摘要
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-aflershocks 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 ofM3.0-5.5 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.
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-aflershocks 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 ofM3.0-5.5 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.
基金
supported bythe Basic Research Foundation of the Institute of Ge-omechanics,Chinese Academy of Geological Sciences(No.DZLXJK200707)
the National Natural Science Foundation of China(No.40674058)
