Interpretation of S-Wave Data from Fanshi-Taipusiqi DSS Profile and Analysis of Correlation between Deep Structural Characteristics and Seismicity

By processing S-wave data from the Fanshi-Huai'an-Taipusiqi DSS profile,which is a three-component,wide-angle reflection/refraction profile,and in the light of the results from P-wave interpretation,two-dimensional(2-D)structures of the crust and upper mantle are presented,including S-wave velocity Vs and the physical parameter of medium-Poisson's ratio a.Taking other geological and geophysical information into account,and with reference to the results from petrophysical experiments at home and abroad,we carried out interpretation and inference with respect to deep crustal structure,tectonics,and lithologic characters.It has been concluded that in the upper and middle crust,a values are mostly not greater than 0.25,and rocks,which generally assume brittle,are mainly composed of granite; the rocks in the lower layer of the upper crust between Yangyuan-Huai'an containing inorganic CO2 itself releases carbon; for the rocks in the lower crust and crust-mantle transitional zone,which are comparatively plastic,a values are mostly greater than 0.26; the rocks in the lower crust are intermediate acid granulite north of the Huai'an,quartz-amphibole gneiss between Huai'an-Yangyuan,and quartzite south of Yangyuan; the M-discontinuity and crust-mantle transitional zone are mainly composed of basic granulite-amphibolite at both ends of the profile and quartz-mica schist-intermediate granulite in the middle area of the profile.On the basis of lateral variation in a values,taking seismic phase characteristics into consideration,the ultra-crustal fault near Yangyuan and Zhangbei and the abyssal fault at Huai'an are deduced.Taking seismicity into account,we interpert that the lower layer of the upper crust beneath Yangyuan is an earthquake-prone area and believe that the rock mass here is comparatively brittle and hard and "floats" over the relatively plastic rock mass under it; affected by the ultra-crustal fault,the regional stress field and the upwelling of material from upper mantle,dislocation and glide may take place in the rock mass where a values vary obviously,thus triggering earthquakes that tend to be concentrated on the brittle and hard side of the rock mass with low a values.