WANG Zhi,
WANG Xu-Ben,
HUANG Run-Qiu et al
.2017.Deep structure imaging of multi-geophysical parameters and seismogenesis in the Longmenshan fault zone.Chinese Journal Of Geophysics,60(6): 2068-2079,doi: 10.6038/cjg20170604
Deep structure imaging of multi-geophysical parameters and seismogenesis in the Longmenshan fault zone
WANG Zhi1, WANG Xu-Ben2, HUANG Run-Qiu3, LIU Guan-Nan2
1. CAS Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China; 2. Key Laboratory of Earth Exploration and Information Techniques of Ministry of Education, Chengdu University of Technology, Chengdu 610059, China; 3. State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of Technology, Chengdu 610059, China
Abstract：Deep structures of seismic velocity and Poisson's ratio are imaged along the Longmenshan fault zone using a large number of arrival-time data of P- and S-waves. Together with magnetotelluric (MT) data analyzing, we consider that the crustal deformation is closely related to the deep seismic velocity, Poisson's ratio and conductivity structural heterogeneities. The generations of the 2008 Wenchuan and 2013 Lushan earthquakes are associated with the crustal deformation process and fluid intrusion from the lower crust of Tibet into the seismogenic layer along the fault zone. Our results indicate that the 2008 Wenchuan and 2013 Lushan earthquakes are located in the high velocity and high resistivity zones within the seismogenic layer. Anomalously slow velocity with low resistivity in the Sichuan foreland basin is in sharp contrast to high-velocity and high-resistivity anomalies in the upper crust in the Songpan-Garzê block. This contrasting feature of the anomalies could reflect the Mesozoic and Paleozoic sediments with thickness variation of several kilometers to more than 10 kilometers in the Sichuan Basin. The tomographic model presented here reveals two crustal bodies with anomalously slow velocity and high conductivity underneath the Longmenshan fault zone which is separated into three contrasting segments by the two bodies. We interpreted these low-velocity and low-resistivity bodies as being associated with extrusion of either fluids or products of partial melting from the lower crust, the upper mantle, or both in the Songpan-Garzê block, which suggests strong variations in the rheological strength of the rock along the fault zone. This finding implies that the coupling between these presumably fluid-bearing bodies and earthquake generation could be extremely complex and that there is dramatic variation from the southwestern portion to the northeastern segment along the fault belt. We then conclude that the 2008 Wenchuan earthquake generation as well as its rupture process is controlled by the structural heterogeneities together with the fluids intruding from the lower crust of the central plateau. The 2013 Lushan earthquake might have been triggered by the high pore-pressure accumulated in source area from both the lower crustal fluids intrusion and the stress transmission of the 2008 Wenchuan earthquake in the seismogenic layer. Our study suggests that the rheological variation in the crust and fluids bearing stress accumulation along the Longmenshan fault zone played a principal role in controlling seismic generation and rupture processes during the 2008 Wenchuan and 2013 Lushan earthquakes.
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