| Investigation of upper crust anisotropy in Ghaen-Birjand region, east-central Iran |
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| 作者姓名： | Mohammad-Reza Gheitanchi Zoya Zarifii |
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| 作者单位： | Institute of Geophysics，Tehran University，Tehran P.O. Box 13145-1137，Iran，Institute of Solid Earth Physics，Bergen University，Allegaten 415007，Norway |
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| 基金项目： | ProjectNo.652/1/502fromInstituteofGeophysicsandtheOfficeofViceChancellorforResearch，TehranUniversity.e-mail:1)mrghchee@chamran.ut.ac.ir，2)zoya.zarifi@ifjf.uib.no |
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| 摘 要： | IntroductionWhenpropagatingthroughananisotropicmedium,ashearwavesplitsintotwo(quasi)shearwaveswithdifferentpropagationspeedsandpolarizedorthogonally.Owingtotherecentdevel-opmentofseismicobservationsystem,detectionofshearwavessplittingwithverysmalldelaytimesbetweenfasterandslowershearwavesbecameavailableandprovidedpowerfulapproachfordetectionofcrustalanisotropy.Crampin(1978)emphasizedtheroleofalignedmicrocracksasacauseofcrustalanisotropyandpointedoutthatforverticallyalignedmicrocracksthedirecti…
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| 收稿时间： | 28 January 2003 |
| 修稿时间： | 18 July 2003 |
Investigation of upper crust anisotropy in Ghaen-Birjand region, east-central Iran |
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| Mohammad-Reza Gheitanchi,Zoya Zarifii.Investigation of upper crust anisotropy in Ghaen-Birjand region, east-central Iran[J].Acta Seismologica Sinica(English Edition),2004,17(5):518-525. |
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| Authors: | Mohammad-Reza Gheitanchi Zoya Zarifii |
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| Institution: | 1. Institute of Geophysics,Tehran University,Tehran,P.O.Box,13145-1137,Iran
2. Institute of Geophysics,Tehran University,Tehran,P.O.Box,13145-1137,Iran;Institute of Solid Earth Physics,Bergen University,Allegaten,415007,Norway |
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| Abstract: | A number of aftershocks of the May 10th 1997, Zirkuh (Ghaen-Birjand) destructive earthquake have been used to investigate the anisotropy in the upper crust by observing shear wave splitting. Particle motion diagram and aspect ratio methods were used as two different approaches to obtain splitting parameters. Clear shear wave splitting was observed on the records of the selected aftershocks, indicating that the media in the region was highly anisotropic. By using particle motion method, the direction of fast shear wave was found 22N19E, while the delay time between the fast and slow shear waves was obtained to be (6516) ms. By aspect ratio method, the direction of fast shear wave was determined to be 35N18E and the delay time between fast and slow shear waves was found to be (4910) ms. For a simple horizontal layer with a thickness about 5 km and uniformly distributed anisotropy, a stress aligned cracks model was used and the result was interpreted in terms of vertical aligned cracks in the direc-tion of N22E, having a density about 0.01. It is assumed that cracks are fluid-filled since they are located in the upper crust. Finally, by using Hudson cracks model for three crack densities 0.005, 0.01, 0.03, the velocity curves of shear wave were plotted as a function of angle between the symmetrical axis of cracks and the azimuth of source to receiver. It was concluded that when shear wave was polarized parallel to the crack surface, the velocity was uniform, but the velocity curve varied clearly if shear wave was polarized perpendicular to the crack surface. |
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| Keywords: | shear wave splitting anisotropy in upper crust Hudson cracks model particle motion diagram aspect ratio method Zirkuh (Ghaen-Birjand) earthquake active faults aftershock activity |
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