Abstract:

 With the broadband seismic records acquired from permanent and mobile digital seismic stations around Longmenshan Fault Zone (LMFZ), the structure of crusts and uppermost mantle was investigated by using integrative seismic and geophysical methods. The study of the deep structure shows remarkable subsection diversity in deep structure and in material properties along the LMFZ. The Hk stack of teleseismic receiver functions was deployed to obtain the crustal thickness and velocity ratio of P and S waves. The result shows that the crustal thickness increases from east to west in general. The minimum thickness is 37.8 km and the maximum is 68.1 km. Across LMFZ from southeast to northwest, the crustal thickness is increasing sharply, from 41.5 km in the southeast to 52.5 km in the northwest. The distribution of Poissons ratio can be divided into three areas: the northern part of SongpanGanzi terrain and the West Qinling orogen, where are of lower Poissons ratio (ν<0.26); the Yangtze platform, where is of lowmiddle Poissons ratio (ν<0.27); and the southern part of SongpanGanzi terrain, the Sanjiang fold system and the Sichuan Basin, where are of middlehigh Poissons ratio (0.26<ν<0.29). The distribution pattern shows that this study can not verify the assumption that there is widespread lower crust flow in the east margin of the Tibetan Plateau because except the southern segment of LMFZ and its vicinity there is no very high Poissons ratio (ν>030) being found in almost of the whole research region. The Poissons ratio is higher (ν>030) in the southern segment of LMFZ, while lower in the northern. The high Poissons ratio (ν≥3.0) in the local area of south segment of LMFZ can be regarded as the evidence of the existence of mafic component increment and/or partial melting. It provided the possibility of the partial melting in the lower crust. The lower crust in the southeastern part of SongpanGanzi terrain may have been of liquidrich or in the status of high temperature and partial melting, which facilitated the eastward movement of lower crust material in the Tibetan Plateau. When the eastward movement of the upper and middle crust was obstructed by the more rigid Yangtze platform, the strain energy accumulated along the LMFZ. As the strain became higher than the critical value, the accumulated strain energy was released with the sharp slip of LMFZ and the Wenchuan Ms 8.0 earthquake occurred. The difference in the behavior of rupture and the distribution pattern of aftershocks may have resulted from the deep structure variation of the fault zone.

Key words: Longmenshan fault zone, thickness of the crust, Poissons ratio, teleseismic receiver function, Wenchuan Ms 8.0 earthquake