Crystal P-wave velocity structure in Lower Yangtze region: Reinterpretation of Fuliji-Fengxian deep seismic sounding profile

摘要

The finite-difference inversion method and Raylnvr technique had been employed to interpret the wide-angle seismic reflection/refraction data of the Fuliji-Fengxian deep seismic sounding (DSS) profile in Lower Yangtze region, hence the velocity structure was acquired and conclusions were summarized as follows: (1) The velocity model along this profile can be divided into three large layers vertically (upper, middle and lower crusts) and six blocks laterally, and this velocity distribution agrees with the feature of stable platform. (2) The depth of Mono discontinuity is 30-36 km. The thickness of the upper crust is 10.5 - 13.0 km, where the lateral velocity varies strongly, and the velocity increases to 6.2 km/s~(-1) at bottom. Besides, the velocity distributions in the bottom layer of middle crust and lower crust have an apparent inhomogeneity. The velocity in upper layer of middle crust, lower layer of middle crust, lower crust and uppermost mantle is 5.9-6.2, 6.3-6.4, 6.6-7.0 and 8.06-8.29 km/s~(-1), respectively. (3) On two sides of the Tanlu fault belt (TFB), the mid-crustal velocity structure is quite different, nevertheless no apparent discrimination in velocity distribution and boundary topography exhibits in lower crust, hence it is inferred that the Jiashan segment of TFB had probably cut through whole crust in the Mesozoic, and the fault behaviour in lower crust had disappeared due to the low viscosity produced by the Orogenic extension or crustal balance, while the fault features in the rigid middle-upper crust have been preserved up to the present. (4) The moderate earthquakes with Ms > 5.0 nearby Zhen-jiang are related to the deep faults extending into the lower crust, and the earthquakes were probably induced by the energy been transferred from mantle lithosphere to upper-mid crust along the deep faults, and aggregated at some preferable tectonic positions.