Preliminary Seismological Studies On Deep Behavior And Movement Characteristics Of Major Active Faults In The Interseismic Interval In Western Sichuan, China

Western Sichuan, located in the southeast borderland of the Tibetan plateau, is one of the most seismically active regions in continental China, which is characterized by active tectonic block movement controlled by large-scale,rapidly slipping active faults. Deformation of this region is thought to be localized along the boundary faults. To investigate detailed deep structures and slip behavior of the major active faults in the interseismic interval within this region using microseismicity, I performed double-difference relocations for 10,215 small earthquakes from a catalog of over 13,000 that occurred between 1992 and 2002. The relocated seismicity becomes highly organized in space, compared to the diffuse view seen in the routine locations, and focuses sharply along the surface traces of faults reinterpreted using the ETM satellite image, indicating that the precision of the relative relocations gets improved greatly, which enables me to address some problems in active tectonic deformation within this region.Relationship between hypocentral depth distribution and crustal rheological structureMost relocated events in western Sichuan occur within the upper crust at depths of 0-15km, and fewer at 20-50 km depths. There appears an aseismic layer between 15 and 20 km depth in the western Sichuan plateau, whereas no such aseismic layer in Sichuan basin. On the basis of fault friction law and rock rheological experimental data, the crustal strength envelopes (stress limit) for western Sichuan plateau and Sichuan basin were calculated respectively to investigate the relationship between the hypocentral depth distribution and the rheological structures, and the deformation feature of the aseismic layer as well. The result shows that the hypocentral depth distribution agrees with the crustal rheological structure well as most earthquakes occur in the brittle zone, and the aseismic layer in western Sichuan plateau corresponds to the ductile granite zone within the upper crust at depths of 14-19 km, while the earthquakes in the lower crust to the half-brittle zone of gabbro. The existence of the aseismic layer is likely to be associated with low velocity zone within the upper crust, which features the orogenic crust structure of western Sichuan plateau. It is proposed that the existence of this aseismic layer may result from eastward extrusion of material from the Tibetan plateau and clockwise rotation activity of the Sichuan-Yunnan block.Microseismicity along the Xianshuihe-Anninghe-Zemuhe fault zoneThe Xianshuihe-Anninghe-Zemuhe fault zone is a large left-lateral strike slip fault zone in western Sichuan, which forms the eastern boundary of the Sichuan-Yunnan rhombic block, and the eastern margin of the Tibetan plateau as well. The deep structures delineated by the relocated seismicity correspond to the surface faulting well, especially in the segmentation feature. The variation in the...