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Journal: |
Chinese Journal of Geophysics 2009 No.2
clicks:1033 |
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Title:
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Author:
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LIU Qi-Yuan LI Yu CHEN Jiu-Hui GUO Biao LI Shun-Cheng WANG Jun ZHANG Xu-Qi QI Shao-Hua |
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Adress: |
State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration |
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Abstract:
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On 12th of May, 2008, a devastating MS8.0 event shocking the global world occurred in the Wenchuan region of Sichuan province. Historically, the same kind of earthquakes is very rare inside of the continent. The study on the tectonic environment of this event is crucial for understanding its genesis. From the teleseismic waveform data recorded by the dense large-scale movable seismic array in the western Sichuan by the State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration (CEA), we investigate the S-wave velocity structure of the crust and upper mantle within the depth range of 120 km and averaged Poisson′s ratio over the crust beneath 19 stations distributed along the profile of 31°N by using the non-linear receiver function inversion technique. This profile is about 420 km long and crosses the main shock area.
Our results show the differences of the crustal structure among the blocks of Chuandian, Songpan-Garze and Sichuan basin. Their main features can be summarized as follows: (1)the crust-mantle boundary in the foreland of the Sichuan basin declines westward and has been deformed obviously; The crustal thickness has lateral variations of 46～52 km; the velocity of middle and lower crust has lateral variations; the Poisson′s ratio averaged over the crust reaches up to 0.28～0.31; however, the crust is hard nearby the Longmen Shan faults, where the Poisson′s ratio averaged over the crust is only 0.2; (2)in the Songpan-Garze block, from the west to the east, the crustal thickness becomes thinner from 60 km to 52 km; in the depth range of 14～50 km exists a wedged low-velocity zone with the S-wave velocity of 2.75～3.15 km/s, whose thickness decreases from ～30 km in the western side to ～15 km in the eastern side; the Poisson′s ratio averaged over the crust containing the low-velocity zone reaches up to 0.29～0.31; (3)the crustal structure of the Chuandian block looks simple to the west of the Xianshuihe faults and the crustal thickness reaches to 58 km; but a high-velocity layer with thickness of ～10 km exists in the depth of 26 km; the Poisson′s ratio averaged over the crust is ～0.25; (4)within the crust beneath the Wenchuan earthquake region exists a high-velocity structure of ～4.0 km/s at the depth of 12～23 km, and below it exists a low-velocity structure, where the Poisson′s ratio averaged over the crust is 0.31～0.32; the aftershocks of the Wenchuan earthquake are mainly distributed in the region of the upper crust with high-velocity structure.
Our results manifest that the Songpan-Garze block has a soft and weakened crust, and the westward subduction of the Sichuan basin does not exist. It could be inferred that under the long-term pressure eastward from the Tibetan plateau, the Songpan-Garze block obstructed by the Sichuan basin has been deformed greatly and a large energy of deformation has been accumulated inside before the Wenchuan earthquake and that the upper crust decoupled with the lower crust at the top boundary of the low-velocity zone leads to the listric thrust of the upper crust. The rigid upper crust with high-velocity underneath the Wenchuan earthquake and its adjacent region as well as the obstruction of the Sichuan basin make it possible to accumulate high stress with low strain rate. The vast strain energy accumulated in the long-term deformation within the Songpan-Garze block should be the dynamic source of the Wenchuan great earthquake.
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