Crustal Faults Reactivated during 2010 Mw = 8.8 Maule Earthquake in South Chile

Abstract

On February 27th 2010 occurred the Mw = 8.8 Maule subduction earthquake, filling a seismic gap of south Chile. The uplift trend is mostly typical for subduction earthquakes with decreasing uplift trend from trench to arc in Andes Cordillera. However local perturbations occurred due to the reactivations of crustal faults occurred such as Pichilemu fault (normal), Santa María fault (normal) and Tirua-Mocha fault (reverse). Different kind of faults and seismic behavior evidence complex stress distribution at the overriding South American Plate. In this paper, the activity and seismicity linked of some crustal faults at Maule earthquake rupture área are considered, and the related seismic potential that can increase the seismic hazard. Some questions are the bigger magnitude that can generate these faults and if their activity is related to the interseismic or coseismic phases of the subduction seismic cycle.

1. Introduction

Chile is a seismic country due to its tectonic framework, dominated by the subduction of Nazca Plate beneath South American Plate at approximately 7 cm/yr at N77°E trend. The Wadati Benioff zone has N10°E strike and 20°E dip. Geological hazard consequences of this are earthquakes, tsunami, and volcanic activity. Subduction earthquakes of thrust focal mechanism included the biggest ever recorder, the Mw = 9.5 Valdivia earthquake that occurred on May 22, 1960 and the sixth, the February 27, 2010 Mw = 8.8 Maule earthquake. In a same segment of the subduction zone, Mw = 8 earthquakes occurred every 100 years and an Mw = 9 earthquake every 300 years or more [1, 2, 3, 4, 5, 6, 7, 8, 9]. Figure 1 shows some of the main subduction earthquakes and rupture lengths that occurred in central and south Chile, divided in Central Chile, Concepción, and Valdivia segments. The epicenters of the subduction seismicity are located from 10 km east to the trench to 160 km eastward in the littoral zones and hypocenters between 10 and 55 km being deeper eastward [1, 9]. Other Nazca intraplate earthquakes are due to slab pull with normal mechanism [10, 11, 12, 13]. Epicenters of these earthquakes are located 160–270 km from the trench eastward and hypocenters between 100 and 200 km. The Chillán Mw = 8 earthquake of January 25, 1939 [10] was the deadliest earthquake that occurred in Chile with more than 25,000 persons killed. Other significant slab pull earthquakes are the Punitaqui 1997 Mw = 7.1 [12] and Tarapaca 2005 Mw = 7.8 [14]. Other Intraplate earthquakes are located at South American Plate with strike slip mechanism. These events are concentrated in crustal faults along the axis of Andes Cordillera of N-S strike and vertical dip, right lateral movement [15, 16, 17, 18]. Significant earthquakes of such kind are 2001 Mw = 6.3 Aroma earthquake, 2004 Mw = 6.4 Curicó earthquake, and 2007 Mw = 6.2 Aysén earthquake. Due to the shallow hypocentral depths (≤10 km), in mountain areas, these earthquakes generate significant damages due to the landslides triggered.

Figure 1.

Significant earthquakes in central and south Chile divided in three segments: Central Chile, Concepcion, and Valdivia. Right: years and extension of the ruptures of the earthquakes. Blue years: Central Chile earthquakes; green years: Concepcion earthquakes; and red years: Valdivia earthquakes.

However, crustal faults can also generate significant earthquakes or being activated or reactivated during significant subduction earthquakes. The 2010 Maule earthquake activated two crustal faults: the Pichilemu fault of normal mechanism [19, 20, 21], the normal Santa Maria fault [22, 23], and the Tirúa-Mocha fault of thrust mechanism [5]. In this paper, we assess the seismic potential of such faults.

2. Maule earthquake

Maule earthquake affected an area of more than 650 km length between 33 and 38.6°S. Two main asperities are identified, the northern one with main slip of 18 m between 34 and 35°S with depth between 20 and 40 km, and the southern one with main slip of 10 m between 37 and 37.6°S [3, 24, 25, 26]. Before Maule earthquake, a seismic gap between 34 37° S, named the Pichilemu-Concepcion seismic gap [1, 2] was identified. An earthquake of Mw = 8.5 was expected in this area, and the previous big earthquake located in such area occurred in 1835. This seismic gap is characterized by a lack of coastal seismicity in this area (Campos et al. [1]. About vertical changes, an interseismic subsidence is noted, mainly in littoral zones located close to the trench such as Arauco Peninsula and Santa María Island between 37 and 37.7°S [5, 21].

When finally, Maule earthquake filled this gap, several anomalies that are not expected occurred. The Mw = 8.8 magnitude was bigger than the Mw = 8.5 predicted [1, 2], overlapping rupture zones of recent earthquakes [3, 5]. In the north, the 1985 Central Chile Mw = 8 earthquake ruptured also the zone between 33 and 34°S. Uplift of 50 cm was observed in the littoral zone that contrasted with the subsidence in the same zone, which was observed in 2010 Maule earthquake. The southern rupture zone of Maule earthquake was affected also by the Concepcion Mw = 8.1 earthquake of May 21st 1960, the first earthquake of the great 1960 seismic sequence. The place of Lebu (37.6°S) was uplifted 1.2 m during 1960 earthquake, such uplift was recovered between 1960 and 2010 earthquakes and after Maule earthquake, was uplifted again 2 meters. Mocha island (38.3°S) was uplifted 1.5 m during 1960 earthquake, but unlike Arauco Peninsula and Santa María Island (37–37.7°S), the 1960 coseismic uplift remained [5].

The Maule earthquake and related tsunami provoked around 570 inhabitants killed and hundreds injured with extensive damage. Land changes included coastal uplift and subsidence, being decreasing uplift arcward (toward east). The hinge zone between uplift and subsidence was around 123 km in the area ruptured by the northern asperity and around 135 km in the southern asperity, indicating the shallower position of the northern asperity [5].

3. Crustal faults activity triggered during and following Maule Earthquake

3.1 Pichilemu Fault

On March 11, 2010, a series of earthquakes started at Pichilemu zone (34.3°S/72°W) with an Mw = 6.9 event, followed by several events of Mw = 5.5–6.5 in the next minutes. Then a crustal seismic activity continued in such zone to date. The focal mechanism of this earthquake was normal, with strike NW-SE, and the seismic activity is located in an alignment of such orientation with 50 km length [19, 20, 27]. Farias et al. [20] had fieldwork at Pichilemu after Maule earthquake but before March 11 event, indicating 0.2 m uplift. Quezada et al. [21] visited Pichilemu littoral zone in April 2010, and it exists at that date at a subsidence of around 50 cm. For this reason, the true dip of the NW-SE fault is toward SW. The littoral subsidence of the Pichilemu fault is bigger than the former uplift generated by Maule earthquake. The aftershocks of Pichilemu fault are separated from the events of the Wadati-Benioff plane [20, 27, 28]. This fault is explained because their strike is normal to the maximum extensional axis (T axis) of the rupture in Wadati-Benioff zone at this latitude [19].

3.2 Santa María Fault

A normal fault was activated close to the northern tip at Santa María Island (37°S) of NE-SW strike and visible open cracks and scarp until 30 cm height. The length of the trace is 600 m. Not significant seismicity or aftershocks are recognized near to this fault [27, 28]. One explanation of this fault is that this corresponds to a shallow extension due to a deeper splay fault of thrust mechanism [23]. This hypothesis was refused by Allmendinger et al. [22] that considers that this fault was reactivated due to the orientation of such fault normal to the maximum extensional axis (T axis) of Maule earthquake main rupture, at Wadati-Benioff zone in this latitude. In subduction earthquakes, the distribution of the T axis changes being normal to a curve trenchward concave between both tips of the main fault. It is the same explanation of the Pichilemu fault [19].

3.3 Tirua Mocha Fault

The vertical movements at the southern zone of Maule earthquake (38.3–38.4°S) are not typical trend with decreasing uplift from trench to arc. In fact, Mocha Island experienced an uplift of 0.25–0.3 m, whereas Tirua place, located at the continental margin in front of Mocha Island, experienced 0.9 m uplift. This anomaly was explained by the existence of a splay fault between Tirua and Mocha Island [5]. The activity of such fault began after 1960 earthquake due to the development of a strong asperity at the updip of Wadati-Benioff zone. This splay fault is synthetic, west vergent, also thrust movement. The main deformation due to the interplate convergence was in such fault explaining the permanence of the 1960 coseismic uplift at Mocha Island and the big interseismic uplift at Tirúa. The strong asperity remained even during Maule earthquake, and the main slip occurred along Tirua-Mocha splay fault explaining the abnormal uplift trend. Significant earthquakes of Mw = 6 magnitude occurred in such zone in the following months, evidencing the instability conditions, and finally, on January 2, 2011, an Mw = 7.1 earthquake occurred in such area that generated 0.5 m uplift at Mocha island and 0.2 m subsidence at Tirúa, indicating the final removal of the asperity at the updip of Wadati Benioff zone. Hicks and Rietbrock [29] indicated hybrid mechanism with thrust movement along Wadati-Benioff zone and a crustal normal fault. Quezada et al. [5] indicated that such normal fault was the Tirua-Mocha fault that experienced tectonic inversion because the movement is in a shallower position at the Wadati Benioff zone, being favorable conditions of such opposite movement.

3.4 Seismicity linked to the crustal faults

From the Centro Sismológico Nacional-U de Chile, we collected those seismic events occurred between January 2010 and November 2022 around Pichilemu (34°S), Santa María Island (37°S), and Tirúa-Mocha (38.3°S) areas. Around each of these locations, seismicity is exposed on a map view and along an ENE-WSW profile: in both cases, a subduction azimuth of 82° is considered (Figures 2–7). Distribution of seismicity is shown in circles according to its magnitude range: M < 3.5 cyan color, 3.5 < M < 4.5 yellow color, 4.5 < M < 5.5 red color, 5.5 < M < 6.5 green color, and 6.5 < M < 7.5 blue color. Distribution of seismicity and magnitudes in time are exposed in graphs. Here it can be seen that this data set is fully influenced by aftershocks linked with 2010 Maule Earthquake (Mw = 8.8). Error location for seismic events can be considered up to 10 km around determined hypocenter.

Figure 2.

Seismicity at Pichilemu. It is noted in the profile the crustal seismicity in the littoral zone related to Pichilemu fault.

Figure 3.

Seismicity at Santa Maria Island. Shallow seismicity exists between the trench and littoral, but it is difficult to separate crustal from interplate events.

Figure 4.

Seismicity at Tirúa-Mocha Island. Shallow seismicity exists close to Tirúa that could be related to Tirúa-Mocha fault.

Figure 5.

Frequency of events and magnitude at Pichilemu.

Figure 6.

Frequency of events and magnitude at Santa Maria Island.

Figure 7.

Frequency of events and magnitude at Tirúa-Mocha Island.

3.5 Seismicity linked to the crustal faults

As it can be seen in Pichilemu, Santa Maria Island, and Pichilemu areas, it exists as a continuous seismicity from trench to arc (Figures 2–4). However, the vertical profiles show separated clusters. The slab is clearly defined. Crustal seismicity occurs at Andean Cordillera. This seismicity must be related to the strike slip partitioning and compressive crustal faults linked to the growing of this relief. The magnitudes of these events are decreasing from north to south. At Pichilemu latitude (34°S, Figure 2), the biggest magnitude reached Mw = 5.5, at Santa Maria Island latitude (37°S, Figure 3), the magnitude reached Mw = 4.5, and at Tirúa-Mocha Island latitude (38.3°S, Figure 4), the magnitude reached Mw = 3.5. Slab pull seismicity is visible in all profiles with depth hypocenters bigger than 50 km increasing depth eastward illuminating the slab. The number of events and bigger magnitude also occur at the northern profile at Pichilemu.

Between the trench and the littoral zone, important seismic activity being interplate and crustal exists. The Pichilemu profile shows a big amount of seismicity in the littoral zone linked to the Pichilemu fault with magnitudes reaching Mw = 5.5. West of this cluster until the trench, lesser shallow seismicity is noted. This light is better than the littoral seismic zone linked to the Pichilemu fault. The Santa María Island profile also displays shallow seismicity between the trench and littoral zone, but separate crustal seismicity to interplate one is more difficult, but crustal seismicity exists. Magnitudes are lesser than Mw = 5.5. At Tirúa-Mocha Island profile, crustal activity also exists between the trench and Tirúa, but with fewer events. Despite this, the hypocenters are located close to Tirúa and may illuminate the Tirúa-Mocha fault with some events of magnitude reaching Mw = 5.5.

About the frequency of the seismicity, it displayed the number of events every 7 days and the magnitude every 7 days between 2010 and 2022 (Figures 5–7). At Pichilemu (Figure 5), during all 2010 year occurred until 200 events every 7 days decreasing later stabilizing in 10 events since 2013, with some specific days with more activity. Average magnitude of these events is 3, but the maximum magnitudes decreased from Mw = 7 in 2010 to Mw = 5.5 since 2012 with some isolated events later.

The Santa María profile (Figure 6) shows a maximum of 40 events per 7 days in 2010 decreasing to 5 events, but since 2017, a small increase of the amount of events with some days with a big amount of seismicity exists. It is typically when occurred an event of magnitude between Mw = 5–6, then an aftershock sequence takes place related to the first event. Between 2010 and 2012, the average magnitude was between Mw = 3.5–4.5; then until 2021, it was Mw = 3, and since 2022, it increases.

The Tirúa-Mocha profile (Figure 7) shows important seismic activity between 2010 and 2012. Also occurred significant events of magnitudes of Mw = 6–7. The fourth peak corresponds to the Mw = 7.1 Araucania earthquake of January 2, 2011. Then the average magnitude of the events stabilized at Mw = 3.5, and since 2020, the amount of seismicity increased slowly.

4. Discussion

Maule earthquake triggered the rupture of three crustal faults: Pichilemu, Santa María, and Tirúa-Mocha, all with different behaviors. Pichilemu and Santa María faults with normal mechanism can be explained better due to their orientation normal to the maximum T axis of the rupture along Wadati-Benioff zone. We agree with Allmendinger et al. [22] that Santa María fault is not a splay fault. By contrast, Tirúa-Mocha is a splay fault with thrust mechanism. Such fault can be explained due to the development of a strong asperity in the interplate zone after 1960 earthquake, which remained even during Maule earthquake, being the movement accommodated in that zone at the Tirúa-Mocha splay fault, and the asperity at the updip of Wadati-Benioff zone was removed finally during Araucania Mw = 7.1 earthquake of January 2, 2011. It is not easily distinguished seismic activity linked to Santa María fault and Tirúa-Mocha fault. But the large number of events linked to Pichilemu fault are noted. Many of them are felt by the population of Pichilemu and surrounding areas.

It is not clear if these tree faults can generate their own earthquakes, not related with the subduction seismic cycle. These faults appear to be activated or reactivated due to the stress changes of South American plate, after Maule earthquake. The interplate border is irregular, so, during some earthquakes, different asperities can be developed in successive seismic cycles, some at the updip, some at the down dip of Wadati-Benioff zone. The position of these asperities, especially if such faults are located at the updip, can trigger the development or the reactivation of a splay fault. Such asperities can be of different sizes, and depending on this, the position of the T axis can vary in successive earthquakes, and some rupture lengths can generate a geometry of T axis that can be favorable to the normal reactivation of crustal faults [19].

About seismic potential, Pichilemu and Tirua Mocha fault proved to trigger earthquakes of magnitude Mw = 7. Pichilemu is located 100 km from places in which is located half of the population of Chile, including the capital Santiago and Valparaiso and Viña del Mar big cities. The Tirua- Mocha fault is located less than 200 km from two main cities of south Chile: Concepcion and Temuco.

5. Conclusions

Crustal faults can be reactivated during some subduction seismic cycles. Their activity is poorly constrained. Some faults have persistent seismic activity, and others appear to be reactivated only after a major subduction earthquake. Some are normal, others are reverse splay faults. The irregular shape in the interplate zone can develop different asperities in successive subduction cycles, and in some of them, crustal faults are reactivated. It is necessary to know the existence of these faults and the seismic potential. Due to their shallow depth, these crustal faults can be another source of seismic hazard, even the traces can be across populated areas. It is necessary to improve the knowledge of the activity of such faults.