16.7.1963 ve 28.7.1976 Kafkasya depremlerinin fay düzlemi çözümleri /
Yazar: ONUR TAN
Danışman: DOÇ. DR. TUNCAY TAYMAZ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Jeofizik Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 1998
Konu: Jeofizik Mühendisliği = Geophysics Engineering
Dizin: Deprem = Earthquake ; Fay düzlemi çözümleri = Fault plane solutions ; Kafkasya = Caucasus

Özet:
Bu yüksek lisans tezinde yerkabuğunun aktif olarak deformasyona uğradığı bölgelerden biri olan Kafkaslar'da oluşmuş iki depremin (16.07.1963, t0: 18:27:18.4, Ms=6.5 ve 28.07.1976, t0: 20:17:44.0, Ms=6.1) fay düzlemi mekanizması parametreleri incelenmiştir. Kafkasya ve çevresi yeryüzündeki en genç kıtasal çarpışma bölgelerinden biridir. Güneyde, Arabistan Levhası'nın kuzeye olan hareketi sonucunda Doğu Anadolu ve Kafkaslar'da muhtemelen 3 m.y. önce başlayan sıkışma rejimi bölgeyi yükseltmiştir. Güneyden gelen hareketin büyük bir kısmı Borjomi-Kazbek Fayı (Doğu Anadolu Fayı) tarafından Doğu Büyük Kafkaslar üzerine aktarılır. Büyük depremlerin genelde Doğu Büyük Kafkaslar, Küçük Kafkaslar ve Borjomi-Kazbek Fayı üzerinde yoğunlaştıkları gözlenmektedir. Batı Büyük Kafkaslar ve çevresinde ise sismik aktivite daha azdır. Fay düzlemi çözümünde deprem episantırından 30°-90° uzaklıklar arasındaki analog WWSSN istasyonlarının P ve SH sismogramları ile McCaffrey ve Abers (1988) 'in Nâbelek (1984) 'den uyarladıkları algoritmalar (SYN3, SYN4) kullanılmıştır. Yapılan ters çözüm işleminde istasyonlar azimutal dağılımlarına ve sismik fazlara göre ağırlıklandırılarak referans alet büyütmesi 3000 ve episantır uzaklığı 40° olan referans bir istasyona göre normalize edilmiştir. Ters çözümde deprem odağındaki hareket bir nokta kaynak (kuvvet çifti) gibi kabul edilmiştir. Her iki deprem içinde fay düzlemleri kuzeybatı-güneydoğu yönlüdür. 16.07.1963 depremi kuzeydoğu dalımlı, 28.07.1976 depremi güneybatı dalımlı ters faylanma vermektedir. Ancak doğrultu-atım bileşenleride mevcuttur. Fay düzlemi parametrelerinin hata miktarlarını belirleyebilmek için kabuk modeli, odak derinliği, doğrultu, dalım ve kayma açısı testleri yapılmıştır. 16.07.1963 depreminin doğrultu, dalım, kayma açısı, kayma vektörü azimutu (ve dalımı) sırasıyla, 1. düzlem için 288±5°, 48±5°, 106±5°, 355° (dalım 46°) iken 2. düzlem için 85°, 44°, 73°, 18° (dalım 42°) dir. Odak derinliği 3±1 km'dir. Depremin sismik momenti 2.98xl018Nm'dir. 28.07.1976 depreminin doğrultu, dalım, kayma açısı, kayma vektörü azimutu (ve dalımı) sırasıyla 1. düzlem için 144±10°, 9 -2/+100, 123 -5/+100, 201° (dalım 8°) iken 2. düzlem için 291°, 82°, 85°, 54° (dalım 81°) dir. Deprem odak derinliği 19±2 km'dir. Depremin sismik momenti 1.13xl018Nm'dir. Fay düzlemi çözümleri, sıkışma rejimi nedeniyle çok az doğrultu atım bileşeni içeren ters faylanmalar göstermektedir. Her iki depreminde basınç eksenleri kuzey-güney yönlüdür. 1963 depreminin P ekseninin azimutu 7° dalımı 2° iken bu değerler T ekseni için 268° ve 78° dir. 1976 depremine ait eksenlerin azimut ve dalım değerleri ise sırasıyla P için 25° ve 37°, T için 195° ve 53° dir.

Abstract:
Seismic activity at continental plate boundaries is distributed over zones often many hundreds of kilometres in extent, rather than being restricted to a narrow zone only a few kilometres wide, as observed in the oceanic plate boundaries. On the other hand, the study of active deformation of continental lithosphere through observations of shallow continental earthquakes has been an attractive research topic in recent years, though the number of events studied still remains relatively small, and detailed studies are limited in numbers. Caucasus and surrounding regions are one of the youngest and seismically active continental collision zones on the Earth. This orogenic mountain belt is highest in elevation at the eastern part of the Alpine-Himalayan belt. The highest point on the Caucasus is Elbruz Mountain (5633 m), and the overall average elevation is about 3650 m. The crustal seismicity gives an insight about active faulting, while thrusts on both flanks of the Great Caucasus are to be identified as the most active features. In addition, the absence of any coseismic surface faulting for the earthquakes studied, and the lack of either any previously recognised major seismogenic structures, or evidence of large historical earthquakes in the epicentral area, provides evidence of the significant contribution that blind thrust faults may play an important role in convergent regions. In other words, most of the shortening in the Caucasus appears to occur aseismically. Therefore, it is further thought that the Caucasus region is in the initial stages of continental collision. This dissertation thesis consists of studies of two moderate-large continental earthquakes from the Caucasus region of the Alpine-Himalayan collisional belt. The Caucasus mountains and surrounding regions are one of the most seismically active regions of intense deformation on the continents, and seismicity accommodates continental shortening between the Eurasian and Arabian plates. Earthquake focal mechanisms suggest that observed convergence along the Eurasian and Arabian plate boundaries is further partioned into almost pure shortening by thrusting in the Greater Caucasus, and pure right-lateral strike-slip motion in eastern Turkey. xvThe current work presented in this dissertation represents teleseismic seismological observations, along with previously published field studies in the epicentral regions and other individual research. The earthquakes studied along with the Racha (Georgia) and the Spitak (Armenia) earthquakes are the largest events in the Caucasus in recent decades, and the largest low-angle thrust faulting in a region of continental convergence. The four main strike-slip faults control the local tectonics, while the Anatolian Plate lies between the right-lateral North Anatolian Fault and the left-lateral East Anatolian Fault escapes to the west, the Iranian Block escapes to the east along the Main Recent Zagros Fault. The right-lateral fault displacement continues to the east of Karlıova triple junction (Çaldıran Fault and Tebriz Fault). The fourth strike-slip fault of the Caucasus and the surrounding regions is the left-lateral Borjomi-Kazbek Fault from the Eastern Anatolia to the Greater Caucasus. It divides the Greater Caucasus into two parts: the Western Greater Caucasus and the Eastern Greater Caucaus. Topography and isobaths of the Moho show that there is a left-lateral displacement between the western and the eastern parts. According to the tectonical and geological studies, the subduction of the Tethys occurred between Late Cretaceous and Eosen. A marginal sea developed between the Anatolian-Lesser Caucasus plates and Russian Platform. There was a calk-alkaline volcanic arc at the southern border of the marginal sea. After the beginning of the opening of the Red Sea in Oligosen-Miosen, the Arabian Platform began to its northward motion. This caused the north-south compression of the Tethys. The Tethys Ocean closed about 20 m.y. ago and subduction was shifted to the northern boundary of the marginal sea. The first continental collision between the Arabian and Eurasian plates most probably occurred in Middle-Pliocene (3.5 m.y. ago). This collision uplifts the Eastern Anatolia and the Caucasus regions. The motion of Arabian Plate (25-30 mm/yr) continues to deform the region, and with compression 10 mm/yr obtained from recent GPS measurements. Earthquakes' epicentral distribution also shows the seismically active regions. Although the Lesser Caucasus, the Eastern Greater Caucasus and the Borjomi-Kazbek Fault (North East Anatolian Fault) have lots of earthquakes clustered nearby, the Western Grater Caucasus has a few earthquakes. It is simply due to the Borjomi-Kazbek Fault, which partitions the significant part of the northward motion of the Arabian Plate to the Eastern Greater Caucasus. The structural, geological and seismological studies point out that an earthquake bigger than 7.0 can not occur in this region. The biggest earthquake is the Racha Earthquake (29.04.1991, t0: 09:12:48.1, Ms=7.0). This event took place on the southern margin of the Greater Caucasus near the Borjomi-Kazbek xviFault. Its fault plane solutions show thrust faulting with a right-lateral strike slip component. The other destructive event was the Spitak (Armenian) Earthquake (07.1.1988, t0: 07:41:24.3, Ms=6.7) on the Pampak-Sevan Fault, on the Lesser Caucasus. The solution of this event is similar to that of the Racha Earthquake. In this thesis, the fault plane solutions of the 16.07.1963 (the Western Greater Caucasus, t0: 18:27:18.4, Ms=6.5) and the 28.07.1976 (the Eastern Greater Caucasus, t0: 20:17:44.0, Ms=6.1) earthquakes were analysed using SYN4 package of McCaffrey and Abers' (1988) version of Nâbelek's (1984) teleseismic waveform modeling algorithm. The shapes and amplitudes of P- and SH- waveforms recorded by stations in the range 30°~90°, and for which signal amplitudes were large enough, with synthetic waveforms were compared. The phases recorded the stations in this range, travels in mantle. The P and SH phases are mixed with the core phases (PcP, ScS) far away from 90° while the rays between the source and the station too close to source (A < 30°) travel upper mantle which has high velocity gradient. Synthetic seismograms are generated by combining direct (P or S) and reflected (pP and sP, or sS) phases from a point source embedded in a given velocity structure. Receiver structures are assumed to be homogeneous half-spaces. Amplitudes are adjusted for geometrical spreading, and for attenuation using Futterman's (1962) operator, with t*=l s for P and t*=4 s for SH. Uncertainties in t* affect mainly source duration and seismic moment, rather than source orientation or centroid depth. Seismograms were weighted according to the azimuthal distribution of stations, such that stations clustered together were given smaller weights than those of isolated stations. The inversion routine then adjusts the strike, dip, rake, centroid depth and source time function, which is described by the amplitudes of a series of overlapping isosceles triangles whose number and duration we selected. In summary, the main features of the technique used in this thesis are: 1. direct inversion for the centroid depth; 2. parameterization of the source time function (STF) in terms of overlapping isosceles triangles that has a realistic decay of the earthquake spectrum; 3. determination of the source orientation and seismic moment; 4. finite source effects may be incorporated using either a generalized Haskell model (extended line source); or a propagating line source; or multiple events in space and time. Fault plane mechanisms of 16.07.1963 and 28.07.1976 earthquakes were first determined by Jackson and McKenzie (1984) by using first motion polarity xvnreadings of P waves. These solutions showed thrust faulting for both events. We have collected 13 LPZ, 14 LPN, 13 LPE seismograms from 16 WWSSN stations for the 16.07.1963 earthquake, and 18 LPZ, 20 LPN, 17 LPE seismograms from 20 WWSSN stations for the 28.07.1976 earthquake. However, all of them could not be used within the inversion due to bad quality of microfilm, and due to instrumental calibration problems, due to background noise and due to missing seismograms. The analogue seismograms were then digitized. The clock-timing problems were corrected according to the Jeffreys-Bullen Travel Time Tables. There is no reliable crustal structure studies for the velocity structure of the Caucasus. If necessary the source structures are assumed from other studies of continental earthquakes. After finding a good minimum misfit solution of the 16.07.1963 earthquake, strike, dip, rake and seismic moment values were calculated to be 288°, 48°, 106° and 2.99xl018Nm respectively. The fault plane determined from field studies is known to be dipping NNE direction. To determine the effects of the structure on the minimum misfit solution, the velocity model tests were conducted. Two different models were analysed for this test. One of the models has a top layer with Vp=5.6 km/s, Vs=3.2 km/s and p= 2.7 gr/cm3, h=4 km. The other model has the same velocity and density values, but 8 km layer thickness. Both of the models has the same half-space as main solution. These tests showed that the different structure models did not significantly effect the solution much. After the interpretation of depth, strike, dip and rake tests, the values of the parameters and their error limits are found to be as 3±1 km, 288±5°, 48±5°, 106±5°, respectively. The solution (295740o/115°) of Jackson and McKenzie (1984) are out of the error limits of our solution. The focal parameters (depth, strike, dip, rake) of the 28.07.1976 earthquake, after the minimum misfit solution and the error tests are found to be 19±2 km, 144±10°, 9 -2/+100, 123 -5/+100, respectively. The seismic moment is 1.13xl018Nm. Structure model test did not significantly effect the solution much. The calculated synthetics using Jackson and McKenzie's (1984) solution (84°/llo/90°) gives reversed polarity for some of the stations. On the other hand, the synthetics of Harvard-CMT solutions (107o/15°/91<'/21 km/2.59xl018Nm) gives anomalous amplitudes. The first motion polarities from long- and short-period records are consistent with minimum misfit solutions. The azimuth of the slip vector of the 16.07.1963 earthquake is 355°, and that of 28.07.1976 is 201°. The direction of pressure (P) axis of the 16.07.1963 (Western Greater Caucasus), 07.12.1988 (Spitak-Armenia) and 29.04.1991 (Racha-Georgia) earthquakes are 7°, 6° and 2°, respectively. But the 28.07.1976 (Eastern Greater Caucasus) has a P axis with 25° of azimuth. This suggest that while the compression on the Lesser xvmCaucasus and Western Greater Caucasus is NS direction, and is approximately NNE-SSW on Eastern Greater Caucasus. The other important point for the eastern part is the dip angles. The fault plane solutions from the other independent studies shows that the dips of the thrust are very gentle in the eastern part of the Greater Caucasus. In other words, low-angle thrust faulting plays major role in region of continental convergence, such as observed in the Caucasus region. Furtermore, the details of the geology of earthquakes should be studied in a more sophisticaded manner in order to correlate with regional geology and global tectonics.