SPATIO-TEMPORAL STRESS FIELD VARIATION IN THE CENTRAL LONGMENSHAN FAULT ZONE AFTER THE 2008 WENCHUAN EARTHQUAKE FROM FOCAL MECHANISM SOLUTIONS OF SMALL EARTHQUAKES |
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| Authors: | JIANG Min CHEN Jiu-hui Yasuto Kuwahara Reiken Matsushita |
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| Institution: | 1.State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China;2.Active Fault and Earthquake Research Center, AIST, Japan |
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| Abstract: | We determined the focal mechanism solutions(FMS)of aftershocks of the 2008 Wenchuan earthquake using the waveform data recorded by the Western Sichuan movable seismic array. We further obtained the spatio-temporal variation of the stress field by inverting the stress tensors from these data. The results show that the FMSs of the small earthquakes are primarily reverse faulting with considerable number of normal and strike-slip. The proportion of reverse type earthquakes clearly increases with time, and the spatial distribution of the FMSs is closely related with local geology and the characteristics of the deep faults. The stress tensor inversion results reveal that the orientations of the maximum horizontal compressive stress(SH)shortly after the mainshock in each area are mainly in EW to NWW-SEE directions, while in the area along Xiaoyudong-to-Lixian aftershock branch it is in NE-SW direction in shallow crust. This implies that the stress field at the early stage after the Wenchuan earthquake is mainly controlled by the dynamic stress change caused by the rupture propagation, and the conspicuous factor determining the SH direction is the characteristics of the rupture plane. Temporal variations of the stress tensors show that the stress regimes at depth changed from a mixture of reverse and strike-slip faulting to pure reverse, implying that local Coulomb stress caused by the main shock is released through strike-slip faulting and gradually recovers into the background stress field. The change of stress in the shallow subsurface follows that in the deep subsurface with observable time delay. |
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| Keywords: | Longmenshan fault zone focal mechanism stress field spatio-temporal variation |
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