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.
美国国家科学基金会（NSF）主要支持基础研究活动及其产生的跨学科的新知识和新技术,并培养世界一流的科学家和技术专家队伍。NSF地球科学部（Directorate for Geosciences,GEO）主要支持海洋科学、大气与地球空间科学、固体地球科学领域的研究以及基础设施与教育,深化对整个地球系统的理解。GEO不仅在理解、预测和帮助美国响应环境事件和环境变化中发挥着关键作用,而且在帮助决定更好地利用地球资源中也起到了重要作用。 相似文献
Surface waveform modeling has played an important role on many continental-scale studies of upper mantle ve-locity structure, but it was seldom applied to the Chinese mainland study. The present study firstly analyzed sur-face waveform fittings for eight wave paths crossing through four different regions of the Chinese mainland (east-ern, central, northern and western China), and then inverted for 1D path-averaged S-velocities for these paths. The inverted crustal and upper-mantle S-velocities showed obvious region-related features, which are well consistent with known geotectonic units and previous research results. These results indicate that surface waveform modeling is a reliable method to get crustal and upper-mantle velocity structure. Furthermore, this method has a prominent advantage in detecting upper-mantle structure compared with fundamental-mode surface-wave dispersion method. 相似文献
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