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| Depth and region dependence of b-value for micro-aftershocks of the May 12th, 2008 Wenchuan earthquake and its tectonic implications |
| Mei Feng, Meijian An, Lin Zhao, Jizhong Zhang |
| Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China |
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Abstract Micro-aftershocks with magnitude range of 1.5-4 around the Wenchuan earthquake epicenter, the southern part of the Longmenshan fault zone, exhibit good frequency-magnitude linear relationships, thus enabling b-value analysis. The average b-value for micro-aftershocks of M1.5-4 from July to December of 2008 in our local study region is about 0.88, similar to the b-value for all aftershocks of M3.0-5.5 from May, 2008 to May, 2009 along the whole Longmenshan fault zone. The similarity between the local and regional b-values possibly indicates that the southern part of the Longmenshan fault zone has similar seismogenic environment to the whole Longmenshan fault zone. Alternatively, it may also imply that b-values derived from all events without consideration of structural variation can not discriminate local-scale tectonic information. The present study shows that the b-value for the Wenchuan earthquake micro-aftershocks varies with different regions. The b-value in southwest of the Yingxiu town is higher than that in the northeast of the Yingxiu town. The high b-value in the southwest part where the Wenchuan earthquake main shock hypocenter located indicates that the current stress around the hypocenter region is much lower than its surrounding area. The b-values are also dependent on depth. At shallow depths of <5 km, the b-values are very small (~0.4), possibly being related to strong wave attenuation or strong heterogeneity in shallow layers with high content of porosity and fractures. At depths of ~5-11 km, where most aftershocks concentrated, the b-values become as high as ~0.9-1.0. At the depth below ~11 km, the b-values decrease with the depth increasing, being consistent with increasing tectonic homogeneity and increasing stress with depth.
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Received: 03 June 2009
Published: 10 December 2009
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| Fund:the Basic Research Foundation of the Institute of Geomechanics, Chinese Academy of Geological Sciences (No. DZLXJK200707) and the National Natural Science Foundation of China (No. 40674058) |
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Corresponding Authors:
Mei Feng
E-mail: mei_feng_cn@yahoo.com.cn
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Yu Wang, Shengli Ma, Toshihiko Shimamoto, Lu Yao, Jianye Chen, Xiaosong Yang, Honglin He, Jiaxiang Dang, Linfeng Hou, Tetsuhiro Togo. Internal structures and high-velocity frictional properties of Longmenshan fault zone at Shenxigou activated during the 2008 Wenchuan earthquake[J]. Earthquake Science, 2014, 27(5): 499-528. |
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