Abstract
In this study, we analyzed the seismic radiation and deformation characteristics of the 2011 Van earthquake during aftershock events with the support of estimated dynamic parameters (seismic b‐value and radiation efficiency, ηR), 3D crustal cross sections of aftershock hypocenters, and deformation styles of Lake Van basin. The resulted variation in the b‐value exhibits two dramatic changes in the b‐value: one (b > 1) during the first 100 days of the mainshock and the other (b < 1) in the last 70 days of the mainshock. The constant b (b = 1) indicates a seismically active time interval and transitional variation in the b‐value from high to low. The estimated b‐value (b >1) reveals that the aftershock sequence comprised a large number of the small and same‐sized events of the Van mainshock due to the extreme material heterogeneity within the rupture zone. This indicates a general decrease in shear stress and increasing complexity in the focal area. The small value (ηR < < 1) of ηR implies that the amount of energy mechanically dissipated during the Van rupture process is large. This reveals that the microscopic breakdown process dominates the rupture dynamics and the whole Lake Van basin. The 3D crustal images of hypocenters suggest that the Van event originated in a strongly heterogeneous fractured setting with the aseismic sedimentary section of Lake Van. The high b‐value combined with the low radiation efficiency (ηR) shows a strongly faulted‐fractured sediment‐rock formation filled with gas‐fluid. This suggests that the seismic energy is intermittently released in the discrete form of aftershock events which is controlled by nonuniform and highly heterogeneous stresses, associated with the deformation style of Lake Van. The frequent redistribution of flickering stresses and nonlinear deformations in the rupture area increase the b‐value and decrease the radiation efficiency.
Keywords
- eastern Anatolia
- Van earthquake
- aftershocks
- seismic b‐value
- radiation efficiency
- micromechanisms
- nonlinear deformations
1. Introduction
Eastern Anatolia is a seismically active accretionary complex region where the Arabian and Anatolian plates collide (Figure 1a). The Anatolian‐Arabian plate collision takes place along a deformation zone, the Bitlis Thrust Zone (BTZ) (Figure 1a) [1–3]. A westward extrusion of the Anatolian plate is along the two major strike‐slip faults: the dextral North Anatolian Fault (NAF) and the sinistral East Anatolian Fault (EAF) zones. These faults join each other at the Karlıova Triple Junction (KTJ) in eastern Anatolia (Figure 1a). The east‐west trending Mus‐Lake Van (MRB) ramp‐shaped lake basin is a conspicuous tectonic feature in the region and roughly reflects the N‐S compression and W‐E extension (Figure 1a) [1].
Two intraplate (midplate‐related) earthquakes took place within an accretionary complex area, near the Lake Van basin (Figures 1 and 2a), the former 23 October 2011 Van event (
The Van earthquake (
Seismic scaling relation shows that the way of quantifying the mechanical efficiency of earthquakes is with the radiation efficiency parameter (
The aftershocks shown in Figures 1b and 2 associated with the 2011 Van mainshock still continue to occur today and there were several thousands of events from October 2011 to July 2012. In this study, it can be found that the aftershocks of the Van mainshock play a key role in the appearance of strong seismic coupling between basin‐bounding faults, faulting style, and active deformation of Lake Van (Figures 2 and 3) and upper crustal seismogeneity [12–14, 25, and the references therein]. This allows for a detailed investigation of thousands of events in the focal region and provides an objective path to obtain an improved understanding of the stress state of the 2011 Van earthquake. Aftershock magnitudes of
2. Structural setting and tectonic elements of Lake Van Basin
The Lake Van basin is the eastern continuation of the MRB and was separated from it by the Nemrut Volcano (NV) with parasitic volcanic domes (PVD) [1, 2] (Figures 1a and 3). Deformational patterns of the lake have been formed as a result of the tectonic structure of eastern Anatolia [14, 29]. Lake Van was formed through a combination of normal and strike‐slip faulting and thrusting [1, 2, 29]. The strike‐/oblique‐slip deformation in Lake Van caused distinct strike‐slip sedimentation, extensional magma propagation through boundary faults (see Figure 2), upper crustal seismicity, and hydrothermal activity [12–14, 29–33].
The multichannel seismic reflection profiles collected from Lake Van basin [12–14, 21, 25, 34, 35] and bathymetry data [36–38] revealed that the lake basin is dominated by a deep Tatvan basin (TB), north‐eastern delta (NE‐delta), and a south‐eastern delta (SE‐delta) (Figure 3). The lake is completely bounded by steep, oblique boundary faults, namely the northern boundary fault (NM‐bf, transpressive), southern margin boundary fault (SM‐bf, transtensive), western margin boundary fault (WM‐bf, transtensive), and Çarpanak margin boundary fault (ÇM‐bf, transpressive) (see Figure 3 for the sense of shear). In the current study, ÇM‐bf is currently named because of its prominent location close to Fault‐Kalecik Fault (F‐KF) or Van Fault [39]. These marginal faults have a basinward, downthrown side (TB in Figure 3), and gently folded sedimentary sections on the downthrown side of these faults together with some small splay faults (Figure 3).
The location of the 2011 Van mainshock and epicentral distribution of its aftershocks (Figures 1 and 2) occurred in a wedged‐shape area (Figure 3) that is transpressively uplifted by major boundary faults, namely ÇM‐bf (F‐KF) in south and NM‐bf in north (Figures 2 and 3). These faults are prominent structural elements, controlling the dextral faulting regime in the east of the lake and also the 2011 Van mainshock. Ref. [40] gave field evidence for possible dextral faulting, extending roughly in the E‐W direction, east of the eastern shore of the Lake Van basin (Figure 3). These faults have been mapped by several studies such as Lake Erçek Fault (LEF), Kalecik fault [40, 41], inferred Edremit fault (EF) [40, 42], Gevas fault (GF) [43, 44] (Figure 3). These faults are dextral oblique‐slip faults lying east and south‐east of the Lake Van basin [9, 45, 46].
High heat flows, hydrothermal discharges, and CO2‐emissions in Lake Van may give some evidence of seismic ductile events (high
3. Data and methodology
3.1. Used data
The earthquake catalog published by Kandilli Observatory and Earthquake Research Institute [50] of Turkey was used to determine the seismicity pattern of aftershock events as a function of time and develop the clean images of the seismic
3.2. Frequency‐magnitude relationship
The frequency‐magnitude distribution (Eq. (1)) defines the relationship between the frequency of occurrence (foo) and the magnitude of earthquakes. The size distribution of earthquakes is adequately described by the G‐R relation [52]:
where
The frequency‐magnitude distribution (Eq. (1)) and the variation in the
Although the
In this study, the
where
In this study, the standard deviation (
where
Detectable minimum magnitude (
The depth‐dependent variations of the
3.3. Seismic scaling relations
Static stress drop Δ
This ratio can be interpreted as proportional to the energy radiated per unit fault area and per unit slip [74, 75]. In this study, following the approach of Ref. [19],
The radiation efficiency,
where,
The computed
If
4. Results
4.1. The variation in the b ‐value as a function of time
In this study, the estimated
The resulted variation in the
The variation in the
Time and depth‐dependent differences in
As shown schematically in Figure 7, the high and low
4.2. The frequency‐magnitude statistics of the aftershocks
We estimated the
The variation in the magnitude of completeness (
Ref. [27] calculated the exponential and linear plots of the
Ref. [20] reported that
In the previous studies, the observed
4.3. The hypocentral cross sections
A prominent aseismic zone can be observed in the deep central basin of Lake Van (TB), where no remarkable events are recorded (Figure 3). Aseismicity suggests that there is clear evidence of seismic quiescence (SQ) in this area (TB) (Figure 3). This may imply a strong structural asymmetry between the seismic (ÇSZ‐LEF‐LE) and aseismic zones (TB) in Lake Van (Figure 3). This asymmetry is also illustrated by the depth distribution of the epicenters at upper crustal depths (Figure 8). In the hypocentral cross sections in Figure 8, aseismic (TB) and seismic zones (ÇSZ‐LEF‐LE) with strong crustal asymmetry are referred to as seismic quiescence and uplifting (U) zones, respectively (which can be seen by comparing Figure 3 with Figure 8). The aseismic (TB) or seismic quiescence zones are distressed and declustered, while seismic zone (ÇSZ‐LEF‐LE) or the uplifted (U) area are highly stressed and clustered. The SQ zone extends over a thick sedimentary volume of TB surrounding the focal zone, while the U zone is rooted in a fault‐bounded block basement (ÇSZ). Previous seismic reflection studies reported that the deep central basin (TB) of Lake Van is thick depositional province (600 m), onlapping onto the shallowing delta settings (NE‐delta), toward ÇSZ where the deposition is sharply terminated, reaching a thickness of sediments of 150–250 m. Sediment termination is occurred due to the fault‐bounded block uplift (ÇSZ) [12–14]. This finding reflects a typical pattern of strike‐/oblique‐slip deformation and sedimentation [1, 2, 85] and is in good agreement with the high and low
Time‐dependent variability in the
These results indicate that the variation in the
5. Interpretation and discussion
The time‐dependent variability of the
The resulting variation in
5.1. Stress state as a function of time
The magnitude characteristics of the aftershocks, including the low
High‐resolution images of the
5.2. The variability in the b ‐value and the radiation efficiency (η R)
As stated above, the high
Previous studies have stated that a large number of smaller and/or same‐sized aftershock events with a long‐time duration can represent the small values of the radiation efficiency parameter (
5.3. The shallow gas and microseismicity
Various shallow gas indicators have been observed from multichannel seismic reflection and chirp data from Lake Van [21]. Strong evidence of the shallow gas accumulations in Lake Van includes acoustic blanking, pockmarks, seismic chimneys, bright spots, mound‐like features, and enhanced reflections (Figures 9b and c).
The enhanced reflections observed at more than 200 locations suggest the presence of free gas in Lake Van with seismic chimneys, suggesting the emission of gases (Figure 9b). Some of these chimneys have pockmarks (Figure 9c), which may be vertical vents or conduits for active fluid emission. Faults, fractures, and fissures may provide migration pathways for deep gas in TB. The acoustic blanking (Figure 9c) indicates that changes in the hydrostatic pressure may control the formation and preservation of the gas‐charged zones. The mound‐like features (mud volcanoes) (Figure 9c) suggest active gas emission and venting activity. Bright spots, which are a characteristic of gas accumulations, indicate gas‐charged zones (Figure 9c). These pockmarks are only seen in the northeastern part of the lake. The absence of pockmarks in other parts of the lake (Figure 9c) can be due to a higher permeability of the lake sediments [14, 21].
These observations show that the thicker and unconsolidated soft sedimentary section, and the deformation style of Lake Van (turbiditic wedges, debris flows, tephra deposits, and progradational clinoform packages and the steep oblique‐slip boundary faults) [12–14, 21]) have the potential to provide ideal conditions that allow the sediments in TB (Figure 9a) to act as a gas and/fluid reservoir (Figures 9b and c). This suggests that a gas‐soft sediment mixture modulates the preferential emission of deep‐sourced fluids through the faults, fractures, and/or fissures in Lake Van and in particular aseismic TB (Figure 9a). The abrupt changes in the sedimentary section hamper the fluid transport in the sediments of TB. This process of the gas‐sediment‐fluid mixture in Lake Van has the potential of many smaller and same‐sized magnitude aftershocks (microseismicity) that increases
The epicenters of aftershock events are densely clustered in a wedge‐shaped narrow area, bounded by the west‐east extending ÇM‐bf, F‐KF and NM‐bf. This narrow area is a fault‐bounded, uplifted block by ÇM‐bf and NM‐bf (ÇSZ‐LEF‐LE) (Figures 3, 9a, and 10). ÇSZ is interpreted to have been cut by an inferred LEF extending from LE in the east to Lake Van in the west (Figures 3 and 9a). The deformational features of Lake Van, especially ÇSZ, ÇM‐bf, and NM‐bf, are structurally related to the two mainshocks of the Van and Edremit events (Figure 2a) and the variability in the
This study for the first time takes into account the low seismic radiation (high seismic deformation), considering the
6. Conclusions
In this study, the time and depth‐dependent variability in the
The variation in
The main result of the 2011 Van and Edremit earthquakes is that they considerably lowered the general shear stress in the area (high
The small radiation efficiency (
Finally, the seismic uniqueness of the 2011 Van earthquake must be considered. This research is the first to take into account the low seismic radiation (high seismic deformation) and examine the
Acknowledgments
The author thanks all members of the National Earthquake Monitoring Center (NEMC) at the Kandilli Observatory and Earthquake Research Institute (KOERI, Turkey) for the continuous seismological data. The author is grateful to Prof. Dr. Alper Çabuk for helping in using and processing the earthquake data and the seismological laboratory, Prof. Dr. G. Berkan Ecevitoğlu for providing the aftershocks monitoring FORTRAN code and commenting on concluding remarks of this study. The author offers sincere thanks to Leader of Lake Van Project seismic survey, Prof. Dr. Sebastian Krastel (Kiel, Germany) for providing multichannel seismic reflection profiles (International Continental Drilling Program, ICDP‐PaleoVan Project‐2004 funded by Germany Money Foundation, “Deutsche Forschungs Gemeinschaft‐DFG”) collected from Lake Van basin, Prof. Dr. A. M. Celal Sengör for commenting on faulting style and tectonics of the study area and Prof. Dr. Elena Kozlovskaya (Oulu, Finland) for providing the Laboratory of Applied Seismology (Seismic Handler Manual software), University of Oulu. Also, the author offers his greatest thanks to editors and the two anonymous reviewers for their constructive comments and suggestions which helped to improve the manuscript. Some figures are generated by Generic Mapping Tools (GMT) code developed by [89].
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