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Journal: |
Earthquake Science 2009 No.6
clicks:801 |
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Title:
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Author:
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Mei Feng; Meijian An; Lin Zhao; Jizhong Zhang |
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Adress: |
Institute of Geomechanics,Chinese Academy of Geological Sciences,Beijing 100081,China |
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Abstract:
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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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Comment: |
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