Jeofizik

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1-D local crustal structures from seismological data in the Cyprus Subduction Zone and Antalya bay
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2013., 2013.) Perk, Şükran.; Karabulut, Hayrullah.
The eastern Mediterranean is a tectonically complex region, where long-term subduction and accretion processes have shaped the overall evolution. Recently, many seismic tomography studies have shown subducted slabs of the Neo-Tethyan lithosphere, continuing its subduction in the Hellenic trench, stalled in the Cyprus trench and being torn near the intersection between them. Antalya bay is a key region located on the western flank of the Cyprus Subduction Zone (CSZ), close to the junction between the Hellenic and Cyprus Arcs. Here deep earthquakes are nucleated, which otherwise cannot be seen anywhere else along the CSZ. For this reason, we focus our attention specifically to the Antalya Bay area but also the remaining parts of the CSZ. Several regional studies have been carried out to define the velocity structure beneath the region but none have been able to locate the CSZ. One of the main reasons for this was the lack of incorporation of a wide seismic network in those regional studies. We compile a large catalog of seismicity and relocate earthquakes to infer 1D local crustal structure using the clusters of seismicity. We used seismic data between 2005 – 2011 which are recorded at more than 250 seismic stations operated by several agencies and portable deployments. The data-set is composed of over 10,000 events and earthquakes can be grouped in several distinct clusters. We defined five of these clusters, where the total number of events is more than 4500, among which we selected over 2000 events with the highest data quality. 1-D local P-wave velocity models are developed using this high quality data-set and the earthquakes are relocated using the local velocity models. The compiled and reanalyzed data will contribute to perform local earthquake tomography. Moreover, obtained local velocity models represent a fundamental step towards an improved seismic tomography studies in a very crucial region in the eastern Mediterranean.
3-D crustal structure of the Isparta angle region from local earthquake tomography
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2011., 2011.) Çakır, Ersin.; Türkelli, Niyazi.
Isparta Angle is a seismically active and complex zone in terms of tectonic and geologic features. A temporary seismic network consisting of nineteen three-component broadband stations were installed around Isparta Angle (IA) and surroundings to address some of the important questions. Detailed crustal structure of the Isparta Angle were obtained using the seismic data collected over two years from July 2006 to June 2008. Results of this study will contribute to beter characterize the crustal structure and the seismicity of the region. In this study, local earthquake data that were collected from temporary broad-band seismic array operated in the region, data from permanent stations of Kandilli Observatory and Earthquake Research Institute (KOERI), permanent stations of Disaster and Emergency Management Presidency (DEMP) and Süleyman Demirel University (SDU) in order to determine the upper crust seismic P wave velocity structure of IA and the surrounding regions by three-dimensional (3-D) Local Earthquake Tomography (LET) method. S-wave arrival times were not included due to strong attenuation and higher picking errors of S-phases. The result of tomographic processes, 3-D velocity model, was compared with the seismological and tectonic features of the region and also compared with the results of the previous studies in the region. The results suggest that low-velocity zones beneath Isparta Angle in the depth range between 0-10 km can be related to alluvial deposits, and the velocity variation below 20 km depth can be related with the transition from upper crust to lower crust.
3-D crustal structure of the Simav - Küyahya and surrounding regions
(Thesis (M.S.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2017., 2017.) Duran, Nuray.; Konca, Ali Özgün.
Kütahya and surroundings are quite complex structures from a tectonic and geological perspective. On May 19, 2011, an earthquake of magnitude M = 5.9 occurred in Simav. The purpose of this thesis is determination of the 3-D seismic velocity structure applying the local earthquake tomography method, using earthquakes that occurred between the years 2010 and 2015 in Kütahya Simav and surroundings. The signifigance of this study is that the first 1-D and 3-D tomographic inversion for Simav, Kütahya and surroundings is done by this study. The region's 1-D velocity model, 3-D velocity model and crustal structure are obtained. In this study, the 3-D velocity distribution up to 30 km depth of upper crust of the study area is determined with tomographic inversion methods. For this purpose, by creating data sets, travel time of phases were prepared by tomographic inversion format, appropriate model structure has been created, Vp, Vs and Vp/Vs models that obtained with appropriate resolution parameters and tomographic inversion, has been interpreted with geological and tectonic features of the area. At 30 km depth, P and S waves velocity structure of Kütahya, Simav and surroundings were obtained. The Graben system of the region is supported by the velocity cross-section obtained. The depth of the graben is determined as 20 km. The relationship between the hypocentre of the recorded earthquakes in the region and Vp, Vs, and Vp/Vs anomalies has been revealed with horizontal and vertical cross-sections. Comments are made about the structural differences of the region’s P wave and S wave velocity information and the physical properties of the rock’s Vp/Vs ratio information. Fault systems, potential fault zones and earthquake activity of the region are discussed with the combined results of the study.
3-D P wave velocity structure of Marmara Region using local earthquake tomography
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2013., 2013.) Işık, Sezim Ezgi.; Gürbüz, Cemil.
The 3D P wave velocity model of upper and lower crust of the Marmara Region between 40.20 - 41.20 N and 26.50 - 30.50 E is obtained by tomographic inversion (Simulps) of 47034 P wave arrivals of local earthquakes recorded at 90 land stations, 30 OBO stations and 14162 shot arrivals recorded at 35 OBO stations (Seismarmara Survey, 2001) between October 2009 and December 2012. We first obtained a 1D minimum model with Velest code in order to use it as an initial model for 3D inversion with 648 well located earthquakes located within the study area. We first developed a 1D model by VELEST code then used it as an initial model for 3D tomography. After trial 3D inversions we decided to create a more adequate initial model for 3D inversion. Choosing the initial model we estimated the 3D P wave velocity model representing the whole region both for land and sea. The results are tested by making Checkerboard Test, Restoring Resolution Test and Characteristic Test, and the reliable areas of the resulting model is defined in terms of RDE, DWS, SF and Hit count distributions. By taking cross sections from the resulting model we observed the vertical velocity change along profiles crossing both land and sea. All the profiles crossing the basins showed that the high velocities of lower crust make extensions towards the basin area which looks like the force that gives a shape to the basins. Under the north of the Central Basin and Çınarcık Basin high velocity zones appeared which look like resisting walls holding the north of the basins between 5 km and 15 km. The same is also observed under the southern Tekirdağ Basin between 5 km and 15 km. These extensions of lower crust towards the basins appeared with an average velocity of 6.3 km/s. These extensions might be the result of the deformation due the shear in the region and it is also interpreted that the development of these high velocities coincides with the development of the basins. Thus, both the basins and the high velocity zones around them might be resulted from the entrance of the NAF into the Marmara Sea when also a shear regime is dominated due to the resistance of the northern Marmara Region (Yılmaz, 2010). The seismicity is observed between 5 km and 15 km after the 3D location of the earthquakes. The locations of the earthquakes improved and the seismogenic zone is determined well, between 5 km and 15 km. The depths of the pre-kinematic basement and crystalline basement showed great differences under the sea. It is observed that the velocity under sea becomes compatible with land after 8 km.
3-D P-wave velocity structure beneath eastern Turkey applying local earthquake tomography (LET) method
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2005., 2005.) Teoman, Uğur Mustafa.; Türkelli, Niyazi.
Eastern Turkey is a seismically active region exhibiting a complex structure in terms of both tectonic and geologic· features. A temporary seismic network consisting of 29 broadband three-component broadband stations were installed in the scope of Eastern Turkey Seismic Experiment (ETSE) to address the important questions. Detailed and accurate evidences relevant to the crustal and upper mantle structure of the region were obtained using the seismic data collected in between October 1999 and August 2001. Results of the ETSE project had a considerable contribution to understanding and interpreting the tectonic evolution along with the seismicity of the region. In this study, local earthquake data from the ETSE project were used in order to determine the upper crust seismic P wave velocity structure of Eastern Turkey and the surrounding regions by three-dimens}onal (3-D) Local Earthquake Tomography (LET) method. As far as resolution is concerned, S waves were not included in this study due to strong attenuation, insufficient number of S phase readings and higher picking errors with respect to P phases. LET is the 3-D imaging process ofthe velocity structure of a target volume by using the travel time data of the earthquakes recorded within the seismic network in an active region. To provide accurate tomographic results, a high quality data set, initial hypocentral parameters and a minimum one-dimensional (1-D) velocity model that adequately represents the region are required. Initially, data compilation and earthquake locations were determined. Initial locations of 9*4 events were performed by HYPOCENTER algorithm using an initial 1-D velocity model, which was obtained from the previous studies in the region. Following the earthquake location procedure, higher quality events were selected in order to construct the 1-D minimum velocity model for the region. As a basic data selection criteria, events with azimuthal gap (GAP) ::; 200° and number of observations (P) ~ 8 were selected from the initial data set (6978 P-phase readings). 1-D velocity model was calculated by VELEST algorithm performing a simultaneous and iterative 1- D velocity inversion. New hypocentral parameters and station corrections were also calculated in addition to the minimum velocity model. Three different trial velocity models were used in order to construct an initial velocity model based on the results of a set of inversions with ten iterations and four calculation steps. Each velocity model were constructed in 0-42 depth range in accordance with the crustal structure of the region, however, accurate results were obtained down to~ 15-20 km due to the depth distribution ofthe events. The relocation of the earthquakes was performed by VELEST via the 1-D minimum velocity model and the relevant station corrections. In the final step, various stability tests were applied to check the resolution capability minimum 1-D velocity model. As a result of these tests, it was suggested that 1-D minimum velocity model adequately represented the region. After the derivation of the most appropriate velocity model, 3-D tomographic inversions were applied to the final data set. Events with GAP :::;; 180° and P > 8 were reselected from the data set. Number of events for the tomographic inversion reduced to 504 after applying this criteria (Total number of 6742 P-phase readings). 3-D tomographic inversion was iteratively and simultaneously performed by SIMULPS 14 algorithm using node-grid geometry for model parameterization. Considering the event-station distribution, a horizontal grid with 30x30 km grid spacing was chosen. In vertical direction, depth values taken from the 1-D velocity model were used. Modeling was made down to 42 km. Tomographic inversions with four iterations and two processing steps were initiated after determining the appropriate control parameters and the damping factor. At the end of these processes, 3-D P wave velocity model and the resulting hypocenters were determined. A significant reduction in data variance (- %50) and in residuals (-%50) was observed during these processes. In order to assess the solution quality and the resolving power of the 3-D model, tests with the synthetic data were performed. Critical parameters affecting the resolution estimates were calculated and mapped along with absolute velocities (Vp) and% perturbations relative to the 1-D initial velocity model in both horizontal and vertical cross-sections. Consequently, after the tomographic applications, the compatibility of the results with the tectonic and seismological features of the region were evaluated and also compar~d with the results of the previous studies in the region. The differences between the initial and the final hypocentral parameters were emphasized in various cross-sections. XV
3-D velocity structure of eastern Marmara region from local earthquake tomography
(Thesis (M.S.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2008., 2008.) Denli, Alper.; Gürbüz, Cemil.
Local earthquakes located in 40-41.5 N and 28-30.5 E geographic coordinates between the dates January,2003-October,2007 were selected to use for local earthquake tomography. Data were obtained from short period and broadband seismic stations dispersely located in the region. This study was conducted to reveal 3-D P wave velocity structure in the target area. To prepare data set, relocation procedure was done. To determine the 1-D velocity model for the study region, earthquakes which have azimuthal gap less than 1800 and recorded at least by 7 stations were selected. Obtained 1-D velocity model has 32 km depth. Reliable solutions were observed to 17 km due to the depth distributions of the earthquakes. To consolidate the robustness of the velocity model, shifting test was applied and quarry blast data were used. Results show that the 1-D velocity model represents the study region. Using reference earth model for local earthquake tomography was decided. Later on, observing more confidential parts of the velocity changes in the study region, synthetic models were produced for the checkerboard test. At the same time, control parameters were set to obtain reasonable solution after running tomography algorithm. 3-D tomographic inversion based on damped least square inversion was applied to 14329 P wave arrival times and the results of 3-D tomographic inversion were tried to correlate with geologic verifications in the region. Generally low velocities change between 5.3-5.7 km/s through vertical extension of the faults. Vertical extensions of the NAF branches are observed between 2 -15 km depths when the resolution parameter of the data set taken into account. These properties are relatively similar for extracted profiles. Location of the vertical extensions of the fault zones mainly show the same depth range similar to done previous studies in the region.
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.
A rapid estimation of moment magnitude Mw for the October 23, 2011 Van earthquake using strong-motion records
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2014., 2014.) Kılıç, İrfan.; Özel, Nurcan Meral.
It is well known that the Richter scale (local magnitude scale, ML) based on the measurement of the amplitude in a narrow- band time domain saturates for large crustal earthquakes, and the possibility of observing clipping on seismograms is high. Therefore it is insufficient to use the local magnitude for reliable magnitude estimation. A typical example of this situation is confronted in case of the October 23, 2011 Van Earthquake ( ML 6.6 vs MW 7.2) . Here MW is the moment magnitude obtained from the far-field displacement spectrum of body waves. On the other hand, a better match is expected in the range of 3 < ML < 7 because in this range the empirical relation between seismic moment M0 and ML is almost the same with the relation between M0 and MW (Thatcher and Hanks, 1973; Hanks and Kanamori, 1979). Hence, there is a1234E need to find a way for the determination of MW from near-field records too.The aim of this study is to determine moment magnitude of earthquakes very soon after occurrence, before any source or focal mechanism inversions have been performed. In this respect (Delouis et al., 2009) proposed the MWSYNTH method. In this method, observed displacement spectra are compared with synthetic spectra computed for a variety of finite dimension source models scaled with MW. In this study we have tried to obtain a rapid estimation of Mw for the October 23, 2011 Van Earthquake from the strong-motion records in the epicentral distances about 120 km using MWSYNTH method. Using strong motion records of only two stations we obtained an agreeable estimation of moment magnitude MW=6.9. The result may be improved (approach more seriously to the reference moment magnitude MW ref=7.15) by doing computation for more stations and it is foreseen that a correct recovery of MW for the Van Earthquake within about 100 sec after origin time (recording+computing time) will be possible if the procedure is automated.
A review of the tectonics of the aegean region
(Thesis (M.S.)- Bogazici University. Kandilli Observatory and Eartquake Research Institute, 1998., 1998.) Komut, Tolga.; Küleli, H. Sadi.
The Aegean region is a remarkably deforming part of the Alpine-Himalayan orogenic belt, which has the highest seismic activity in Europe. An extensional deformation regime has led to subsidence of the continental crust over all the Aegean region behind the south Aegean consumption boundary. The region that is mainly under pure shear stress is an internally deforming part of the counterclockwise rotating (relative to Eurasia) Anatolian plate. To understand the tectonics and dynamic evolution of the region many geoscientists have collected numerous data and interpreted them. In the present study, all available data have been compiled into a unique database. For this purpose, as a first step, raw and processed information have been compiled and classified with their sources, date information and criticism of their accuracy and limitations. This step allows the comparison of similar types of study and data in order to· understand basic problems more clearly. As the second step, all interpretations of the studies are discussed and critically reviewed by considering associated data and methodology on a regional scale. The goal of the work is to determine the problems with highest priority, find the gaps in the database.
A tomographic image of the fault zone on the north anatolian fault
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2006., 2006.) Afacan Ergün, Tuğçe.; Karabulut, Hayrullah.
During the fall of 2003, seismic data were collected in the eastern Marmara Region on the North Anatolian Fault Zone near izmit rupture zone by a controlled source experiment. The fault was clearly exposed on the surface and the thickness of the sediments in the basin is expected to be greater than several hundred meters. The data were acquired along a 1.2 km long nmih-south profile with a total of 50 recorders equipped with 4.5 Hz geophones. 3-component geophones were used near the fault zone to detect trapped waves. The shot spacing was 5 m and receiver spacing was varying from 1Om to 20m. More than 180 shots were fired using a vibroseis. Data were recorded continuously with 1OOHz sampling rate. . This study is a step towards a determination of a shallow P wave velocity structure on the NAFZ near izmit rupture. We applied regularized inversion technique to the first arrival travel times. More than 6500 picks from 129 shot-gathers were used in the analysis. A shallow P wave velocity image (<200m) from travel time tomography was obtained in the izmit basin. The results indicate a fault zone of approximately 100 m thick. A velocity contrast was detected between the fault zone and surrounding blocks. The P wave velocity on the southern block varies between a range 1.4 - 1. 7 km/s and 1. 7 - 2.0 km/s on the northern block the velocity decreases to 1.4 km/s within the fault zone. 3-component recordings and fan shots indicates the presence of the fault zone consistent with the . tomographic image. Previous seismological studies in the region also show that the thickness of the fault zone is on the order of approximately 1OOm.
Accurate location of hypocenters using double difference and active fault structures in Gökova bay
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2014., 2014.) Eskiköy, Figen.; Aktar, Mustafa.
Double Difference Algorithm, HYPODD, is used for relocating the earthquakes in the Gökova Bay. The aim of this thesis is two folds: first we look for the best choice of inversion parameters which determine the performance of HYPODD at local scale, and second, as a by product of the test data used in the study, we determined the active faults in the Gökova Bay. We used four year (April 2006-December 2009) seismic earthquake recordings and relocated 972 events with magnitudes between 1.5 and 4.5. The inversion part of HYPODD package can be run by using both catalog and cross-correlation data. In this study both methods were used. We have observed that correlation based inversion gives a better picture only if the events in the cluster are very close to each other (<3km). When stations are sufficiently high in number (>4 stations) and well scattered around the seismic zone at moderate distances (i.e. <60 km), we observed that the performance is high, and do not critically depends on the control parameters. The improvement in using hypoDD and in particular correlation based applications is mostly apparent when depth sections are analyzed. The other important observation is that the choice parameters and therefore the final performance entirely depend on the geometry and the distance of event pairs. The parameters MAXSEP, MINLNK, MINOBS are very critical and a conservative selection of these parameters will lead to a drastic reduction of the data set. Separating the data into clusters or not is a matter which entirely depends on the data. If data shows isolated clusters with distinct character each, it would be unrealistic to use a single set of control parameters for all of them, and clustering is recommended. In term of active fault geometry of the faults in Gökova, it is clear that an offshore fault parallel to the northern boundary is well confirmed. The fault extends from midway between Ören and Çökertme to land close to Akyazı, roughly 27045’W to 28020’W. The depth section of this fault is vertical in the central part, but shows a possible south dipping in the east. At the western end the fault shows a change in strike and turns south with a strike direction of roughly 360. This fault continues to 36045’N, midway between Cos Islands and Datça Peninsula.
An application of the coda methodology for moment - rate spectra using three broadband stations in Turkey
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2003., 2003.) Eken, Tuna.; Türkelli, Niyazi.
In this study, an empirical magnitude calibration method developed by Mayeda et al., (2003) was tested by using both local and regional dista1tce earthquakes that occurred along the North Anatolian Fault Zone as well as throughout the broader region of Turkey. The method is based on source spectra that are derived from time-domain amplitude measurements of coda envelopes for 14 consecutive narrow frequency bands ranging between 0.02-8.0 Hz. Previous application of the methodology to earthquakes in the western United States and Dead Sea Rift regions (Mayeda and Walter, 1996; Mayeda et al., 2003) show that the moment magnitudes, Mw(coda), based on coda envelopes are significantly more stable and unbiased than conventional narrowband regional magnitudes which are obtained from direct phase measurements. Coda envelopes have a number of properties that make it a desirable choice for sparse station monitoring: 1) the coda envelopes are nearly insensitive to the radiation pattern and directivity of the earthquake source, 2) the coda is not as sensitive to lateral crustal heterogeneity because of the crustal averaging due to scattering, 3) clipped data can be used for fitting the envelopes after the clipped portion of the seismogram. During the calibration study, the empirical magnitude calibration method was applied in order to calibrate the three broadband stations, ISP, ISK and MALT in Turkey. Therefore, 182 common events, which mostly occurred along· the North Anatolian Fault zone near the Marmara region and recorded at both stations ISP and ISK, 137 common earthquakes, which are distributed over the broader region of Turkey recorded by stations ISP and MALT, were used. After applying the method to both pairs of datasets, consistent source spectra that were validated by equivalent seismic moment estimations from long period waveform modeling and derived moment magnitudes (Mw) were obtained. This study resulted in amplitude measurements that are a factor of 3 to 4 less variable than distance-corrected direct wave measurements (i.e., Pg, Lg, and surface waves). Upon comparing our coda-derived moment magnitude, Mw(coda)'s with those from long-period waveform modeling, Mw(waveform), a standard deviation of 0.17 for ISP-ISK and 0.14 for ISP-MALT station pair was 'Observed comparable to the results of previous studies (Mayeda and Walter, 1996; Mayeda et al., 2003). After calibrating the stations ISP, ISK and MALT, for some recent earthquakes such as the Piiltimtir earthquake of January 27, 2003, Mw = 6.1, the Urla earthquake of the Apri110, 2003, Mw = 5.7 and the Bingol earthquake of the May 1, 2003, Mw = 6.4, Mw(coda) values were estimated. The successful application of the method is remarkably important considering we are studying a much larger region with significant lateral complexities. With these calibrations we can extend the measurement of stable Mw to significantly smaller events, which could otherwise not be waveform, modeled due to poor signal-to-noise ratio.
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.
Analysis of 15 December 2000 and 3 February 2002 Sultandağı - Afyon earthquakes
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2006., 2006.) Aksarı, Doğan.; Karabulut, Hayrullah.
We analyzed two moderate size of earthquakes (Mw=6.0, Mw=6.5) occurred in Sultandağı-Afyon, southwestern Turkey on December 15, 2000 and February 3, 2002. Both earthquakes took place on the Sultandağı fault (SF) having normal mechanism with slightly different orientations and same directivities. Both earthquakes ruptured to the northwest of SF. The distance between the epicenters of two events was less than 16 km. We relocated the large magnitude aftershocks and used the closest broadband station (ISP – Isparta Station) to reveal the differences in the directivity. Coulomb stress changes for December 15, 2000 earthquake were calculated and its effect on February 3, 2002 event was explored. It was observed that the December 15, 2000 event increased Coulomb stress up to 4 bar in the area of February 3, 2002 and favored the 2002 rupture. Following both earthquakes, we observed seismic activity triggered in 40 km far from the first mainshock and 25 km from the second. It is considered that the activity took place as a result of dynamic triggering.
Analysis of microearthquakes with correlation method at salavatli geothermal area, Aydın
(Thesis (M.A.) - Bogazici University. Institute for Graduate Studies in the Social Sciences, 2013., 2013.) Kalkan, Esra.; Gürbüz, Cemil.
Induced seismicity typically refers to minor earthquakes and tremors that are caused by human activity that alters the stresses and strains on the Earth's crust. Earthquakes which are smaller magnitude are called micro-earthquakes. Micro-earthquakes could be observed in areas which includes energy technologies that involve injection or withdrawal of fluids from the subsurface. Micro-earthquakes are the result from these kind of process. Geothermal areas are good example of that kind of areas. So we chose Salavatlı, Aydın geothermal area to study. The main aim of this study is to find similar micro-earthquakes with correlation method to check if they come from same source or location. So the data which was collected from 9 broad-band stations which were installed at the study area,between June of 2010 and April of 2013 were used for this aim. 977 micro-earthquakes were located with SEISAN. 815 of them could be used for the correlation analysis due to quality of data. GISMO correlation toolbox was used for this process. The duration of waveforms are 10 seconds because the longest micro-earthquake in this study is 8 seconds. We took lower limit as 0.9 for the correlation co-efficient. 34 event of similar waveforms were found. When we examined location of all micro-earthquakes, we saw that they were scattered over the study area. This might be caused from wrong P or S wave pickings or inadequate crustal velocity model. So, firstly, we decided to check the initial velocity model by using VELEST software. For the VELEST processing 334 best located micro-earthquakes were selected on the base some criterias. After getting minimum 1D velocity model for the study area, all micro-earthquakes were relocated again and then mapped. The location of most of them altered and get better. Then with final 1D velocity model, events of similar waveforms were map. In general, when monthly location of micro-earthquakes were checked, the direction of the injected water from re-injection wells could be clearly seen on the seismicity maps.
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.
Calibration of various magnitude scales in Turkey using broadband data
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2003., 2003.) Görgün, Ethem.; Aktar, Mustafa.
The local magnitude, ML, for earthquakes of Western Turkey and surrounding regions are determined using synthetic Wood-Anderson seismograms derived from newly available broadband recording from the Boğaziçi University Kandilli Observatory and Earthquake Research Institute (BU KOERI). Seismograms obtained from various types of broadband instruments are transformed to Wood-Anderson type using SAC (Seismic Analysis Code) routines and script files that are developed for this purpose. Wood-Anderson peak amplitudes are measured on 1560 three-component, obtained from 200 earthquakes in the distance range of 8 to 785 km. The earthquakes ranged from ML = 1.0 to 6.4 and were recorded at about 7 stations in the region. ML magnitudes can be obtained from the horizontal and vertical component. Scattering of each ML magnitude in each station is analyzed and their dependence on magnitude and distance are discussed. The station magnitude correction values are found. Some improvements are proposed for the magnitude determination procedures that are actually used at BU KOERI Seismological Laboratory. Finally we have compared our ML computations with the one obtained by other studies using different approaches or different waveform data. We have noted that our results agree well with ML magnitude determinations done at TUBITAK MRC, but deviates significantly from the MD calculations of BU KOERI Seismological Laboratory.
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.
Crustal structure of southwestern Anatolia using p-receiver function analysis
(Thesis (M.S.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2008., 2008.) Özakın, Yaman.; Aktar, Mustafa.
The crustal structure in Aegean has been subject to many geophysical studies. To enlarge the picture of Aegean and African Moho’s, a P receiver function analysis has been conducted on data collected for 15 months from 12 seismic stations in south-western Anatolia. The results agree with that of various geophysical studies made on the same area. The calculated Moho depth increases from 20 km.’s in FETY, which is due to the south of Gökova Bay, to 29.4 km. in MLSB, which is due to the north of the bay. The trend of the slope appears to be in the NW-SE direction. In stations located on the southern side of the Gökova Bay, it is also possible to observe a second conversion phase which is attributed to Moho of African plate. The slope of the Moho of the African plate appears to follow the NE-SW line in the region.
Crustal structure of the Isparta Angle and surrounding regions using p-receiver function analysis
(Thesis (M.S.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2008., 2008.) Kahraman, Metin.; Türkelli, Niyazi.
Isparta Angle is located on southwestern Turkey and has a complex structure which is a consequence of collision between African and Anatolian plates. In order to further understand this complexity, we deployed a temporary network consisting of nineteen broad band instruments in addition to KOERI’s permanent seismic stations in the region. Crustal velocity and Moho depth variation were figured out along north – south and east – west profiles formed by fourteen temporary and permanent stations. We implemented two different receiver function methods to the recorded events. Firstly, we applied joint inversion of receiver functions and surface wave group velocities in order to model lithospheric velocity structure of the region. Receiver functions are sensitive to shear wave velocity contrast and vertical travel times, however surface wave dispersion curves are sensitive to shear wave velocity averages. Combining these different properties of shear waves may bridge resolution gaps associated with each individual data set. Secondly, Moho depth was calculated by applying H-K stacking algorithm. We found that the shear velocities for crustal and upper mantle thicknesses vary between 1.95-4.05 and 4.09-4.14 km/s, respectively, and in addition to this, Moho depth is varying between 30.5 and 40.7 km. beneath stations on the east – west profile. On the other hand, the shear velocities for crustal and upper mantle thicknesses vary between 1.41-4.05 and 3.97-4.23 km/s respectively, and the calculated Moho depth is between 35.5 and 47.0 km. beneath stations located on the north – south profile. Compared to the other geophysical studies done in the same region, this is the first study to understand unusual aspect of Isparta Angle.

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