems typically involve a network of seismometers to detect undersea earthquakes, tide gauges and deep-sea pressure sensors to measure changes in sea level and wave characteristics, and computer models to forecast tsunami propagation and inundation. Upon detection of a potentially tsunamigenic earthquake, the warning system analyzes the seismic data to estimate the earthquake’s location, depth, and magnitude. If the event is deemed to have high tsunami generation potential, a tsunami warning is issued, typically through a range of channels to ensure broad dissemination. This may include sirens, broadcast media, text messages, and social media alerts [81, 82].

11.2 Coastal defenses and their effectiveness

A range of coastal defenses can be employed to mitigate the impacts of tsunamis, from natural systems such as mangroves and coral reefs to engineered structures such as seawalls and breakwaters. The effectiveness of these defenses varies widely and depends on the size of the tsunami, the specific characteristics of the coastal area, and the design and maintenance of the defense structures. Engineered defenses, such as seawalls and breakwaters, are designed to absorb the energy of incoming waves and prevent or reduce inundation. While these structures can be effective against smaller tsunamis, they may be overwhelmed by larger events, as was the case in the 2011 Tohoku Earthquake and Tsunami in Japan. Moreover, these structures can be expensive to build and maintain, and they can have negative impacts on coastal ecosystems and beach accessibility. Natural defenses, such as mangroves, coral reefs, and sand dunes, can also protect against tsunamis by absorbing wave energy and reducing the speed and height of incoming waves. These natural systems offer the added benefits of enhancing coastal biodiversity, sequestering carbon, and providing recreational opportunities. However, their effectiveness is limited by their extent and density, and they may not provide sufficient protection against large tsunamis [21, 83].

11.3 Community preparedness and education

In addition to warning systems and physical defenses, community preparedness and education are critical components of tsunami risk mitigation. This includes developing evacuation plans, conducting regular drills, educating community members about tsunami risks and response strategies, and building resilient infrastructure. Evacuation plans should include clearly marked evacuation routes and assembly points, and they should be designed to ensure that all community members can reach a safe location within the available warning time. Regular drills can help to familiarize community members with these plans and identify potential issues before a real event occurs. Education programs can raise awareness of tsunami risks, provide information on natural warning signs and appropriate response actions, and promote a culture of preparedness. These programs can target various audiences, including students, residents, and visitors, and they can utilize a range of formats, from school curriculums and community workshops to informational signs and online resources [84, 85].

11.4 Case study: tsunami warning and mitigation in Japan

Japan, as a country located in a highly seismic region, has developed one of the most advanced tsunami warning and mitigation systems in the world. This includes a dense network of seismometers and sea-level monitoring stations, sophisticated tsunami forecasting models, extensive coastal defenses, and comprehensive community preparedness programs. In the wake of the 2011 Tohoku Earthquake and Tsunami, Japan has made significant improvements to its tsunami warning system, including installing additional deep-sea pressure sensors and developing improved tsunami prediction models. Despite these advances, the 2011 event underscored the limitations of engineered defenses and the importance of timely and accurate warnings. The event also highlighted the need for continued improvements in community preparedness, particularly in terms of evacuation planning and public education [86, 87].

12. Emerging research and future directions

The past two decades have seen significant advancements in our understanding of tsunamis and their triggers. This progress has been driven by several major tsunami events, which have provided new data and insights, as well as technological advances in fields such as seismology, oceanography, and numerical modeling. In this section, we review some of the key developments and outline potential directions for future research.

• Recent Developments in Understanding Tsunami Generation: Several recent developments have improved our understanding of the processes involved in tsunami generation. These advancements have been facilitated by an increase in the quality and quantity of data available for analysis, as well as advancements in numerical modeling techniques. A significant development is the recognition that non-seismic sources, such as landslides and volcanic eruptions, can also generate tsunamis. This understanding has been advanced by events such as the 1958 Lituya Bay landslide in Alaska and the 2018 Anak Krakatau eruption in Indonesia, which generated significant tsunamis. Researchers are now investigating the specific conditions under which these non-seismic events can trigger tsunamis, and how the characteristics of these tsunamis may differ from those triggered by seismic events [88, 89]. Another area of advancement is in our understanding of how the characteristics of seismic events, such as their magnitude, depth, and type, affect tsunami generation. For instance, researchers have found that larger magnitude earthquakes are more likely to generate tsunamis and that the depth and type of the earthquake can also play a crucial role. In addition, there have been significant advances in the modeling of tsunami generation and propagation. These models incorporate complex bathymetric and topographic features, as well as realistic representations of the seafloor displacement caused by seismic events. Such models can help researchers better understand and predict the behavior of tsunamis and are an important tool for tsunami hazard assessment [21, 90].

• Directions for Future Research: There are several areas that warrant further exploration in future research. One of these is a deeper understanding of non-seismic tsunami sources. While significant progress has been made, there is still much to learn about the conditions under which landslides, volcanic eruptions, and meteorological events can generate tsunamis and how these tsunamis may differ from those caused by seismic events. Another area of potential research in the development of more accurate and comprehensive models of tsunami generation and propagation. Recent advancements in computational power and artificial intelligence offer promising opportunities for the development of high-resolution, multi-dimensional models that can accurately simulate complex tsunami behaviors [91, 92]. In addition, future research could focus on the impact of tsunamis on coastal and inland areas. This could involve investigations into the process of tsunami run-up and inundation and the development of methods for assessing the vulnerability of coastal communities and infrastructure to tsunami hazards. Finally, there is a need for research that translates scientific understanding of tsunamis into effective mitigation strategies. This could involve the development of effective tsunami warning systems, the design of tsunami-resistant infrastructure, and the implementation of community education programs to enhance public awareness and preparedness for tsunamis [93, 94].

• Improved Understanding of Non-Seismic Tsunami Sources: While seismic events remain the most significant source of tsunamis, recent events have underscored the importance of non-seismic triggers, such as landslides and volcanic eruptions. There remains much to learn about these triggers, particularly in terms of their frequency, geographic distribution, and tsunami-generating potential. Future research in this area could provide valuable insights for hazard assessment and mitigation efforts [90].

• Advancements in Tsunami Modeling: There is an ongoing need for advancements in tsunami modeling. This includes the development of more accurate and comprehensive models of tsunami generation, propagation, and inundation, as well as the integration of these models into real-time warning systems. Recent advances in high-performance computing and artificial intelligence offer promising opportunities in this regard [92].

• Enhanced Coastal Defenses and Evacuation Planning: Effective tsunami mitigation requires a multifaceted approach that combines physical defenses, such as seawalls and tsunami evacuation buildings, with well-planned evacuation strategies. Research in this area could focus on the design and evaluation of these measures as well as the development of innovative solutions, such as floating tsunami shelters [85].

• Strengthening Community Preparedness and Response: Given the potentially devastating impacts of tsunamis, it is critical to enhance community preparedness and response. This could involve research on risk perception and behavior, the development of effective education and awareness programs, and the evaluation of communication strategies for tsunami warnings [94].

13. Conclusion

In this comprehensive exploration of tsunami generation and impact, we have delved deeply into both seismic and non-seismic sources, underpinning the significant threats they pose to global coastal communities. Tsunamis, distinguished from regular ocean waves by their extraordinary energy and far-reaching impacts, are complex natural phenomena that demand our undivided attention and rigorous scientific investigation.

• Seismic activities, particularly undersea earthquakes, have been recognized as predominant triggers of tsunamis. Throughout this chapter, we have explored how the magnitude, depth, and type of seismic activity influence tsunami generation. The 2011 Tohoku Earthquake and Tsunami served as a key case study, highlighting the destructive potential of seismic tsunamis and the profound effects they can have on ocean floor topography. We have also delved into the dynamics of foreshocks, mainshocks, and aftershocks and their respective roles in tsunami genesis.

• In the realm of non-seismic triggers, we have examined the potent potential of landslides, volcanic eruptions, and meteorological events in generating tsunamis. Each of these triggers presents unique challenges and necessitates further research for a comprehensive understanding. The case study of the 1958 Lituya Bay Landslide and Tsunami underlines the profound potential for destruction these triggers can possess.

• We have navigated the complex terrain of modeling tsunami generation from these diverse sources. Despite the inherent challenges, advances in technology and methodology are continually enhancing our ability to predict and respond to these devastating events. This chapter has also underscored the crucial roles of bathymetry and tectonic structures in tsunami generation, emphasizing the necessity of considering these factors in our predictions and preparations.

• When it comes to non-seismic triggers, our discussion has underscored their significance in the broader context of tsunami generation. The devastating impact of the 1958 Lituya Bay Landslide and Tsunami, examined as a key case study, highlights the importance of extending our focus beyond earthquakes to include other triggers such as landslides, volcanic eruptions, and even meteorological events. However, these non-seismic sources pose unique challenges due to their varied nature, thereby necessitating specialized investigation and predictive modeling techniques.

• The endeavor to model tsunami generation from non-seismic sources is undoubtedly complex but vital for improving our understanding and predictive capabilities. Advances in technology and computational methods have propelled progress in this domain, but the path toward accurate and reliable prediction is paved with continuous exploration and learning.

• Moreover, the intricate roles of bathymetry and tectonic structures in tsunami generation cannot be overlooked. The bathymetric features of an area can dramatically influence how a tsunami forms and propagates, while tectonic structures may serve as hotspots for both seismic and non-seismic tsunami triggers. This understanding has significant implications for risk assessment, disaster preparedness, and mitigation strategies in coastal regions worldwide.

• In the face of these complexities, this chapter has reinforced the need for interdisciplinary collaboration. From geophysicists and oceanographers to engineers and disaster management experts, the collective efforts of many disciplines are required to piece together the jigsaw puzzle that is tsunami genesis. Our quest for understanding is far from over, but each new piece of knowledge takes us one step closer to the goal: the protection of life and property against the devastating impacts of tsunamis.

• It is crucial to acknowledge that non-seismic triggers also hold the potential to generate tsunamis, sometimes with catastrophic effects. Landslides, volcanic eruptions, and meteorological events can induce tsunamis, contributing significantly to our understanding of these natural disasters’ diverse origins. The 1958 Lituya Bay Landslide and Tsunami serve as a testament to the substantial impacts of these non-seismic triggers, further reinforcing the need for comprehensive investigations and advanced modeling techniques that consider these distinct sources.

• Modeling tsunami generation, particularly from non-seismic sources, remains a complex yet essential endeavor, pivotal for enhancing our predictive capabilities and devising effective disaster management strategies. As technology advances and computational methods become more refined, the progress made in this realm can significantly improve the accuracy and reliability of our models.

• The comprehensive roles of bathymetry and tectonic structures in tsunami generation warrant profound attention. These aspects can significantly influence the formation, propagation, and subsequent impact of tsunamis, with important implications for disaster risk assessment, preparedness, and mitigation efforts in vulnerable regions worldwide.

• In closing, this chapter highlights the importance of continuous vigilance and research in our quest to understand tsunamis. Although significant strides have been made in our comprehension of these natural disasters, there is still much to uncover. By persistently enhancing our research techniques, refining our models, and advancing our disaster management strategies, we can progressively mitigate the powerful impacts of tsunamis. It is hoped that the insights shared in this chapter will inspire ongoing research and contribute to a deeper understanding of tsunamis, ultimately promoting the safety and well-being of vulnerable coastal communities worldwide.

• In conclusion, this chapter underscores the vastness of the field of tsunami studies and highlights the essentiality of continuing research efforts. It emphasizes the necessity or not only understanding the geophysical mechanisms of tsunamis but also the critical need for preparing societies to better cope with the impacts of these often-devastating natural events. The hope is that the knowledge gleaned from this chapter, combined with future research efforts, will lead to more accurate tsunami prediction methods, more effective mitigation strategies, and ultimately safer coastal communities.

Acknowledgments

The author would like to express gratitude to all the researchers and scientists who have contributed their valuable work to our collective understanding of tsunamis. Their diligent efforts form the foundation upon which future research can be built.