Ph.D. Theses

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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.
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.
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.
A new insight into the crustal structure of the central Anatolia to eastern mediterranean from a wide angle seismic data
(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2016., 2016.) Denli, Alper.; Özel, Nurcan Meral.
As a part of the CyprusArc project a seismic wide angle reflection/refraction profiles, the 300 km and 45 km long north-south trending profiles extended from Cihanbeyli in Central Anatolia to Anamur in eastern Mediterranean and in southern Cyprus, respectively, in March 2010. The seismic experiment was comprised of two land explosions of 1125 kg explosives onshore and 98 cubic liters airguns offshore. 76 three-component and 119 vertical-component sensors were deployed along ~300 km distances between Cihanbeyli and Anamur with an average spacing of 1.25 km. 25 three-component sensors and 25 vertical component sensors were installed along 45 km distances on land at southern Cyprus with an average spacing of 1.25 km. Appropriate band pass filter was applied for each controlled sources to pick the arrival times. Modelling of the CyprusArc profiles data show that a Moho depth of 38 km at the northern end of the profile which increases 45 km through the southern end of the profile from central Anatolia to eastern Mediterranean. An average P-wave velocity is 6.5 km/s beneath Tuz Golu basin till approximately 23 km depth. P-wave velocity of some rock materials which brought into the open by Taurus Mountains is 5.5 – 5.6 km/s till 5 km thickness. A high velocity block (average P-wave velocity is 6 km/s) between 120 -150 km offset, till 8 km thickness probably correspond to ophiolite complex belong to Troodos. 2-D crustal P-wave velocity model shows crustal thinning between south Turkey and Cyprus from 45 km to 30 km. Final 2-D P-wave velocity models were further refined by generating synthetic seismograms to observe the theoretical travel times and amplitudes of the various arrivals. Additionally, 2-D gravity modelling was done to check robustness of the unresolved part of models by seismic phases and the all results were correlated with geology, tectonics and previous investigations in the study area.
Attenuation structure in central Anatolia using Belbaşı - Keskin borehole array
(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2018., 2018.) Şemin, Korhan Umut.; Karabulut, Hayrullah.; Özel, Nurcan Meral.
The Multiple Lapse Time Window (MLTW) method has been applied to inves tigate the dominant attenuation mechanism of Central Anatolia region by separating scattering attenuation and intrinsic absorption that are a↵ecting the seismic wave am plitudes. A total of 177 local earthquakes with magnitudes varying between 2.5 – 4.7 and hypocentral distances between 5 to 150 km recorded during 2008-2011 by two borehole type broadband seismometers as well as KOERI seismic stations were se lected according to the criterion deﬁned by SNR > 3 (Signal-to-Noise Ratio). The single station approach of the MLTW allowed us to characterize the lateral variations of attenuation in the region by calculating the attenuation around each station indi vidually for frequencies 1.5, 3, 6, 8, 9 Hz. Moreover, average attenuations were also estimated representing the whole region of Central Anatolia. Final results were com pared with other studies conducted in di↵erent regions around the world. Results of this study show that for frequencies 3 Hz and higher the intrinsic absorption is more prominent than scattering attenuation for the whole of Central Anatolia, especially at south and southeastern parts due to Quaternary volcanism. Comparison of attenuation with di↵erent regions indicates that the Eastern Anatolia has higher attenuation than Central Anatolia whereas Western Anatolia has comparable values of attenuation.
An investigation of Anatolian - African subduction zone in southwestern Turkey : lithospheric structure beneath Isparta angle and the surroundings from rayleigh wave phase velocity inversion
(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2016., 2016.) Teoman, Uğur Mustafa.; Özel, Nurcan Meral.
Geodynamics of Turkey is complicated by the tectonic interactions between Africa, Eurasia and Arabian plates leading to high seismic activity and internal deformation beneath this region. Subduction of African Plate beneath Western Anatolia along Hellenic and Cyprus Arcs even more complicates the overall picture. In this sense, Isparta Angle (IA) plays a key role in understanding the neotectonic development of the Eastern Mediterranean. In this research, our goal is to put constraints on the upper mantle structure beneath IA and the surroundings via Rayleigh wave tomography method. In this regard, we adopted a phase velocity inversion technique named as “Two-plane wave method”. With the use of this technique, we will be able to effectively map the three-dimensional velocity structure and amplitude variations to a certain extent. In August 2006 - September 2009 time frame, we recorded teleseismic earthquakes (30 <  < 120) with magnitudes greater than 5.5 using the permanent stations of Kandilli Observatory and Earthquake Research Institute (KOERI), Süleyman Demirel University (SDU) and IRIS/GEOFON together with temporary stations deployed with support from Missouri University and Boğaziçi University Research Fund (BAP). Following the detailed analysis of vertical component seismograms, we calculated a one-dimensional dispersion curve which served as an input for two dimensional (2-D) phase velocity inversions. Phase velocity maps were displayed in several cross sections at various periods. We also performed other series of inversions to determine the shear wave velocity distribution down to 250 km. Furthermore, construction of a 3-D shear velocity model enabled us to address the significant issues regarding the complex slab geometry of Anatolian-African subduction. These velocity anomalies provided us insights on the key elements that define the nature of subduction such as slab detachment, slab tearing, asthenospheric upwelling, and volcanism etc. The outcomes have been compared to most recent and previous studies to make reliable interpretations.
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.
Crust and upper-mantle imaging by using P and S receiver functions in different tectonic regimes
(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2015., 2015.) Kahraman, Metin.; Türkelli, Niyazi.
P and S receiver functions (RF) are effective tools to solve crustal and upper-mantle velocity contrasts. In this respect, RFs are utilized to image three different tectonic regimes of Anatolia. Firstly, western segment of the North Anatolian Fault Zone (NAFZ) is inspected by data from a dense broadband network (Dense Array for North-Anatolia - DANA) of 71 seismic stations with a nominal station spacing of 7 km in the vicinity of the 1999 Izmit earthquake. High resolved 2-D cross-section images reveal previously unkown small-scale structures and fault geometries in the crust and upper-mantle. Secondly, N-S extension dominated Western Anatolia (WA) is observed by 47 permanent broad-band stations. Totally, 3563 high signals to noise ratio P wave RFs with cut-off frequency of ~1 Hz are obtained among 43146 teleseismic earthquakes. Crustal differences, sharp Moho changes and low velocity zones are defined by 2-D cross-sections in the region. Lastly, Isparta Angle (IA) is imaged by data from a temporary and permanent broad-band seismic network that is composed 42 seismic stations. 4501 P wave RFs are used to resolve upper crustal and Moho depths and 946 S wave RFs are operated to figure out lithosphericasthenospheric boundary (LAB). Migrated P wave RFs cross-sections present Moho anomalies and African slab in the crust of IA. On the other hand, migrated S wave RFs cross-sections show variation of LAB boundary between ~50 to ~90 km depth range.
Tsunami hazard in Turkey and surroundings
(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2014., 2014.) Necmioğlu, Öcal.; Özel, Nurcan Meral.
This study is an attempt to provide an overall assessment of the earthquake origin tsunami hazard in Turkey and its surroundings (Eastern Mediterranean, Aegean and Black Seas). To achieve this, I have first investigated the earthquake focal mechanism parameter variations effect into tsunami generation with special focus on Eastern Mediterranean. My analysis shows that, given the difficulty in accurately determining all focal mechanism parameters, tsunami hazard studies should look at a range of parameters, taking into consideration the maximum generated tsunami. If this broad study scope is not possible due to computational limitations, at least sensitivity studies, should be conducted, and parameters should be selected that would lead to maximum tsunami generation. An option would be to consider only strike and rake variations in the scenario databases as the key criteria in determining the worst-case scenario for a given forecast point, and if applicable, various earthquake depth ranges should be considered. Following this work, I have compiled all available information concerning earthquake source parameters and their regional characterization and attempted to define characteristic static earthquake source parameters for all tsunamigenic earthquakes in the region. I have identified a set of tsunami scenario input parameters in a 0.5° x 0.5° uniformly gridded area in the Eastern Mediterranean (both for shallow and intermediate depth earthquakes), Aegean and Black Seas (only shallow earthquakes) and calculated tsunami scenarios using SWAN-JRC (Annunziato, 2007) with 2-arcmin resolution bathymetry data for the range of 6.5 – Mwmax with a Mw increment of 0.1 at each grid in order to realize a comprehensive earthquake origin tsunami hazard analysis in the region based on the definition of the characteristic earthquake source parameters from a compiled set of sources such as existing moment tensor catalogs and various reference studies, together with the Mwmax assigned in the literature, where possible. Results from 2415 scenarios show that in Eastern Mediterranean and its connected seas (Aegean and Black Sea), shallow earthquakes with already Mw ≥ 6.5 may result in 0.5m coastal wave height, whereas same level of wave height could be expected only with Mw ≥ 7.0 for intermediate depth earthquakes. The distribution of maximum wave heights calculated indicate that tsunami wave heights up to 1m could be expected in northern Aegean, whereas in Black Sea, Cyprus, Levantine coasts, northern Libya, eastern Sicily, southern Italy, and western Greece, up to 3m wave height could be possible. Crete, southern Aegean, and the area between northeast Libya and Alexandria (Egypt) is prone to high level of tsunami hazard with Hw > 3m wave heights calculated. Considering that calculations are performed at a minimum bathymetry depth of 20 m, these wave heights may amplify by a factor of 2 at the coastline according to Green’s Law. On the other hand, it should be emphasized that while the Green’s Law provide an empirical approximation, the non-linear dynamics of the tsunami at the coastal zones may lead to deviations in actual wave heights. As an overall conclusion, my analysis indicate that in Black Sea, locations in the southern coasts of Crimea, northwestern coast of Turkey, Bulgarian coast and southeastern coasts of Romania are prone to considerable (1m < Hw < 3m) level of tsunami hazard, whereas the eastern Black Sea coasts could be considered as low (Hw < 1m) tsunami hazard zones. Concerning Eastern-Central Mediterranean and Aegean Seas, my study indicate that locations in, around and orthogonal to the Hellenic Arc are prone to high (Hw > 3m) level of tsunami hazard, whereas the tsunami hazard should be identified as considerable (1m < Hw < 3m) for the rest of the Eastern Mediterranean, Southern Aegean, Tripoli (Libya), eastern Sicily, Calabria, western coasts Greece, Antalya Peninsula and Bay western and southern Cyprus and southern Levantine coast. Tsunami hazard can be classified as relatively low for the northern Aegean, Tunisia, western Sardinia, southwest coasts of Italy, and western and northern coasts of Sicily. The results of the modeling are in accordance with the historical tsunami events in the study area. It should be emphasized that these conclusions are valid only for the earthquake sources considered in this study excluding any possibility of a triggered submarine landslide. In addition to the work describe above, I have also exploited several reliable tsunami catalogs (Ambraseys, 1962 and 2009; Soloviev, 2000; Fokaefs and Papadopoulos, 2007; Altinok et al., 2011, Papadopoulos et al., 2012) by performing a cross-comparison of the events in order to create a harmonized database for the study area. The reliability of the tsunami occurrence has been defined as Unlikely (U), Questionable (Q), Probable (P) and Definite (D), where improbable events were removed. There are 145 events in the harmonized database, where 41 events are defined as definite, 30 events as possible, 58 events as questionable and 16 events as unlikely. 44 events include contradicting information from selected source databases.
Imaging the upper mantle beneath Turkey and surrounding regions
(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2012., 2012.) Kömeç Mutlu, Ahu.; Karabulut, Hayrullah.
This study includes two interdependent sections. The first section presents an analysis of Pn travel times to determine Pn velocity, Pn anisotropy and crustal thickness variations beneath Turkey and surroundings. Between 1999 and 2010, more than 50 000 Pn arrivals are compiled from 700 regional earthquakes by 832 stations of permanent and temporary networks operated in the study area. A regularized least squares inversion method is used to estimate crustal thickness variations and image velocity perturbations in the uppermost mantle. The results reveal features that correlate well with the surface geology and the active tectonics of the region. The Pn velocities show very fast (> 8.4 km s-1) and very slow (< 7.6 km s-1) anomalies indicating a heterogeneous lithospheric structure. The average velocity of 8.0 km s-1 is determined from a linear fit to Pn travel times. Relatively uniform Pn velocities (7.9-8.1 km s-1) are observed in the Western Turkey. Large velocity contrasts are located at subduction and suture zones. A sharp transition in the central Anatolia is apparent from the uniform Pn velocities in the west to lowest velocities (< 7.6 km s-1) in the east. The lowest velocities coincide with the volcanics of the easternmost Anatolia and the Central Anatolian Volcanic Zone. Beneath the Dead Sea Fault Zone and Dinarides-Hellenides, the upper mantle velocities are also low (< 7.8 km s-1). High Pn velocities are observed beneath oceanic lithosphere such as Mediterranean Basin (> 8.3 km s-1), western Black Sea basin (> 8.3 km s-1), Adriatic Sea (> 8.3 km s-1), and Zagros suture zone (> 8.3 km s-1). Large velocity contrasts are observed at subduction, suture zones and across the North Anatolian Fault. Pn anisotropy has maximum amplitude of ±0.8 km s-1 in the study area corresponding to 10 per cent anisotropy. The coherent and largest anisotropic anomalies are observed in the western Anatolia, Aegean Sea, and Cyprian Arc. A significant anisotropic pattern is observed in the Cyprian Arc region. Pn anisotropy in western Anatolia, Aegean Sea and Greece correlate well with the present state of tectonic deformation and GPS velocities. The Dinarides-Hellenides exhibit arc-parallel anisotropy. In Western Anatolia, anisotropy is aligned in N-S direction along the major principal strain orientation. Along the North Anatolian Fault, the anisotropy directions are E-W, aligned with the fault geometry in the western part while no correlation is observed on the central and eastern parts of the fault. Anisotropy in Eastern Anatolia is complex and the directions are varying strongly in the region of low Pn velocities. The absence of anisotropy is apparent in an area dominated by the neogene volcanism. Low Pn velocities and absence of clear anisotropic pattern beneath Eastern Anatolia may have resulted from thermal anomalies in the uppermost mantle possibly due to delamination processes. Large positive station delays are observed along the southern coast of Anatolia, Eastern Anatolia and beneath Dinarides-Hellenides while large negative station delays are observed in Western Anatolia and the Marmara Region. The majority of the stations in Central Anatolia show small station residuals indicating the average crustal thickness of 35±2 km. Western Anatolia and the Aegean Sea have crustal thicknesses between 28±2 and 33±2 km. In Greece, the crustal thicknesses are increasing from 33±3 km from the western coast to a maximum of 48±3 km beneath Dinarides-Hellenides. The large crustal thicknesses (40-48 km) are also observed along southern coast of Anatolia. In eastern and Southern Anatolia the average crustal thicknesses are 40 km and 36 km, respectively. In the second section of this study, shear wave splitting on records of core-refracted (SKS) phases are obtained. Waveform data from 850 teleseismic earthquakes occurred between 1999-2010 at epicentral distances between 84° and 130° with magnitudes greater than 6.0 are analyzed. A total number of 4163 splitting measurements are obtained from 217 broadband seismic stations located in and around Turkey. The anisotropy parameters measured from SKS are consistent with the results of similar studies conducted in North-Central Anatolia, Eastern Anatolia and Aegean. Fast direction polarizations are dominantly in NE-SW direction in the Eastern Anatolia. In the Marmara Region, fast polarization directions are in NNE-SSW direction with greater lag times. There is a relatively sharp change in the fast polarization directions form NE-SW to NW-SE at the Antalya Bay, Isparta Angle Region (~30°E). SKS measurements are non-uniform in Central and Northern Greece. There are progressive changes in the fast splitting directions as well as delay times from Eastern Turkey to the Aegean. The change in the fast splitting directions from NNE-SSW in the eastern Anatolia to N-S in the Aegean may be the result of the retreat of the Hellenic slab. Through the North Anatolian Fault, shear wave splitting directions are aligned NE-SW.
Surface wave tomography of Turkey and surroundings
(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2010., 2010.) Cambaz, Musavver Didem.; Karabulut, Hayrullah.
Seismic wave velocities can be obtained by using active or passive sources with appropriate arrays. Seismic reflection and refraction surveys using active sources are the most traditional ones. However the cost as well as inapplicability in urban areas, limits active source reflection and refraction methods in the crustal investigations. Seismic body waves and surface waves emitted from earthquakes are also widely used in seismology in order to constitute the images of the subsurface. However the insufficient path coverage between sources and stations may be the limiting factor. This amounts to the obstruction of obtaining high resolution images in crustal studies with earthquake data. In order to overcome the shortcoming of these techniques, a relatively new concept of “Passive Imaging Technique” is proposed to obtain the surface wave velocity structure of the Earth. Generally, not only in seismology but also in other disciplines which deal with signals, accept noise as an undesired component of the signal. It is commonly believed that noise obscures data and does not contain useful information. However recent developments changed this judgment by indicating that long term correlations of ‘ambient noise’ can also be used as seismic source. This method promises significant improvements in the resolution and accuracy of crustal and upper mantle images. Green’s functions between station pairs can be extracted from long term correlations of seismic recordings. Shear wave velocity distribution can then be obtained from the Green’s functions using the conventional imaging methods. In the frame of this thesis, for a better understanding of the character of the seismic noise, a comprehensive noise analysis has been performed for permanent and temporary broadband stations operating in Turkey and surrounding areas. Power spectral densities (PSD) were computed in the frequency range of 100 sec to 10 Hz. Probability Density Functions (PDF) as a function of noise power, have been analyzed for the stations with available data. Noise maps have been constructed from the power spectral density estimates of selected stations in the region in order to characterize the temporal and geographical variations. Diversities in noise spectra due to different sensors, installation properties and geographical variations are discussed. Ambient seismic noise records are used to determine the group velocity variations in Turkey and surrounding regions. A database for noise correlations was constructed from the continuous recordings of 156 permanent and temporary broadband stations during 2006-2009. The cross correlations of the ambient seismic noise are calculated to determine surface wave Green’s function for station pairs in the region. In order to obtain the group velocity maps from earthquakes a waveform database was formed from 285 earthquakes with magnitudes Mw>4.5 recorded by more than 270 broadband stations. Love and Rayleigh wave group velocity dispersion curves are computed and group velocity maps of Turkey and the surrounding regions have been obtained from local and regional earthquakes. Results from ambient noise were compared with the group velocity maps obtained from earthquakes. The group velocity maps were interpreted in relation to the known geological and tectonic structures in the region. The study shows the existence of significantly different crustal types in the area. Low group velocities at shorter periods (10-20 sec) are observed in local sedimentary basins, the Eastern Mediterranean and the Black Sea. The Eastern Anatolia region is also characterized by low group velocities while Pontides and Bitlis-Pötürge massif display higher group velocities. The Central Anatolia exhibits uniform velocity distribution indicating more homogenous crust. The Isparta Angle is marked by a wedge shaped-low group velocity anomaly. High velocities observed on the maps are associated with metamorphic, magmatic arcs along the orogenic belts of Pontides, Pötürge massif and crustal thinning in the Aegean region. At larger periods (40-50 sec) the Anatolian Block shows low and uniform group velocity distribution while its surroundings display higher group velocities with the exception of the eastern Mediterranean Region.
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.
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.
Lithospheric structure of the western Turkey and aegean region
(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2019., 2019.) Afacan Ergün, Tuğçe.; Karabulut, Hayrullah.
Aegean-Anatolia region undergoes an intense internal deformation as evidenced by the existence of major active faults, intense seismic activity and the marked thinning of the crust. It makes the region center of attraction to the study the interaction between the deep structure with the surface deformation. The aim of this study is to provide constraints on the crustal and uppermost mantle structure by using seismic data of permanent broad-band network of Kandilli Observatory and Earthquake Research Institute (KOERI-RETMC), and a temporary array of Seismic Imaging beneath Aegean-Anatolia Domain (SIMBAAD) experiment. Seismic stations of Republic of Turkey Prime Ministry Disaster and Emergency Management Presidency (AFAD), Incorporated Research Institutions for Seismology (IRIS) and previous experiment called Western Anatolia Seismic Recording Experiment (WASRE) were used to complement the network. In this regard we present two high resolution lithospheric images along a ~650 km transect crossing western Anatolia at 28°E longitude from the Black Sea to the Mediterranean and a ~550 km transect crossing central Anatolia at 30.5°E longitude. A total of 5250 receiver functions are computed from the records of teleseismic earthquakes at 40 broadband seismic stations for each of the profiles with an average spacing of ~ 15 km. Lateral variations of crustal thickness, Vp/Vs are inferred from both H-K, and common conversion point stacks (CCP). In order to have a better idea on the accuracy of the estimated crustal parameters we also performed a search scheme based on the Neigboorhood Algorithm. The receiver functions are inverted for a 1-D layered medium to determine the layer thicknesses, Vs and Vp/Vs. The CCP images reveals a longwavelength variations of Moho depth from ~31 km in the Thrace basin to ~25 km beneath the Marmara Sea, ~25 km beneath the Menderes Massif and ~20 km on the coast of the Mediterranean on the western Anatolia transcent. On the eastern transect, a smooth Moho topography is observed with a sharp discontinuity at depths ranging from 34 km beneath the Black Sea coast, ~35 km beneath the Sakarya Zone with mafic composition to 43 km beneath the Antalya Bay on the central Anatolia profile. The Moho of the subducted African lithosphere is imprinted between ~40 and ~60 km depth at the southern end of the western Anatolia profile, dipping northward where the subducted Cyprus lithosphere is observed dipping northward with an angle of 40◦ between ~40 and ~100 km depths beneath the Antalya Bay on the central Anatolia transect.
Determination of upper mantle heterogeneity beneath Aegean-Anatolian region from travel time tomography
(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2019., 2019.) Aksarı, Doğan.; Karabulut, Hayrullah.
The objective of this work is to determine the heterogeneities of the upper mantle in the Aegean-Anatolian domain using teleseismic tomography. A waveform dataset was prepared from 798 teleseismic earthquakes with magnitudes greater than 5.5 between January 2004 and December 2015. 417 stations from permanent and temporary networks with more than 64,000 direct P phases are used in the computations. The relative travel times of P waves with respect to the ak135 (Kennett et al. 1995) earth model are computed using waveform cross-correlations technique. The tomographic images are computed as perturbations with respect to ak135 earth model. An algorithm named as fast marching method (FMM) (Sethian, 1996a, 1996b) based on the solution of Eikonal equation is used in the forward computation of the travel times. The inversion is performed using subspace inversion scheme. Trade-off curves are plotted and several synthetic tests are performed in order to select optimum parameters (damping and smoothing) for tomography and the resolution and model roughness were investigated. The tomographic images obtained to a depth of 700 km. The computed tomographic images show a heterogeneous upper mantle structure in the Aegean-Anatolian domain. The results are similar to the previously published images mostly but provides higher resolution for the study area. Both Hellenic and Cyprus subductions are imaged to the depth of 700 km. The tear (Pliny-Strabo Tear) between two subduction zones is clearly observed reaching to 660 km discontinuity. A smaller scale tear (Antalya Bay Tear) is also observed on the Cyprus slab around Paphos Transform Fault. The Anatolian plate is underlined by low velocity mantle material with thickness increasing from west to east. The northern block of the North Anatolian Fault (NAF) is observed as high velocity body observable to a depth of 100-200 km. NAF has a sharp velocity contrast between the north and south.
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.
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.
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.
Three-dimensional resistivity modelling and interpretation of geothermal fields in the Gediz graben by magnetotellurics
(Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2018., 2018.) Cengiz, Özlem.; Diner, Çağrı.
The Gediz Graben hosting several geothermal systems is one of the most promising grabens in terms of temperature and production rate of western Anatolia. In order to provide the most comprehensive understanding about the geothermal systems situated in the graben, specifically about the reservoir types, heat sources and structural controls, 253 MT sites were installed at four different areas of the graben to delineate the electrical resistivity distribution at depth. The wide-band MT data were analyzed by phase tensor analysis, and then the data at 31 selected periods in the range from 0.001 s to 1000 s modeled in three-dimensions (3D). The resulting models reveal three different reservoir types, namely (i) a classical geoelectrical distribution of a high temperature geothermal system, with a prominent highly conductive hydrothermal alteration zone sitting above a more resistive deep reservoir zone, (ii) a deep reservoir zone characterized by fractures within metamorphic rocks in the highly resistive basement and (iii) a shallow reservoir (aquifer) corresponding to the hot springs in the shallow sedimentary layer existing in the Gediz Graben. The heat source of the geothermal systems may be attributed to the heat transfer from the interior of the Earth to the upper crust as a consequence of crustal thinning resulted from the extensional tectonics accompanied by magma intrusions into crust in western Anatolia. The 3D models bring out a well-defined interface between the sedimentary cover and underlying metamorphic basement owing to high resistivity contrast between two layers, characterizing the Gediz detachment fault (GDF). The geothermal fields formed along the southern margin of the graben are spatially coincident with the intersecting zone of two fractures, namely the GDF and high angle normal faults, and the circulation of geothermal fluids in reservoirs are dominantly controlled by these fractured zones and major faults. The crustal scale main graben-bounding fault (MGBF) acts as a conduit through which fluids and heat are transported from deeper parts of the crust to near surface. The meteoric waters percolating deep into the crust through the north dipping normal faults are probably heated up by magmatic intrusions, and some of geothermal waters containing meteoric and magmatic fluids rise up to surface through the permeable faults, in particular through the lower bounding sub-horizontal GDF. Furthermore, 3D resistivity models suggest a thick sedimentary layer (2500-3000 m) in the middle part of the graben basin. The thickness of the sedimentary layer decreases gradually on the northern and southern margins of the graben and becomes much thinner towards the eastern end of the graben. 3D resistivity models also delineate an undulating basement topography under the conductive sedimentary fill of the graben.
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.

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