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2009-11-13 – SEMINAR by Gulten Polat: Crustal Seismic Anisotropy: Implications for Understanding Crustal Dynamics of Marmara region in Turkey

13 November, 2009 (15:30 GMT), 5 Merrion Square, Dublin 2.

Speaker: Gulten Polat, DIAS.
Title: Crustal Seismic Anisotropy: Implications for Understanding Crustal Dynamics of Marmara region in Turkey.

Abstract:

Shear wave splitting manifested as leading shear wave polarization, that is, parallel alignment of leading shear wave particle motions from a variety of sources, has been observed at a number of seismograph stations in the Marmara region, Turkey. We evaluated shear wave anisotropy from micro earthquakes recorded by 29 stations installed in the TURDEP (Multi- Disciplinary Earthquake Research in High Risk Regions of Turkey) project and also 33 permanent stations from KOERI in the Marmara Region. We applied four methods to investigate crustal anisotropy and compare the
resolution obtained: (1) Aspect ratio, (2) Cross-correlation, (3) Systematic analysis of crustal anisotropy and (4) SPLITshear modified by Ludek Vecsey, to the events from 2006 to 2008 recorded by three component digital seismometers. The results from the methods are compared in order to acquire acceptable splitting parameters. In order to explain the observed seismic anisotropy in the crust, three phenomena are taken into account: stressinduced microcracks primarily aligned in vertical or subvertical planes; cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes; and intrinsic rock anisotropy resulting from preferred orientation of minerals. The polarization of the faster shear waves is dominantly NW-SE and shows a nearly uniform distribution of each part of the study area that separated into four areas in terms of tectonic structure and seismicity. The polarization direction of the fast split shear wave is nearly parallel to the direction of the alignment cracks and hence parallel to the maximum horizontal stress direction in this region. Preliminary analysis indicates that the majority of local events exhibit some degree of splitting and that splitting patterns, while complicated, are coherent. We observed that the results are also consistent with neotectonic stress in the study area. These results have important implications for understanding crustal dynamics of the complex regime of Marmara region and the large overall spatial variations of the results in all parts of the study area are related to multiple structures and geodynamic mechanisms in the Eastern part of the study area.