M.S. Theses

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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.
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.
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.
Identification of triggered earthquakes in the gulf of Gökova
(Thesis (M.S.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2010., 2010.) Güner, Gülçin.; Aktar, Mustafa.
Triggering mechanism is described as the consequence of the stress transfer on the faults. Triggered earthquakes can either be close to the triggering one as well as far from it. The study done in this thesis is to search for the static and dynamic triggering effects in the Gulf of Gökova using three different catalogues. One of the catalogues is prepared within this study (July, 2007) and the others are from NEMC and USGS (2001-2009). Triggering effect due to local, regional and global earthquakes are considered. First, a high accuracy analysis is carried out using the catalogue produced within the framework of this thesis for a period of two months. Mean values of stress amplitudes with their variances and standard deviations are evaluated for each seismic station (BLCB, BODT, CETI, DALT, DAT, ELL, MLSB, OREN, TURG and YER). These stress variations in the Gulf are compared with the seismicity rates. Teleseismic events are seen to give signs of triggering. A second analysis is made using a catalogue of longer duration (2001-2009) but lower resolution (completeness, M>2.5). This second analysis also shows evidences for triggering by teleseismic events.
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.
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.
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.

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