Ph.D. Theses

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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.
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.
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.
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.
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.
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.
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.

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