Ph.D. Theses
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Browsing Ph.D. Theses by Author "Konca, Ali Özgün."
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Item Dynamic earthquake rupture simulations in the sea of Marmara(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2019., 2019.) Korkusuz, Yasemin.; Konca, Ali Özgün.; Özel, Nurcan Meral.The 1912 Mürefte and 1999 Izmit M"7.4 earthquakes are the last devastating events of the western and eastern sections of the Marmara region, respectively. The center of the Sea of Marmara, the region between locations of these two earthquakes, is prone to creating another large earthquake. The main objective of our study is to determine 3D dynamic earthquake rupture scenarios, considering non-planar and heterogeneous stress distribution in the Sea of Marmara. Recent studies show that some segments of the North Anatolian Fault (NAF) beneath Marmara are partially creeping. In this study, it is the first time that we attempt to generate realistic earthquake scenarios by putting constrains on initial stress on the fault using regional stress from earthquake focal mechanisms, in addition to stress release during past earthquakes and strain accumulation during interseismic period using geodetical measurements on slip-rate and locking depth at various segments along the NAF beneath the Sea of Marmara. In order to constrain the regional stress in addition to our previous five cluster analysis a new earthquake cluster is analyzed in the Central Marmara Basin. We use 3D Finite Element Method (PyLith) for dynamic earthquake simulations and tetragonal mesh for better smoothing at the fault bends, which allows us to implement nonplanar fault geometry and initial stress heterogeneity using slip-weakening friction law. We place constraints on initial shear stress from geodetic and seismic studies of locking depth and interseismic strain accumulation. We consider 80 rupture scenarios and calculate slip distribution, rupture velocity and moment magnitude in addition to slip-rate and traction on the fault surface, and displacement and velocity on the ground surface. We find that for the most scenarios possible earthquake magnitude does not exceed Mw7.2. In addition, in none of the possible scenarios we obtain super-shear rupture velocity. We find that depending on the location of the initiation point, asperities in the partially creeping segments and loaded initial stress, the rupture may not extend into the Prince’s Island Segment.Item Dynamic rupture process of the 1999 Düzce earthquake(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2018., 2018.) Bekler, Feyza Nur.; Konca, Ali Özgün.; Özel, Nurcan Meral.Rupture process of large magnitude earthquakes have been generally performed by using a kinematic approach. A typical set of input parameters for kinematic approach includes; fault length, fault depth, rupture velocity, slip distribution and rise time defining the slip velocity time function. Kinematic models have been quite successful in obtaining detailed slip distribution maps of large earthquakes. However, the kinematic models have their own disadvantages. One major disadvantage is that the physics of the kinematic inversion scheme is incomplete. One uses representation theorem and Green’s functions approach to obtain slip distribution without considering the forces and the frictional properties on the fault interface. In fact, it is not clear whether the kinematic models of earthquakes with the inverted slip and rise time distributions are physical plausible. This lack of physical constraint on physical properties and the force balance leads to lack of long-term behavioral property of the fault. Dynamic modeling has been proposed as a new perspective to explain complexity of source parameters, rupture radiation pattern and slip distribution. One way of understanding the dynamic and kinematic mechanism of the earthquake source is to model how the rupture process improves. Hence, proper understanding of this process and appropriate modeling approaches play an important role in seismic hazard and seismic mitigation estimations. On the other hand, the modeling of a dynamic rupture process of an earthquake may provide information on how the limitations on the source can be understood.Item Interseismic behavior along the North Anatolian fault in the Marmara region using 3D structure(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2022., 2022) Yılmaz, Zeynep.; Konca, Ali Özgün.A series of earthquakes occurred along the North Anatolian Fault (NAF) during the 20th century, primarily migrating from east to west. The only part of the NAF that has not broken is under the Marmara Sea. The Main Marmara Fault (MMF), the NAF’s northern branch, is the most active one, with the highest slip rate amongst the several branches of the NAF. Since the seismic gap of ~150 km is beneath the sea, the geodetic data is not sufficient to constrain the full fault coupling, particularly in the Central Marmara. Nevertheless, the current data does imply that the GNSS vectors along the northern coast of the Marmara Sea are smaller than expected. One interpretation is that the MMF has heterogeneous interseismic coupling with creeping and locked segments. Another explanation is that the fault is locked, but the strain is asymmetrically localized around the MMF as a result of the deep basins. In this study, the competing effects of weak interseismic locking of the MMF and deep basins around the fault are studied by developing a 3-D finite element model for the Marmara Region, which includes a realistic topography, the 3-D geometry of the main fault, and basins, and using the geodetic data as a constraint. Our findings show that the deep basins confine the interseismic strain in the fault vicinity, and using a homogeneous half-space model leads to a slight underestimation of the locking depth. Our 3-D model shows that while the basins have some effects on strain localization, the heterogeneity of interseismic coupling is necessary to explain the observed GNSS data. We infer a change in the locking depth at the Ganos Bend between the strongly coupled Ganos and the weakly coupled Western Marmara. Seismic studies also indicate that these two segments vary considerably in background seismicity. The 50 km creeping segment coincides well with repeating earthquakes and higher rates of diffuse seismicity. Variations in regional stresses and earthquake focal mechanisms, including the 2019 Silivri earthquake sequence, are compatible with the dilatational quadrants in the region due to the loading caused by the interseismic creep of the Western Marmara.Item Seismic interactions and related faulting revealed by moderate to large earthquakes in the SE Aegean region(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2022., 2022) Eskiköy, Figen.; Konca, Ali Özgün.; Ergintav, Semih.Southern Aegean and Western Turkey are mainly dominated by significant exten sional regime and crustal thinning. Segmented normal faults and horst graben systems with varying orientations characterize the region. In these types of extensional re gions, seismicity is quite diffuse and fault networks can be quite complex. Thus, it is challenging to obtain the exact distribution of active faults and understand how they interact with each other to accommodate the extension. In order to interpret the fault structures of the region and their interaction, we focused on Gökova and Kuşadası Bay earthquakes due to the existence of recent high-rate seismicity. For the fault interaction interpretations, we used multidisciplinary data set based fault geometry and fault slip models, source mechanism solutions, InSAR time series analysis. We analyzed the 2017, Mw 6.6 Bodrum − Kos earthquake aftershock progression by using InSAR time series and 3 moderate sized earthquakes that occurred in Ula, which is located on-land in the east of G¨okova Bay. Our goal is to reveal the active fault structures on eastern edge of the Gökova Bay, Ula region, by InSAR data modeling of the events, regional seismic waveform inversions and last ten years Mw ≥ 4 magnitude waveform based focal mechanism solutions. As part of this study, 2020 Mw 7.0 Samos earthquake and its aftershocks were also studied. We also applied cluster analysis to identify the changes in waveforms and obtained focal mechanism solutions for most clusters.Item Studying seismotectonics of Eastern and Southern Anatolia using earthquake mechanisms(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2019., 2019.) Güvercin, Sezim Ezgi.; Konca, Ali Özgün.The Anatolia-Aegean domain provides a unique opportunity to explore plate interactions where oceanic subduction, continental collision and transform plate motions are observed simultaneously. High seismicity rates and diversity of the earthquake source mechanisms are the result of the accommodation of these relative plate motions. As the initial tectonic buildup involves the amalgamation of different tectonic units, it is natural that lithospheric segments with varying structural properties in this relatively small region also contributes to the complexities of the observations. Understanding interactions of these plates and related deformation requires an integrated analysis of various observations such as seismic tomography, earthquake slip models, geodetic observations and stress changes along with the seismicity and earthquake source mechanisms. In this thesis, 3 case studies in different tectonic settings are presented: the continental collision in the east, the extension due to roll back in the west and the transition between extension and compression. For these 3 case studies, the relation of earthquake source mechanisms to other seismological and geodetic data is used to better understand the present state of the seismotectonics of Easternmost Mediterranean including eastern Anatolia. The October 23, 2011 Mw7.1 Van, Eastern Anatolia earthquake which is on an EW trending thrust fault, in a region under N-S compression due to the convergence of the Arabian plate toward Eurasia. The three faults were activated during and after the coseismic rupture. The earthquake source mechanisms with consistent orientations are grouped in three clusters. An average fault mechanism is calculated for each cluster by the summation of moment tensors. The triggered faults have experienced Coulomb stress increase due to co-seismic rupture revealing a mechanism which accommodates NS shortening in the region. The June 20, 2017 Mw 6.6 Bodrum-Kos earthquake which occurred on an E-W trending normal fault is related to the roll back effect of Hellenic Subduction. The Bodrum-Kos event revealed that the extension in the western section of Gökova Bay is accommodated by a north dipping fault. Two different fault slip models, dipping to north and south, are used to compute the Coulomb stress changes at different depths. The coherency between the seismicity and the regions of increased stress is used to put a constraint on the dip of the ruptured fault. The gradual change of strikes of aftershock mechanisms from east to west is consistent with the rotation of the strain field region indicating that the observed earthquake pattern during the 2017 earthquake reflects the long term tectonic frame work in the region. In between these compressive tectonics of Eastern Anatolia and extension in the Aegean, Cyprus Arc region acts as a transitional zone which is tectonically less understood. Specifically how the convergence of Nubia toward Anatolia is accommodated remains unclear. By the analyses of novel earthquake source mechanisms, and other seismological and geodetic data, it is proposed that the segmentation of the subducting Nubian Plate has a significant contribution to the lithospheric deformation. The change in the orientations of the earthquake mechanisms around the Isparta Angle determines the eastern boundary of the N-S extension due to roll back of the Hellenic slab and is consistent with the counter clockwise rotation of AnatoliaAegean domain which is revealed by the recent GPS vector field. Thrust mechanism earthquakes along with Bouguer gravity, seismicity, and horizontal GPS velocities reveal the geometry of the subducting slab beneath Antalya Basin towards N-E. We suggest that the Antalya Slab deforms as an isolated block, responding in part to adjacent plates, including the Anatolian Plate that moves toward the west, overriding the remnant Antalya slab.