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??????Ms8.0????????????????????????????????????????????????????????????????????????????????????з?????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????Pn????????7.6~7.8 krn/s?????????????????8.0~8.2 krn/s??????????????Pn????????????ζ??????????????????????????????????????????????????п??????????L?ε?????棬???????????????????????????????????????

DEEP GEOPHYSICAL EXPLORATION OF LONGMENSHANAND ITS ADJACENT AREA

In order to reveal the deep tectonic background of Wenchuan Ms8.0 earthquake, the deep geophysical investigation data were collected and deep structural characteristics beneath Longmenshan area were discussed. The present results show that the Longmenshan tectonic zone is a steep variation belt of crustal thickness.At its two sides, the layers with low velocity or high conductivity are widely distributed, and there is big difference in structures of crust and uppermost mantle. It means that the Longmenshan tectonic zone has a high resistance basement. In western Sichuan Plateau, the Pn velocity is 7.6~7.8 km/s,which is lower than that of Sichuan Basin, where the Pn velocity is 8.0~8.2 km/s.The lower Pn velocity suggests that there is possiblly strong heat exchange between crust and mantle in western Sichuan Plateau. The low velocity-high conductivity layers in middle upper crust are likely to be an L detachment, which served as the deep tectonic conditions for shallow earthquake development in Longmenshan tectonic belt.