Ph.D. Theses
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Browsing Ph.D. Theses by Subject "Geology, Structural -- Marmara Region."
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Item High resolution microseismicity and nearly - repeating events in the Marmara Sea(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2022., 2022) Başarır, Nilay.; Karabulut, Hayrullah.; Özel, Nurcan Meral.The Main Marmara Fault beneath the Marmara Sea has a prominent seismic gap that can produce a devastating earthquake and a serious risk for the surroundings. It is important to scrutinize the seismic activity in region and relate this activity to the deformation of the fault zone. In this study, a new micro-earthquake database is created for the Marmara Sea between 2014-2016 using the data mostly from ocean-bottom seismometers. The detected and located seismicity indicate that Tekirdağ Basin hosts a diffuse activity from ~7 km to about 18 km depth. A high micro-earthquake activity rate predominates beneath the Central Basin, at depths from 3 km to 15 km. The abundancy of earthquakes in the area can be attributed to a creeping zone, considering the conformity with the geodetic observations. On the other hand, Kumburgaz and the western part of Çınarcık Basins show sparse seismicity at depth ranges of 5-19 km and 3-18 km, respectively, signing to a locked fault compatible with the geodetic observations. In addition to micro-seismicity, the repeating events are detected using template matching method on the continuous waveforms from 2008-2021. The clusters of highly correlated detected earthquakes, which are closely spaced or partially overlapped, are attributed to the “near-repeating earthquakes”. The nine nearly-repeating earthquake clusters beneath the Central Basin are observed at 8-13 km depths, suggesting seismic creep behavior together with a high seismicity rate. The fault mechanisms of the near-repeater clusters have strike slip mechanism consistent with Main Marmara Fault zone. The nearly- repeating events have two different patterns of repeating intervals, as long-term and short-term type events. The amount of slip rates from the near-repeater clusters shows varying slip rates but comparable to geodetic rate. The number of near- repeating events decreased significantly after the 2018 and no repeating event is observed during 2019 which Mw 5.8 Silivri earthquake occurred.Item Investigating crustal structure of the Marmara region using local tomography and seismic anisotropy anisotropy methods(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2019., 2019.) Polat, Gülten.; Karabulut, Hayrullah.; Özel, Nurcan Meral.The crustal structure underneath the Marmara region was investigated by uti lizing local tomography and shear wave splitting methods in this study. These regions have high seismicity and are thus of serious importance to seismic risks. The first part of the research was based on travel-time tomography utilizing local moderate and microseismic events occurring in the study area recorded by the Multi-Disciplinary Earthquake Research in High-Risk Regions of Turkey project and Kandilli Observa tory and Earthquake Research Institute. We had chosen 2,131 seismic events and 92,858 arrival times in total, comprising of 50,044 P-wave and 42,814 S-wave arrival times. The mapped earthquakes were gathered in the segments of the fault that has high seismicity. Low velocities were observed beneath the central Marmara Sea at 5 km depth. Also, the 2006 Mb =5.3 Manyas-Kus Golu (Manyas) earthquake had been ret rospectively “stress-forecasted” utilizing changes in time-delays of seismic shear wave splitting to evaluate the time and magnitude at which tension-modified microcracking reaches fracture criticality within the stressed volume where strain is released. We observed that clear decreases in delay-times before the impending event, especially at the station GEMT are consistent with the anisotropic poroelasticity (APE) model of fluid-rock deformation, but we could not observe similar changes at other stations sur rounding the main event. The logarithms of the duration of the tension accumulation are proportional (self-similar) to the magnitude of the impending event. Although time and magnitude of the 2005 Manyas earthquake could have been stress-forecasted, as has been recognized elsewhere, shear wave splitting does not appear to provide direct information about the location of impending earthquakes.Item The crustal structure of the eastern Marmara region using receiver function analysis(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2019., 2019.) Büyükakpınar, Pınar.; Aktar, Mustafa.This study focuses on the crust of the Eastern Marmara in order to understand of how much the structure is influenced by the tectonic history and also by the activity of the NAF. Recent studies have claimed that the crustal thickness varies significantly on the north and south of the NAF, which is assumed to indicate the separation line between Eurasian and Anatolian Plates. The present study aims to reevaluate the claim above, using newly available data and recently developed tools. The methods used during the study are the receiver function analysis and surface wave analysis. The first one is more intensively applied, since the second one only serves to introduce stability constraint in the inversions. Data are obtained from the permanent network of KOERI and from PIRES arrays. The main result of the study indicates that the receiver functions for the stations close to the fault zone are essentially very different from the rest and should be treated separately. They show signs of complex 3D structures of which two were successfully analyzed by forward modeling (HRTX and ADVT). A dipping shallow layer is seen to satisfy the major part of the azimuthal variation at these two stations. For the stations off the fault on the other hand, the receiver functions show a more stable behavior and are analyzed successfully by classical methods. CCP stacking, H-k estimation, single and joint inversion with surface waves, are used for that purpose. The results obtained from these totally independent approaches are remarkably consistent with each other. It is observed that the crustal thickness does not vary significantly neither in the NS, nor in the SW direction. A deeper Moho can only be expected on two most NE stations where a gradual transition is more likely than a sharp boundary (SILT and KLYT). The structural trends, although not significant, are generally aligned in the EW direction. In particular, a slower lower crust is observed in the southern stations, which is possibly linked to the mantle upwelling and thermal transient of the Aegean extension. Otherwise neither the velocity, nor the thickness of the crust does not imply any significant variation across the fault zone, as was previously claimed.