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Earthquake Science

, Volume 22, Issue 6, pp 589–594 | Cite as

Depth and region dependence of b-value for micro-aftershocks of the May 12th, 2008 Wenchuan earthquake and its tectonic implications

  • Mei FengEmail author
  • Meijian An
  • Lin Zhao
  • Jizhong Zhang
Article

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.

Key words

micro-aftershocks frequency-magnitude relationship tectonic implication Wenchuan earthquake 

CLC number

P315.08 

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Copyright information

© Seismological Society of China and Springer Berlin Heidelberg 2009

Authors and Affiliations

  1. 1.Institute of GeomechanicsChinese Academy of Geological SciencesBeijingChina

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