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Source mechanism of the 23 October, 2011, Van (Turkey) earthquake (Mw = 7.1) and aftershocks with its tectonic implications
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2012
Received: 19 December 2011
Accepted: 5 May 2012
Published: 26 November 2012
This study has investigated the rupture process of the 23 October, 2011, Van (Turkey) earthquake (Mw = 7.1) by using inversion of teleseismic waveform analysis and its tectonic implications. Focal parameters of the main shock and 21 aftershocks were obtained by using the first motion polarities of regional P-waves. The first results for the source rupture process were derived from broadband teleseismic P-waves. The main outcomes of the analysis are: (a) the main rupture is located around the initial break point, and the maximum slip amount was 3.6 m; (b) the size of the main fault plane area was about 40 km in length and 20 km in width, the duration of rupture was approximately 19 seconds and the seismic moment of the earthquake was estimated to be 5.53 × 1019 N m (Mw = 7.1); (c) the rupture gradually expanded near the hypocenter and propagated both northeast and southwest, but mainly to the southwest. Tectonic implications of the earthquake were defined by field observations. The 23 October, 2011, Van earthquake occurred on a main thrust fault plane trending NE-SW between Lake Van and Lake Erçek located in the East Anatolian compressional province. This main fault plane and the secondary structural elements were generated by a continental-continental collision taking place in a region located 200 km north of the the Bitlis-Zagros Suture Zone.
The tectonic setting of Turkey and east Anatolia is the main factor in earthquake occurrence. The movement of the Arabia plate towards the Eurasia plate occurring along the Bitlis-Zagros Suture Zone (BZSZ) has been continuing from Serravalien (~12 Ma) to the present (Fig. 1(a)). This time interval is called the Neotectonic period for the East Anatolia region (Şengör and Yılmaz, 1981; Dewey et al., 1986; Koçyiğit et al., 2001). The movement rate of the Arabia plate towards the Eurasia plate is about 20–30 mm/year according to GPS measurements (Reilinger et al., 2006). The effects of the continental-continental collision in East Anatolia prolongs to the north of the BZSZ, as E-W slicing thrusts (Şaroğlu and Yılmaz, 1986). The collision created by the N-S movement of two continental crusts developed some structural elements related to the collision in the East Anatolian plate. The most important ones are the North Anatolian Fault Zone (NAFZ) and the East Anatolian Fault Zone (EAFZ) (Fig. 1(a)). Besides, the impacts of the collision are observed as inter-continental synthetic back thrust faults, right and left strike-slip faults located in a region extending from the BZSZ to approximately 200 km north of the BZSZ (Şengör et al., 1985). Additionally, the secondary tensional cracks and normal faults parallel, or at an angle, to the reverse faults developed during the thickening of the crust as a result of the collision of the continental plates. Similar features has been presented by Friedrich (1993) and Yin et al. (2008).
The Eastern Anatolia region, where all of these structural elements are observed, is defined as a compression region (Şengör and Kidd, 1979; Yılmaz et al., 1987; Yılmaz, 1990). The Lake Van Basin which is a ramp basin developed as a result of the activities of the thrust faults in the Eastern Anatolia compression region includes secondary normal faults and strike-slip faults (Şengör et al., 1985; Bozkurt, 2001; Koçyiğit et al., 2001; Fig. 1(b)).
Historical and Instrumental period earthquakes (Ergin et al., 1967; Alsan et al., 1975; Soysal et al., 1981; Ambraseys, 1988; Eyidoğan et al., 1991; Turkelli et al., 2003).
Macroseismic Lat.-Lon. (°)
Instrumental Lat.-Lon. (°)
Van, Bitlis, Muş
Van, Bitlis, Muş
The purpose of this study is to define the tectonic system that has deformed the region by using the surface rupture data, and fault plane solutions of the main shock and aftershocks of the 23 October, 2011, Van earthquake that occurred in the eastern Anatolian compression region. The source rupture processes of this earthquake were analyzed using teleseismic P-waves collected by the Data Management Center of the Incorporated Research Institutions for Seismology (IRIS-DMC). The source parameters of the 21 aftershocks (3.5 < M < 5.7) have been derived by using the first motion polarities of regional P-waves collected by KOERI. The fault plane solutions of the earthquakes that occurred in the seismogenic zone of the continental crust were carried out in detail. The results of the fault plane solutions were compared with the surface rupture geometry to reveal the active tectonic model of the Lake Van Basin.
2. Field Observations
Another surface rupture of 4-km length, located between Kozluca and Yalnızağaç villages in the west of Lake Erçek, indicates a right step-over transpressional left-lateral strike-slip fault geometry (Fig. 3(c)). The strike of the second fault varies N-S and N15°E. It was mainly followed in fields and grasslands, so the left-lateral offset was only measured as 0.08 m at one location in the village of Yalnızağaç. This fault caused rockfalls at the northwest of Lake Erçek and does not continue northeast of the lake. In addition, triangular facets extending in a N-S direction and left-lateral offsets in the E-W ridges-valleys were observed between high rugged morphology and low plain morphology between the villages of Satıbey and Kolsatan.
Several secondary structural features of the region are observed in the northern block of the main fault. These features are seen in Erciş, and the villages of Gülsünler and Göllü, located around the Van-Erciş Highway. These secondary structural features were observed in Erciş with a N10°–40°E direction as en-echelon tensional ground cracks with a maximum 2-m zone (Fig. 3(d)). The maximum vertical offset was 0.3 m. Although the rupturing plane was not circular, down blocks were back-tilted to the rupturing plane. This surface rupture geometry is observed discontinuously in an east-west direction between the villages of Gülsünler and Göllü. The southern blocks, which moved downward, were observed along the secondary surface ruptures. The same type ruptures had been observed in a delta located in Gölcük-Kavaklı near the western fault end of the 1999 Kocaeli (Turkey) earthquake, and was defined as the Kavaklı normal fault (Barka et al., 2002). In addition to the secondary surface ruptures, landslides and liquefactions were widely observed in the northern block of the region (Fig. 3(e)).
3. Seismological Data Base
4. Focal Parameters of the Main Shock and Aftershocks
1-D velocity models derived by Kalafat et al. (1987) and used in this study.
V p (km/s)
V s (km/s)
In addition, the fault plane solutions were compared with the surface ruptures mapped in the field. The strike-slip faults along with the thrust faults that were also determined by the fault plane solutions of the aftershocks, and tensional cracks possibly related to the normal or left-lateral strike-slip faults, were observed especially in the overlapping northern block due to the NW-SE compression among the micro-scaled continental plates. A similar fault plane solution pattern was observed in the 1952 Kern county earthquake which occurred on the Pleito thrust fault in the north of the San-Andreas fault, which presented both normal and thrust faults with a strike-slip offset component (Webb and Kanamori, 1985).
5. Teleseismic Waveform Analysis of the Main Shock
The Green’s functions have been computed using the Kikuchi and Kanamori (1991) method. The ray is incident almost perpendicularly on the receiver at teleseismic distances, and is not affected by near source crustal effects, so we used a standard JB crustal model to compute the theoretical waveforms. A Q filter was used with the attenuation time constant t p = 1 s for P-waves and t s = 4 s for S-waves. The moment tensor solution indicate that the 23 October, 2011, Van earthquake has a reverse faulting mechanism with a small amount of left-lateral strike-slip component (Fig. 7(b)).
To obtain the slip distribution, a single fault plane was assumed for the waveform analysis. The initial size of the fault plane was taken to be 75 km × 25 km from the aftershock distribution, and the rupture was assumed to start at the hypocenter of the mainshock. According to the right-hand rule, the strike and dip angle-directions were assumed to be 246° and 46°NW respectively, referring to the moment tensor solution obtained in this study.
Theoretical Green’s functions were computed for simple layers and were referred to the Jeffreys-Bullen model, using Kikuchi and Kanamori’s (1991) method for all stations; this is due to the fact that it is generally expected that the observed seismograms are less affected by local site effects in the teleseismic range. The spatiotemporal distribution of slip on the fault plane was inverted by the teleseismic body-wave inversion program developed by Yoshida et al. (1996) and Yagi and Kikuchi (2000). For discretization in space, the fault plane was divided into 15 in the strike direction and into 5 in the down-dip direction (making a total of 75 subfaults with an area of 5 km × 5 km). The slip rate function of each subfault is expanded into a series of 2 triangle functions with a rise time of 1.0 seconds. The rupture front velocity of 3.0 km/s was selected by trial and error, which determines the initiation time of the basis function at each subfault, that minimizes the residuals between the observed and predicted waveforms. To suppress instability or excessive complexity, a smoothing constraint was applied to differences in the moment release.
In the total slip distribution, a large asperity area can be seen with a large slip in the hypocentral area of the fault plane. The rupture is very smooth and gradually expands near the hypocenter and propagates bilaterally in the directions northeast and southwest (but mainly to the southwest). The main moment release areas are located at and around the hypocenter. The rupture front also reached a shallower part of the fault plane (asperity area) about 9 seconds after the rupture initiation. In the moment-rate function, a small peak occurs after about 15 s. This peak is due to asperity in the lower corner of the fault plane (Fig. 8(d)). In some broadband seismograms presented in Fig. 9, this peak is recognizable, too. According to these results, an asperity was broken at 15 s after the focal time of the main shock at a distance of about 40 km away from the hypocenter. A barrier with higher stress (or lower stress) and with a width of about 15 km was located between the asperity and the main slip area. The rupture jumped over this barrier with a velocity comparable to the rupture velocity.
The distribution of the slip vectors indicates that a thrust fault mechanism with a small left-lateral strike-slip component (assuming the NE-SW plane is the active plane) occurred near the hypocenter and in deep parts. However, a small left lateral strike-slip is dominant in shallow parts of the fault plane mainly NE of the epicenter. The moment release and displacements rates in the shallower parts of the fault plane are smaller than the deeper parts of the fault plane. This situation could reduce the chances of a continuous surface rupture. If the rupturing of the fault plane reached the surface, the left-lateral strike-slip component could be mainly seen at the northeast of the main fault.
6. Tectonic Model and Interpretations
The ophiolitic rocks emplaced to the Lake Van Basin along the thrust planes during the Paleotectonic period (before Neogene) constituted the rugged mountainous morphology in the region (Yılmaz et al., 1993; Parlak et al., 2000). This continental-continental collision type plate motion creates thrust faults with lengths less than the length of the BZSZ. Continuation of the compression during the Neotectonic period in the region developed several folds and thrust faults in the Neogene and Quaternary units deposited in the Lake Van Basin.
The thrust faults in the Lake Van Basin have the potential to produce earthquakes with magnitude greater than six. The thrust faults cut the Plio-Quaternary deposits especially in the north of Van city center (Örçen et al., 2004). Thereby, these faults are likely to be active. Additionally, the minimum length of these faults is about 10 km, so each of these faults has the potential to generate an earthquake of magnitude greater than six (M > 6) (Wells and Coppersmith, 1994). One of the thrust faults, which has a fault plane dipping towards the NW, ruptured in the Van Lake Basin and created the Van earthquake. This rupture, which took place along the intra-continental thrust fault, developed secondary structural features such as left-lateral strike-slip faults with transpressional component and tensional cracks. The reason for this is that the widening areas related to the NE-SW left-lateral faulting could occur as a result of the NW-SE compression of the region. The fault-plane solutions of the main shock and the field observations proved that the 23 October, 2011, Van earthquake was generated by an intra-plate thrust fault with a NE strike and NW dip. Additionally, the rupture of this fault plane developed the secondary faults and created the aftershocks and several tensional cracks on the ground surface or beneath the ground surface.
7. Conclusion and Discussions
This study has investigated the rupture process of the 23 October, 2011, Van (Turkey) earthquake by the inversion of teleseismic waveform analysis and its tectonic implications. The teleseismic data set does not provide details of the slip distribution, but it provides the same general characteristics as other data sets such as near field, or strong ground-motion, data (Hartzell and Heaton, 1983; Yagi et al., 2004). Therefore, we discuss the general features of the rupture process and compare the focal mechanisms of the analyzed earthquakes with field observations.
The initiation of rupture is usually described by the first motion polarity solution. However, the rupture direction could change during large earthquakes. The entire rupturing is modeled by slip-distribution modeling, which models the waveform. Therefore, the moment tensor solution is more suitable than the first motion polarity solution in terms of representing the entire rupturing process. Additionally, the strikes of the fault planes obtained from the moment tensor solutions are more consistent with the main surface rupture. The difference between the directions of the fault planes obtained from the first motion polarity solution and the moment tensor solution indicates that the direction of the rupture initiation is different from the entire rupture propagation.
The inverted source process model shows that a large asperity was located on the hypocentral area on the fault plane; with a maximum slip about 3.6 m. The rupture was very smooth and gradually expanded near the hypocenter and propagated bilaterally in the direction of northeast and southwest, but mainly to the southwest. The rupture process of the 23 October, 2011, Van earthquake is characterized by a smooth and bilateral rupture.
First motion results of the focal mechanism of the studied earthquakes.
The surface ruptures observed from Erciş to Alaköy located on the northern block of the main thrust fault were defined as tensional cracks. These surface ruptures could be the structural products of a transtensional left-lateral strike-slip fault, and is an indication of intra crust deformation and suits with the direction of the aftershock pattern. The origin of the tensional cracks observed in the region could be explained by either normal faults or transtensional components, especially of left-lateral strike-slip faults. Both fault types are secondary and “intra-plate” with limited lateral continuation. In other words, the activities of these faults are directly related to the rupturing of the thrust faults in the region. In some regions of the world, there are several examples of both normal and strike-slip faults which developed due to a N-S compression of the region. For example, normal faults that were parallel and semi-parallel to the strike of the Vergent thrust fault System (WTS) developed during the 4 July, 2001, earthquake in the Altiplano region of north Chili in the overlapping continental plate (Somoza, 1998; Farias et al., 2005). In addition, it was indicated that the normal faults could be transtensional faults that created the pull-apart basins in the same region (David et al., 2002). Another example is that several normal faults parallel and semi-parallel to the thrust faults in the north and northeast of the Alborian Basin in Spain developed contemporarily with the overthrust (Roca et al., 2006). Additionally, a graben was formed by the normal faults parallel to the reverse faults of the continental thrust zone and strike-slip faults developed concurrently due to regional compression in the West Europe Carpathian region. Although the reverse faults were dominant in the southeast of the East Carpathian region, normal faults with the same strike were observed in the northeast region (Oszczypko et al., 2006; Picha, 2011). The small-scale normal faults parallel to the reverse fault planes that occurred along with the raising of the hanging wall, and shortening, were determined by the tectonic model of the 2008 Wenchuan earthquake in the Tibet Plateau (China) (You-Li et al., 2010). These normal faults developed as a consequence of both heterogenic stress distribution in the seismogenic zone and thickening and shortening of the overlapping block.
The surface appearances of the normal faults based on the fault plane solutions of the aftershocks are in the form of tensional ground cracks in the study area. These tensional cracks are the secondary structural features developed on the overlapping northern block. The development of the tensional cracks are either related to the normal faults created by the thickening and contraction of the northern block, or the rupturing of the transtensional parts of the intra-continental strike-slip faults created by the compression.
We would like to thank the Rector of Kocaeli University, Prof. Dr. Sezer Ş. Komsuoğlu, and the retired dean of the Engineering Faculty, Prof. Dr. Savaş Ayberk, and Prof. Dr. Mithat Fırat Özer, for their support, and the Rector of Van 100. Yıl University, Prof. Dr. Peyami Battal, for providing us transportation and accommodation during our stay in Van. Also, a special thanks is extended to Assistant Prof. Dr. Özkan Coruk for his contribution during the office work and Serdal Karaağaç for his contributions during the field and office works, and John Pyle for his reviewing effort. We also thank two anonymous reviewers for their helpful comments that improved the manuscript.
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