Jeofizik

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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.
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.
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.
Search of optimal crustal velocities using waveform modelling of local earthquakes
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2005., 2004.) Bekler, Feyza Nur.; Aktar, Mustafa.
One of the concerns of geophysicist during the last decade is to reduce the damage of earthquakes. As a result, seismic hazard studies have become an integral part of long term planning and mitigation. One approach is to calculate pe.ak ground acceleration (PGA) and use this scalar value in building design. However, this method has some disadvantages: it is very simple and it does not give spectral information related to the ground motion. Generally, the effects of faulting type, source mechanism, rupture directivity, asperities are ignored during the calculation of the PGA values. The study in this thesis contributes to the simulation of the ground motion by constructing optimal crustal velocity models based on 1- D synthetic seismogram modeling. For this purpose, different crustal models were generated using the discrete wave-number technique (Bouchon, 1981) and they have been tested by searching the best-fit between observed and synthetic seismograms. Waveforms from 5 earthquakes were analyzed in this study. The selected earthquakes have magnitudes larger than 3.5 and they are located along the main fault zone in the Sea of Marmara. The vertical, radial and transversal components were compared using the cross correlation coefficient between observed and synthetic seismograms. Crustal models having S-layers with fixed depths were used to calculate the synthetics for each selected event. First, the optimum P-wave velocities were searched within predefined velocity limits for each layer. Once the optimum P velocities were obtained, then the S wave velocities have been searched. In general a moderate level of fitting is obtained even for the optimal crustal models. Although numerically the correlation values are quite low, the shapes of the waveforms are roughly close to each other, at least for some selected parts of the total waveform. The degree of fitting is particularly low in the part of the waveform where the 3-dimentional effects in the crust start to dominate, such as the P-arrivals in the transversal component. The performance also degrades with the level of the local noise, which is known to be not negligeable at ISKB station. The use of a clever search algorithm that uses a feedback mechanism to guide the search in a selective parameter space and accelerates the convergence towards the optimum (such as steepest descent, etc) will allow the scanning of wider range of parameter (eg estimating the layer depths in parallel to velocities, etc). This will certainely improve the results.
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
The analysis of 2003 Saros earthquake sequence, North Eastern aegean region and a calibration study of surface wawe magnitude (Ms)
(Thesis (M.S.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2005., 2005.) Mutlu, Ahu.; Karabulut, Hayrullah.
This study includes works on two different topics. In the first part, we analyzed the aftershock activity following Mw=5.8, 6th July 2003 Saros earthquake. The activity took place along the axis of the Saros gulf between the depths of 8 km and 20 km. The mainshock occurred at a depth of 17.5 km on the continuation of the North Anatolian Fault Zone (NAFZ) in the Gulf of Saros. Focal mechanism of the mainshock and largest aftershocks are almost pure right lateral strike slip with minor normal faulting. Strikes of the fault mechanism solutions are aligned with the axis of Saros depression. in the second part of this study, three different surface wave magnitude (Ms) formulae, Prague, Herak and Herak and Modified Prague formulae were used to determine Ms for earthquakes in and around Turkey between the years 1997 and 2004, recorded at local and regional distances. The results of three Ms formulae with different correction factors were compared. It was concluded that Prague formula and Modified Prague formula produce significantly distance dependent estimates while Herak and Herak formula has no significant distance dependency and more robust Ms values. It was also observed that Herak and Herak formula has better correlation with Moment magnitude than the other two formulae.
Paleoseismological studies on Düzce fault and geological data on the seismogenic sources in the vicinity of Düzce area
(Thesis (Ph.D.)- Bogazici University. Kandilli Observatory and Eartquake Research Institute, 2005., 2005.) Komut, Tolga.; Gürbüz, Cemil.
In 1999, two earthquakes ruptured the Northern Almaclk strand (NAIS) of the North Anatolian Fault that includes the Diizce and Aksu faults, producing surface rupture along the northern border of the Almaclk block. Paleoseismological trenching is performed at four sites along the Aydmpmar and Mengencik segments of four-structural-segmented Diizce fault. Six dated-events from eleven trenches that cross cuts the fault zone provided insights on its seismogenic behavior. The excavations from five trenches at three sites expose evidence for six earthquakes that occurred since B. C. 1740. A shallow of watertable, unfavorable trench materials and lack of financial support to perform 3-D trenching made the identification anu characterization of individual paleoearthquakes difficult. However, by integrating date constrains of events from the trenches that were performed along the Diizce fault including previous studies along the easternmost segment, a periodical recurrence model seems to be concordant with the radiocarbon age data. Seven serial surface-rupturing earthquakes including the 1999 Diizce event along the Diizce fault are defined. The model suggests that the Diizce fault is behaving as a single seismic segment at least for the last serial rupture in Holocene time. The additional two older events are showing an irregularity in the serial. Hitherto unrecognized earthquakes probably would eliminate this irregularity. A recurrence interval is estimated by also considering the two older earthquakes. This final sequential model suggests that 1999-type earthquakes repeat each 355±35 year (% 70 probability). The slip rate that was calculated from 350 cm average slir of the 1999 event and the recurrence period is about 9.5±1 mmlyr having % 70 probability. This value is in agreement with geodetic measurements (10 mmlyr) from independent studies. Because short time has elapsed since the 1999 earthquake that occurred along the Diizce fault, the Diizce fault does not appear to have an important seismogenic potential in near future. The Diizce plain is one of the major basins of the Marmara region. There is no active faulting excluding Diizce and Aksu faults that is detected in the basin and surrounding borders according to this study. In addition to this pull-apart and then continuing active formation of the Diizce basin hypothesis is not validated by rigorous data. Therefore, considering these data, it may be suggested that Diizce area is not under a severe seismogenic threat for very near future.
Relocating earthquakes by hypodd in Cınarcık basın and surrounding
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2005., 2006.) Bulut, Fatih.; Aktar, Mustafa.
We relocated part of aftershock activity in <;marclk Basin and surrounding that are associated with the 1999 izmit Earthquake Mw 7.4. We used double difference relocation algorithm (Waldhauser and Ellsworth, 2000) to relocate the aftershocks. The data set was obtained by a temporary seismic network deployed by Bogazi9i University, Kandilli Observatory and Earthquake Research Institute, LGIT (Grenoble) and IPGP (Paris) 10 days after the mainshock (Karabulut et al., 2002). For a better station coverage, additional data set was obtained from a network operated by TUBITAK Marmara Research Center. Differential travel times were calculated using both arrival time readings and waveform cross correlation method. We relocated 1550 of the aftershocks and interpreted the results by classification into three main clusters, namely, Tuzla, Y alova and Central Cluster. Tuzla Cluster is located in the northern scarp of the <;marclk Basin and shows events that are linearly oriented in NW-SE direction. The depth section of this cluster indicates a vertically dipping activity,. The linear trend might imply a secondary strike-slip faulting parallel to the main one. However, the earlier fault plane solutions do not confirm this statement. Y alova Cluster contains a well-developed aftershock activity that is located beneath the north of the Armutlu Peninsula. The depth section of this activity reveals a well defined linearly dipping characteristic which is plunging to the Iiorth with an approximate angle of 56°. The orientation of the seismicity is roughly EW and therefore parallel to the main rupture of 1999 izmit Earthquake. The Central Cluster traverses the total length of the Gulf of izmit and extends into the <;marctk Basin linearly with the orientation of E-W direction. It corresponds to the continuation of the main rupture of 1999 Izmit Earthquake to the west of the Hersek Peninsula. The relocation results obtained by HypoDD reveal seismicity patterns in a more clarified manner, provide more convincing data for models that were proposed before and fmally imply new seismological ideas about the Eastern Marmara.
Seismic anisotropy and mantle flow beneath Turkey
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2006., 2006.) Polat, Gülten.; Türkelli, Niyazi.
In this study, SKS and SKKS phases have been used for the analysis of shear wave splitting in order to investigate polarization anisotropy in the upper mantle beneath Turkey. To determine the shear wave splitting parameters (fast polarization direction and delay time), we have used teleseismic waveforms obtained from 21 broadband stations in Turkey. Shear wave splitting measurements are very important in determining the role of mantle flow in the geodynamics of the African-Arabian-Eurasian collison. The collision of the Arabian and African plates with Eurasia dominates the tectonic framework of the Eastern Mediterranean and Middle East (e.g., McKenzie, 1972; Jackson and McKenzie, 1988). The leading edge of the African plate is being subducted along the Hellenic trench at a higher rate than the relative northward motion of the African· plate, required that the trench moves southward relative to Eurasia proper (e.g., Sander and England, 1989; Royden, 1993). There are two adjacent subduction zones in the region: The Hellenic arc to the west and the Cyprian arc to the east. The Hellenic arc is characterized by a relatively steep, retreating subduction, whereas the Cyprus arc appears to involve a shallow subduction with two major seamounts (the Eratosthenes and Anixamander) impinging on the trench (Kempler and Ben-Avraham, 1987). Extension behind the Hellenic arc is arc-normal, whereas extension behind the Cyprus arc appears to be arc-parallel. Our results in Central Anatolia basically exhibits a NE-SW fast direction, while in Eastern Anatolia we have found a NE-SW fast direction and lag time consistent with the results obtained from temporary broadband stations of the eastern Turkey Seismic Experiment project within the Anatolian plate (Sandvol, 2003). These observations indicate that the anisotropic fabric could be relatively uniform throughout the upper mantle beneath the Anatolian plate. However, in the western Anatolia we have found a N-S fast direction that shows consistency with the directions of lithospheric extension inferred from GPS data. We have also found some evidence of trench parallel mantle flow as inferred from our results and those of Hatzfeld et al. (2001). Given the thin and hot lithospheric mantle beneath Turkey, it is unlikely that any of the observed anisotropy can be attributed to "frozen" or "fossilized" LPO induced splitting. Therefore, we believe that these observed changes in splitting reflect the variations in the asthenospheric flow along the AfricanAnatolian plate boundary.
The crustal structure of the central anatolia using receiver function analysis
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2006., 2006.) Yelkenci, Seda.; Gürbüz, Cemil.
Central Anatolia has complicated crustal and upper mantle structure that is influenced by the thermal and geodynamic process related to motions of surrounding plates. Few studies of the crustal structure of Central Anatolia have been done. Basically, studies of the nature of the crust have been performed for central Anatolia under the Calibration project started in 2002. To further investigate the crustal structure in the region, we present results from receiver function analysis using new data from temporary network which was installed under the same project. These temporary stations operated for period of one and half months between October and November, 2002. In addition, permanent stations ANTO, ISP and MALT were also used to compute receiver functions. Receiver function that is the seismic response of the Earth beneath the seismic station leads to investigation of tectonic and geodynamic processes. In this study, H-K stacking algorithm which gives crustal thicknesses and VpNs ratios as outputs of method has been used for receiver function analysis. Three component teleseismic data with epicentral distances between 30 and 103 degrees has been preferred to obtain single-event RFs and to investigate Moho depth under the Central Anatolia. Consequently, the structure of the region has been reasonably defined and compared with other geophysical studies. As a result of the RF analysis, although it has found that Moho depth decreases from about 43 km eastern part of central Anatolia to about 35 km western part, we observed sharp changes for some regions in which Tuz GOlti basin, central Anatolia core complex and !sparta that may prevent smooth changes in crustal thickness. Furthermore, we found almost high VpNs ratios that can indicate thermal structure in the study area. Additionally, we acquired information concerning the thickness of central Anatolia from relocating local earthquakes, which were recorded by the temporary stations during recording periods. Then obtained outcomes were discussed with results of RF analysis. In the light of this study, new treaties based on waveform modeling technique in the future can allow us getting detailed investigation concerning crustal structure of the Central Anatolia.
The crustal and upper mantle shear wave velocity structure beneath eastern Turkey using inversion of surface wave dispersion
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2005., 2006.) Can, Birsen.; Gürbüz, Cemil.
In this study, shear wave velocity structure of the crust and upper mantle across a portion of Eastern Turkey was obtained with one station method, using group and phase velocity dispersion of fundamental mode Rayleigh and Love waves. The S wave velocity structure of the Eastern Turkey is estimated beneath the 29 broadband seismological stations that were deployed during the 1999/2001 ETSE experiment. Rayleigh wave phase velocity is measured between 10 and 85 second periods. Phase velocity is 2.75 km/s for 10 second period and it is 3.9 km/s for 85 second period. On the other hand, Love wave phase velocity is measured between 10 and 50 second periods. Phase velocity is 2.8 km/s for 10 second period and 4.40 km/s for 50 second period. According to the results of S wave velocity inversion? it is found that; between 20 and 30 km depth there is a low velocity layer, which has S wave velocity of approximately 3.4 km/s. The thickness of the crust is calculated as 45 km. S wave velocity, gradually increases between 30 km and 45km.
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.
High frequency ground motion scaling in eastern Turkey
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2006., 2006.) Şemin, Korhan Umut.; Gürbüz, Cemil.
A regional study of ground motion scaling parameters is presented for the region surrounding the eastern Turkey. The data set used in this study consists of 100 earthquake events from Eastern Turkey Seismic Experiment (ETSE) network with the magnitudes between (ML and Mw) 3.0 - 5.5. All selected events are in the upper crust. In order to emprically obtain the scaling relationships for high frequency S-wave motion, regressions are carried out on three component broadband seismograms, all recorded within a hypocentral distance of 400 km to emprically obtain the scaling relationships for the high ' frequency S-wave motion. The signals were processed to examine the peak ground velocity and Fourier velocity spectra in the frequency range of 0.3 - 10 Hz. Random vibration theory (RVT) is used to test estimates of the peak ground motion in the time domain. Comparison of the two regressions indicated both regression results display consistent shapes. Geometrical spreading function g(r), characterized by: g(r) = r-Lo for r ~ 40 km, r0 is used for 40~ r ~ 100 km and { 0 · 5 is used for r 2: 100 km. A very low quality factor, Q(f) = 50fl·75 is used to described the anel~stic crustal attenuation in the region. Excitation terms are well matched. An effective high-frequency, distance independant spectral parameter, Kerr= 0.035 sec, is obtained in this study. Both regressions show that eastern Turkey region has very high attenuative properties.
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.
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.
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.
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.
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.
Seismic site studies in Pendik - Tuzla regions in Istanbul
(Thesis (M.S.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2010., 2010.) Er, Leyla Aslı.; Gürbüz, Cemil.
The objective of this study is to identify soil amplification and soil fundamental frequency in the area between Pendik and Tuzla, the southeast of Istanbul city, and showing how the values of soil amplification, dominant period and frequency will change according to layer thickness overlying the bedrock. Pendik - Tuzla regions were selected according to variation of bedrock depth which changes from 65 to 150 meters and topographical changes from north to south representing a valley type structure. According to geological formations, and topographic structure, 64 measurement points were selected for a horizontal/vertical spectral ratios method, (Nakamura's method) in Pendik–Tuzla area. In the same area, OYO Company collected data using Suspension Ps logging and Refraction Microtremor methods. I used these data to calculate amplification and dominant period using Shake 91 program for different 56 measurement points. Also, dominant periods were calculated using empirical formulas. The results obtained from site measurements and model studies were correlated with each other and were displayed in contour maps and interpreted according to geology, topography and the geological distributions of bedrock depths and the formation types in the investigated area. According to the H/V and model studies, amplifications on the three regions, Aydınlı, Orhanlı and south of the study areas were found much more bigger than surrounding stable areas. These three regions consist of approximately reaching up 120 m thickness of Cenozoic aged, Sultanbeyli and Kuşdili Formations, overlying the Paleozoic bedrocks. Besides, the highest fundamental soil period values calculating from both studies are estimated in these regions. As a result through investigation and significant observation of both of these studies, thickness of sedimentary layers overlying bedrock and soil dynamic properties of these structure layers have a great effect on the evaluation of seismic site studies, soil transfer function and dominant period and frequencies.
Rapid determination of earthquake magnitude using ElarmS
(Thesis (M.S.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2010., 2010.) Coşkun, Zeynep.; Aktar, Mustafa.; Özalaybey, Serdar.
Earthquake Alarms Systems, ElarmS, is designed with the goal of providing warning of forthcoming ground shaking during earthquakes. The event magnitude is found to scale with the maximum predominant period, τp max, which is estimated, using the frequency content of the first few seconds of the P-wave arrival from the vertical component of velocity records. I tested ElarmS offline using 242 earthquakes with magnitudes between 0.5 and 5.1 occurred across The Gulf of Gökova, Turkey. The test data for small magnitude events were obtained from the detailed re-processing of August 2007 with additional data from temporary stations. The larger events were directly taken from NEMC catalogue. I found that events smaller than 3.0 did not have clear relation between τp max and event magnitude, on the other hand, events with magnitude larger than 3.0 showed a scaling relation between τp max and event magnitude. The relation obtained for Gokova was consistent with the ones obtained in California and Japan. The average magnitude error between real local magnitude and ElarmS magnitude was about  0.91 magnitude units when single closest station to the epicenter was used. Once data from the two closest stations available the error drops to  0.62, when six station data available the error drops to  0.49.

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