M.S. Theses

Permanent URI for this collection

https://digitalarchive.library.bogazici.edu.tr/handle/123456789/11711

Browse

Now showing 1 - 20 of 49

Stress drop analysis of earthquakes in the vicinity of Ayvacık geothermal reservoir
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2023., 2023) Yıldıran, Eda.; Konca, Ali Özgün.
An earthquake swarm occurred in Çanakkale-Ayvacık, where started with an earthquake of M=5.4 magnitude on February 6, 2017, and more than 4000 earthquakes were recorded in 2 months. A significant number of these earthquakes were larger than Mw = 4. Most of the activity occurred on the southwest-dipping Tuzla Fault, which is a normal fault with WNW strike. Given that the fault region is a geothermal reservoir where active geothermal power plants operate, it is important to examine the characteristics of these earthquakes more closely. The activity started near a geothermal power plant and expanded along the Tuzla fault. In this thesis, the stress drops of earthquakes that occurred in the Ayvacık Region during the 2017 earthquake swarm was studied. Stress drop is a crucial macroscopic parameter for earthquakes and in geothermal regions, and variations in stress drop might indicate changes in stress conditions, especially pore pressure. The corner frequencies of 106 earthquakes with magnitudes greater than ML ≥ 3.0 were calculated from the P waveforms using 2 stations in the vicinity of the activity. We also calculated the moment magnitudes of 139 earthquakes with local magnitudes greater than 2.8 using P wave spectra. Our analysis shows that the stress drop of the earthquakes before and during the 2017 activity is quite variable. In addition, in comparison to the earthquakes that occurred before (∼ 3 MPa), the average stress drop is higher with mean values of about 7 − 9 MPa. We infer that the higher average stress drop during the 2017 activity might be related to a change in the pore pressure, where an increase in pore pressure might have led to a decrease in effective normal stress leading to higher slip values and therefore higher stress drops.
Inversion for elasticity tensor of focal region using machine learning algorithms
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2023., 2023) Ünal, Yılmaz.; Diner, Çağrı.
The moment tensor is an essential tool in seismology to examine the structure of seismic sources. The deformation at the focal region, using combinations of force couples arranged in a 3 x 3 matrix, is represented by the moment tensor. The moment tensor can be expressed as a linear combination of the eigenvectors of the anisotropic focal region’s elasticity tensor. The eigenvalues of a vertically transversely isotropic (VTI) elasticity tensor from a occurring moment tensor of a focal region can be obtained, and the precision of this determination depends on the degree of anisotropy. Machine learning optimization involves iteratively enhancing a machine learning model’s precision by reducing the error level. Choosing an algorithm that can effectively sample the search space and identify optimal solutions is necessary to optimise a function. Many algorithms are available for function optimization, but it is crucial to set a baseline to determine the practicable solutions for a given problem. This thesis defines a new objective function (misfit function). The function is proposed for obtaining the elastic parameters of an anisotropic focal region, and these parameters are calculated by using machine learning algorithms such as Grid Search, Random Search, Simulated Annealing, and Nelder-Mead.
Electrical resistivity structure beneath MT. Erciyes Central Anatolia, Turkey
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2022., 2022) Yazıcı Bozkurt, Ruken.; Tank, Sabri Bülent.
The Central Anatolia (CA) is an intermediate belt between the Pontides and the Anatolide-Tauride blocks. Numerous basins, folds, faults, volcanic fields and rich mineral resources in the region have attracted the attention of geoscientists for years. Mt. Erciyes is a Quaternary-aged stratovolcano located in CA, which has not been activated for a long time. in this thesis, I aimed to perform three-dimensional (3 - D) imaging of its electrical properties using magnetotellurics (MT) and interpret its structure. MT method has been extensively used in the exploration of volcanic areas. This study can be considered as the first systematic application of the MT performed in the vicinity of Mt. Erciyes. Between 2013 and 2018, deploying 48 wideband MT soundings forming a grid surrounding the volcano and its location as part of the Continental Dynamics/Central Anatolian Tectonics project (CD/CAT). The obtained data set was used for determining the dimensionality of the survey area, and the realistic electrical structure of the region was achieved with 3 - D solution algorithms including 38 station data. The results showed that the regional geoelectric strike direction is ~ N25° E which coincides with the structural strike of the region. For determining the dimensionality of the MT data, phase tensor analyses were performed. The observed wide-band data were inverted using a 3 - D inversion algorithm known as the MODEM © in two distinct ways; with and without topography information to reveal the electrical structure. The 3 - D inversion results revealed three distinct conductivity features: a widespread conductive anomaly found beneath the volcano and two conductive areas in the vicinity of the volcano. When the final model is examined, it is possible to observe that the conductive regions are separated from each other. lf the final model is examined together with the geological data, it is noticed that the boundaries of the conductor coincide with the faulting in the region.
Symplectic geometry and Hamiltonian Monte Carlo method
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2022., 2022) Öztürk, Feyza.; Diner, Çağrı.
Hamiltonian Monte Carlo (HMC) method is an application of a non- Euclidean geometry to an inverse problem. HMC is a probabilistic sampling method with the basis of Hamiltonian dynamics. One of the main advantages of HMC algorithm is to draw independent samples from the model space with a higher acceptance rate than other Markov Chain Monte Carlo (MCMC) methods. In order to understand how higher acceptance rate is achieved, I have studied HMC in the light of symplectic geometry. Hamiltonian dynamics is defined on the phase space (cotangent bundle), which has a natural symplectic structure, i.e. a differential two-form which is non-degenerate and closed. Hamiltonian function is defined on the phase space, which corresponds to the sum of misfit and the square of the generalized momentum. By using the non-degeneracy property of symplectic form, a vector field can be found in which Hamiltonian function is invariant along the integral curves of the vector field. The invariance of the Hamil tonian function results in high acceptance rate, where we apply accept-reject test to satisfy detailed-balance property. In this thesis, we define some basic concepts and theorems in symplectic geometry, then describe the relation between symplectic geometry and HMC, namely Hamiltonian dynamics. Lastly, we show an implementation for HMC algorithm to a 2D-tomography problem and analyze the tune parameters for application of HMC.
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.
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.
Resolution of isotropic percentage in moment tensor inversion of tensile sources
(Thesis (M.S.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2019., 2019.) Öter, Gökçe.; Diner, Çağrı.
Moment tensor solutions are commonly used in order to understand earthquake source mechanism. Moment tensor can be decomposed into three components, namely isotropic (ISO), double-couple (DC) and compensated linear vector dipole (CLVD). It is well-known fact that tensile sources generate non-DC components and those earth quakes can be deﬁned as the combination of both tensile and shear motion on a fault. In this thesis, resolution of the isotropic part in the moment tensor are considered for tensile sources. For that reason, synthetic waveforms are created by using full moment tensors with diﬀerent isotropic percentages and those waveforms are inverted with gCAP method. Afterwards, a range of diﬀerent isotropic values, with a step of t = 0.1, are forced in the moment tensor inversion process in order to investigate the change in variance reduction as the isotropic percentage deviate from its true value. Inversions of the full waveform are performed in diﬀerent distances and depths for three moment tensors with diﬀerent isotropic percentages, namely 2%, 5% and 14%. Inver sions results of those original moment tensors and moment tensors with manipulated isotropic percentages are expressed. Those results are compared to each other in terms of changing isotropic percentages, depth and variance reduction in diﬀerent stations. The results can be summarized as ﬁrstly, inversion is not really sensitive to the isotropic component of the moment tensor because isotropic component has small en ergy compared to the whole waveform. Secondly, earthquakes with relatively high isotropic percentages are less sensitive when inversions are performed for high values of manipulated isotropy. Finally, it is observed that the error in the depth of the earthquake is very sensitive to isotropic percentage.
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.
How does anisotropic focal region change the structure of the moment tensor?
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2021., 2021.) Poyraz, Dilay.; Diner, Çağrı.
In this study, we show how the invariants of the moment tensors change for different orientations of sources in a vertical transversely isotropic (TI) focal region. The invariants of the moment tensors, namely their norms, traces and eigenvalues, have physical interpretations such as seismic moment, isotropic-component and radiation pattern, respectively. Hence it is important to know how these values change for a given elasticity tensor of the focal region. These invariants strongly depend on the strength of anisotropy which is related with the variation of the eigenvalues of TI elasticity tensor from its closest-isotropic elasticity tensor. We plotted the values of the projection for each 10-gridded orientations of slip and normal vectors around the unit sphere in order to see the distribution properly. Thus, we plotted the projection of source tensor d onto different eigenspaces. In doing so, various materials were used for data of elastic parameters and eigenvalues such as shale, dry-cracks etc. Then the norms of the elasticity tensors of various materials has been calculated and plotted. The maximum and minimum points in the norm plots and the ratios of the maximum and minimum eigenvalues of various materials were found to be similar to each other. The eigenvalues of moment tensor for different anisotropic focal regions have been shown. Thus, the relation between isotropic component amount and γ values were illustrated. Lastly, the strength of anisotropy is evaluated and related with the invariants of the moment tensors.
Probabilistic tsunami hazard analysis for Tuzla test site using Monte Carlo simulations
(Thesis (M.S.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2018., 2018.) Bayraktar, Hafize Basak.; Karabulut, Hayrullah.; Özer Sözdinler, Ceren.
In this study, time-dependent probabilistic tsunami hazard analysis (PTHA) is performed for Tuzla in the Sea of Marmara, Turkey, related to the possible rupture of Prince Island Fault within next 50, 100, 500 and 1000 years. The study associates the probabilistic seismic hazard analysis with tsunami numerical modelling. Synthetic earthquake catalogue, which includes 100 events, was generated by Monte Carlo (MC) simulation technique. This catalogue includes earthquakes having magnitudes between Mw 6.5 and 7.1 (that comes from the maximum moment magnitude range this fault system can generate), of which probability of occurrence and associated tsunami wave heights are calculated. Tsunami numerical code NAMI DANCE using Non-Linear Shallow Water Equations are used for tsunami simulations. According to results of the tsunami numerical modelling, distribution of probability of occurrence with respect to minimum and maximum water surface elevations and maximum ow depth on land are represented. Moreover, tsunami inundation maps of Tuzla region are prepared with regard to probability of exceedance of 0.3 m ow depth and probability of exceedance of 0.3 m tsunami wave heights at synthetic gauge points for di erent time scales. This was observed that tsunami wave heights up to 1 m have 65% probability of exceedance for 50 years return period and this value reaches up to 85% in Tuzla region for next 100 year. Inundation depth also exceeds 1m in Tuzla region with probabilities of occurrence of 60% and 80% for next 50 and 100 years, respectively. When return period increases by 500 and 1000 years, earthquake occurrences demonstrate very high probabilities. Furthermore, inundation maps show that the probability of occurrence of certain ow depths ranges between 10% and 75% when exceedance threshold of wave height considered as 0.3 m for 50 and 100-year time exposure. The maximum inundation distance calculated among entire earthquake catalogue is 60 m in this test site. Looking at the situation for the synthetic gauge points, only few of them has a probability between 75% - 85% when probability of exceedance of 0.3 m wave height is considered for next 50 years and this probability value increases up to 90% for 100-year exposure time. Probabilities are observed more than 95% for 500 and 1000-year return periods due to the short return periods of characteristic faults in Marmara Sea.
The role of crustal fluids in tectonics of north-central Turkey inferred from three-dimensional magnetotellurics
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2011., 2017.) Özaydın, Sinan.; Tank, Sabri Bülent.
Central Pontides have been an active margin to Laurasia since the Paleozoic times. They consists of geological footprints of several episodes of subduction-accretion events and collisional tectonics. For these reasons, they have been a great laboratory for examining the evolution of Anatolia and the Tethyan oceans for decades. Many geological studies have been made with di erent methodologies, however there is still a lack of geophysical results in the region. In search of this potential, 26 wide-band (320 - 0.00055 Hz) magnetotelluric measurements were deployed to depict the geo-electric properties in a crustal range. The data were collected to form a 190-km-long pro le, passing through C ank r Basin, _Izmir-Ankara-Erzincan Suture Zone (_IAES), Tosya Basin, North Anatolian Fault Zone, Central Pontides Super-complex, Kure Complex and the Pontide Arc. Phase tensor analyses and 3-D inversions were applied to this data to develop a model that accurately display the geo-electrical characteristics of the region. Phase tensor analyses point out that the regional geo-electric strike angles t within the N80 E {u100000} N90 E range with the exception of N75 E for the data near C ank r Basin and K z l rmak Fault. Lack of oblique conductors in the data reduces the dimensional complexity and helps the 3-D inversion schemes to be more accurate for a pro le based measurement. Two distinct 3-D models were developed: (1) A model that covers whole frequency range and stations, (2) another model that aims to picture the area in the vicinity of North Anatolian Fault (NAF). For the second model, data from only ten stations (stations from 9 to 19) were used down to a frequency of 0.035 The nal models attained from the 3-D inversions were compatible with the rheological environment around the NAF that lacks moderate seismicity and ensures a deeper brittle to ductile transition by two resistive features appearing on both sides of the fault. This characteristic is present at the southern side of the fault via obducted ophiolites and intra-oceanic arc units, while the other side is made up of metamorphic rocks of Central Pontide Supercomplex. Both of the bodies are geological units, which are devoid of e ective porosity that can enhance mechanical strength along the fault and inhibit uid ow towards the fault zone. The NAF model demonstrate that the Tosya Basin appears as a conductive syncline where its basement is placed between 3.6 and 4.3 km. Fault zone conductors were found within this model matches well with the spatial positions of the NAF and related subsidiary faults. South of the suture zone, a buried thrust belt is visible with several norwarddipping conductive-resistive interfaces. C ank r Basin appears as a conductive zone with several resistive interruptions. Projected earthquakes correlate well with the resistivity variations indicating fault-like structures. Beneath the northernmost features in the model, the resistive characteristics appear in deeper structure con rming the crustal thickening in the region. North of the fault, CPS shows itself as a downward convex shaped resistive body that demonstrates heterogeneous conductive features near the surface. C angalda g Complex, placed at the tip of the CPS, exhibits highly resistive values beneath the Kastamonu Basin. A large conductive anomaly, which appears to have an upwelling feature was found beneath this region. Although the source of this uid-rich zone is not resolved or clear, existence of this conductive region might have important implications on the seismic nature, rheological attributes and geological evolution of the area.
Investigation of electrical conductivity characteristics at the vicinty of Genos fault, northwest Turkey by magnetotellurics
(Thesis (M.S.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2017., 2017.) Karaş, Mustafa.; Tank, Sabri Bülent.
As constituting the westernmost end of North Anatolian Fault, 45-km-long Ganos Fault acts as an active segment which represents a seismic gap. In this thesis, the ﬁrst systematic magnetotellurics observations are made to reveal the electrical conductivity characteristics of locked Ganos Fault. Near the epicenter of last major event, 1912 Mu¨refte Earthquake; audiomagnetotelluric (AMT) data were collected through north south aligned almost-continuous proﬁle including twelve stations to decipher the shal low conductivity structure. Then, thirteen wide-band magnetotellurics (MT) stations surrounding the AMT proﬁle as a grid were installed to investigate deeper structure. The dimensionality analyses performed on both AMT and wide-band MT datasets indi cate a considerable agreement with previous geological and seismological observations. While the geo-electric strike angles were determined with Swift’s method following Groom and Bailey decomposition, they were also calculated with phase tensor analy ses independently. Both methods conﬁrm each other and veriﬁed the strike directions as∼N70oE for the AMT and∼N60oE for wide-band MT data. Two- (Rodi and Mackie, 2001) and three-dimensional (Siripunvaraporn et al., 2005; Egbert and Kelbert, 2012) numerical modeling routines were used for inverse modeling. All modeling attempts illustrate highly conductive anomalies representing so-called “fault zone conductors” along the Ganos Fault. Three-dimensional models indicate that ﬂuctuations in spatial extents of the FZC are observed along the fault. Subsidiary oblique faults around Ganos Fault which are represented as conductive structures with individual mechani cally weak features merge in a greater damage zone and they constitute a wide ﬂuid bearing environment. By concentrating on the southern side of the fault, the damage zone shows an asymmetry around main fault strand, which exhibits a distributed con duit behavior for ﬂuid ﬂow. A highly resistive block represents the ophiolitic basement beneath younger formations at a depth of 2 km, where the mechanically weak to strong transition occurs. Below this transition, the overt resistive structures at both side of the fault imply that the absence of ﬂuid pathways through the seismogenic zone might be related to lack of seismicity in the region.
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 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.
Studying the finiteness of large earthquakes with higher order moments of the moment tensor
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2016., 2016.) Örsvuran, Rıdvan.; Konca, Ali Özgün.; Diner, Çağrı.
Finite fault parameters of the large earthquakes can be obtained using kinematic finite fault models which consist of a collection of subfaults. Yet calculating each subfault individually costs time and the fault model needs to be defined a priori for the inversion process. Alternative representation of the source is defining it as a moment tensor density distribution. In this case higher order moments of the distribution can be calculated. Higher order moments can also be used to estimate first order finite-fault parameters and it has the advantage of having less number of unknowns. This characteristic would allow for more rapid fault parameter solutions. The aim of this thesis is to develop a measure on the approximation of finite fault models using higher order moments up to degree two. Synthetic seismograms for both finite fault sources and their higher order moment approximations are generated using infinite homogeneous isotropic medium to identify the similarities between the waveforms. The effects of the receiver azimuths and distances are investigated using using different frequency ranges. It is found that higher order moments can give an approximation of waveform broadness or pulse width rather than the overall shape of the waveforms. Higher order moments also improved the point source approximations at frequencies that are beyond the corner frequency of the event. Fault type, fault strike direction and receiver azimuth influence the higher order moment solutions while distance is an insignificant factor at least for the whole-space medium which is considered in this study.
Numerical modeling of exhumation of high pressure - low temperature metamorphic rocks: a case study of Tavsanlı and Afyon zone in western Anatolia
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2016., 2016.) Bodur, Ömer Faruk.; Konca, Ali Özgün.; Göğüş, Oğuz.
Despite many numerical modeling works on burial and exhumation of crustal rocks, the formation of metamorphic rocks in High Pressure / Low Temperature (HP/LT) region and mechanism of their exhumation are poorly constrained. The metamorphic rocks in Tavsanli and Afyon Zone have high P/T ratios and they are represented by blueschist and greenschist to blueschist transitional rocks, respectively. The mechanism of burial and exhumation of crustal rocks were investigated by performing numerical experiments such as subduction of oceanic plate under the continental margin, intra-oceanic subduction and post-collision. Rheological constraints on exhumation of HP/LT rocks are presented. The numerical modelling results for subduction settings indicate that lithospheric mantle deforming in viscous rheology is the key factor to preserve the temperature of buried crustal rocks in subduction channel, whereas continental lithospheric mantle deforming in frictional plastic rheology gives metamorphic rocks in higher pressures and temperatures. The weaker crust and viscous rheology imposed in continental lithospheric mantle provides metamorphic rocks with P-T conditions comparable to Tavsanli Zone; strong crust and viscous rheology in continental lithospheric mantle gives metamorphic rocks with P-T conditions in agreement with Afyon Zone. In post-collision setting, the numerical modelling results show that weaker crust with high Moho temperature are the necessary conditions for exhumation of crustal rocks in high temperature / high pressure region.
Variations of source parameters due to anisotropic focal region
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2016., 2016.) Ertuncay, Deniz.; Diner, Çağrı.
Seismic sources in anisotropic medium have more complex moment tensor structures compared with the moment tensors of isotropic medium. Shear sources in an isotropic focal medium generate pure double-couple (DC) moment tensors. However in an anisotropic medium, shear sources can generate moment tensors with DC, compensated linear vector dipole (CLVD) and isotropic (ISO) components. The DC, CLVD and ISO percentages of a moment tensor depend on the magnitude and the orientation of the anisotropy. In this study, we choose five fault types namely, left/right strike slip, normal, reverse and dip-slip faults in a medium of different anisotropy classes; transversely isotropic, orthotropic and monoclinic. We rotated the anisotropic elasticity tensors of the medium for every possible orientation and evaluate the moment tensors of each cases. Then moment tensor decomposition is applied and DC, CLVD and ISO components are found. We plot the DC, CLVD and ISO percentages of the moment tensors generated by different fault types and anisotropy classes. By using the DC components, first we obtained fault plane orientation then we calculate the deviation from the original fault mechanism. Effects of anisotropy of the source region on calculated fault parameters are found. Distance from isotropic space of given anisotropic elasticity tensor and P/S wave velocity anisotropy percentages are measured. These percentages are proportional to the distane from isotropy. There is a correlation between distance to isotropy and P wave anisotropy with variation of fault plane parameters and percentages of non-DC components of earthquake source.
The magnetotlluric phase tensor method for imaging the north Anatolian fault zone
(Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2016., 2016.) Bekin, Ekrem.; Tank, Sabri Bülent.
Magnetotelluric data often suffer from localized (three-dimensional) heterogeneities by a real and frequency independent phenomenon known as “galvanic distortions”. There have been numerous techniques developed for avoiding the galvanic distortions and thus, determining the dimensionality of the observed data. Groom and Bailey’s decomposition and the phase tensor analyses are used for avoiding galvanic distortions and Swift’s analysis, Bahr’s parameters are some of the most common and conventional dimensionality determination approaches. The phase tensor is a relatively more recent approach compared to the rest and is useful for studying in a distortion-free medium. The main objective of this study is to develop MATLAB® scripts for calculating and imaging the phase tensor ellipses and applying them to observed data collected at the central part of the North Anatolian Fault Zone (NAFZ). Final images are compared to other geophysical study results performed within the same area.

Browse

Recent Submissions