Ph.D. Theses
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Browsing Ph.D. Theses by Author "Aktar, Mustafa."
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Item Modelling 3D seismic wave propagation in Marmara region(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2016., 2016.) Yelkenci, Seda.; Aktar, Mustafa.This study focuses on the modeling of 3D seismic wave propagation in the east of the Marmara Sea in particular for the city of Istanbul, which is identified as one of the megacities with the highest seismic risk in the world. For the first time, an attempt is made for creating a 3D seismic model and for testing the new model with real data. In the frame of constructing 3D velocity model, previous crustal studies of Marmara region and all other available field data, including surface and borehole measurements, are compiled to form a collection of 1D models. Each 1D model relates to a specific location point inside the study area. We have used interpolation methods, in particular Delaunay triangles approach, in order to fill in the no-data zones, which separate the 1D observation points. Elastic wave propagation is simulated inside the newly created 3D model using finite difference approach. An open source code called Wave Propagation Program (WPP), which operates on parallel processing environment, is used for that purpose. We have tested the performance of the 3D model with real data using the earthquake of September 29, 2004 (Ml=4.1) occurred in Çınarcık Basin, which was recorded by 18 permanent broadband stations and 100 strong motion stations. A detailed analysis of the source properties of the event is done, both for the location and the fault plane solution. Real and synthetic waveforms are compared both in time and frequency domains. Matching of the waveform shapes are studied in detail. In each case improvement of 3D model over 1D counterpart is discussed. A more quantitative evaluation of 1D and 3D performances is carried out using waveform correlation. The final result shows that a considerable improvement is achieved with 3D model both in terms of amplitudes and P and S arrival times. The finite difference method is also applied to specified basin structures filled with soft sediments of low shear velocities. Sabiha Gökçen Airport area in Pendik, is studied in detail because its basement geometry and sedimentary cover are well-known. The analysis, performed both in the time and frequency domain, helps to understand the characteristics of the 3D wave propagation inside the basin and the site effects related to it.Item Source properties of micro - earthquakes in eastern Marmara and their connection to the structure of the Çınarcık basın(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2019., 2019.) Can, Birsen.; Aktar, Mustafa.This study mainly focuses on the source properties of micro-earthquakes in Eastern Marmara and their connection to the structure of the Çınarcık Basin, in particular. Throughout this study, Prince Islands Real Time Earthquake Monitoring System (PIRES) Arrays data have been used, which is the closest land site locations to the North Anatolian Fault (NAF) in the Marmara Sea. Only a limited number of small magnitude earthquakes occur in the Çınarcık Basin. Therefore, earthquakes only within an epicentral distance of ~20 km to the arrays have been evaluated considering that Signal to Noise Ratio (SNR) decreases abruptly for further distances. Special methods have been developed and adapted to the PIRES in this study. In this context, advantages of the arrays have been used in all aspects. Array based cross correlation method has been developed for the optimal detection of the small magnitude events which show similarity. Using this method, a systematical search of the foreshocks and aftershocks activities has been performed. This has led to a large improvement of the detection level and revealed large number of earthquake clustering. It became possible to extract many small magnitude events that are buried in the background noise or in the coda of previous events and therefore were missed by the land stations. Since, the main target was to evaluate the performance of the surface arrays against the boreholes, various noise cancelation tools are developed based on the stacking of repetitive observations. These procedures are used for the estimations of the fracture properties of the small events inside the Çınarcık Basin. The fracture properties that have been analyzed are the seismic moment, fracture radius, stress drop, energy and occurrence statistics. Tests are performed to see if the fracture properties are changing in space and time, or show any other characteristic behavior that may be connected to a particular location in the study area. Variations are observed between the stress drop and location of the events. Similarly, foreshock and aftershock occurrence statistics seems also to vary across the Çınarcık Basin. Since, the present data is rather restricted, it is expected that the interpretations are only preliminary. The results obtained imply that this type of analysis will probably be part of the real time monitoring processes in the future, for the purpose of early warning systems.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.