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Zhu Jieshou;Cso Jiamin;Liu Shunhua;Jiang Guofang;Yu Min;Jiang Hualin Chengdu College of Geology Seismological Bureau of Sichuan Proviuce  
A program of explosion seismic investigation was carried out by using twomine blasts in Dukou and Huidong, Sichuan province. By processing the dataof seismic wave arrival times the model of P-wave velocity structure of crustand mantle is obtained. The thickness of upper crust is about 21 km and itconsists of Mesozoic-Cenozoic sedimentary rocks, Palaeozoic and Precambrian flysch suite and metamorphic folded groups. The middle crust of 14 km thickis a low velocity zone suggesting granitic or equivalent composition. The lowercrust of about 17 km thick is indicated by a layer of basalt. The P-wavevelocity at the top of upper mantle is only 7.8 km/sec in this area. whichis lower than other areas of China. The average crustal thickness is changed rapidly from 52 km at the eastof the Longmenshan and Jinhe-Qinghe great fracture zones to 60 km at thewest of it. We conclude that the great fracture is through the Mohorovicicdiscontinuity. It is evident that there are strong relations between the seis-micity and the Ganzi-Chuxiong rhombic block moving south southeastward.According to the crustal model we conclude that the brittle blocks are cutby great fracture zones, which seem floating on the low velocity zone of themiddle crust. It can be driven by regional stresses and causes frequent earth-quake activities.
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