Judgement and interpretation of S-wave data on the Beijing-Fengzhen DSS profile

By use of S-wave information from three-component records of wide-angle reflection/refraction Beijing-Fengzhen DSS profile (340.0 km) and its adjacent area, several main S-wave phases (wave groups) of the crust and the top of upper mantle have been picked up and identified with digital processing technique. With reference to the results from P-wave interpretation, 2-D structures, including S-wave velocity vs and Poisson ratioμ, are calculated. After a comprehensive study with information from near-vertical reflection, geology and other geophysical explorations, the deep structural configuration of this region is presented and relevant inferences are drawn from the above result: crustal thickness along the profile tends to increase westwards, from 35.0 km at Shunyi to 42.0 km at Fengzhen; there exist three low velocity bodies in the lower crust east of Yanqing, in the middle crust below Yanqing-Zhuolu and in the upper crust west of Tianzhen, respectively; from east to west, there is also a crust-mantle transitional zone about 5.0–9.0 km thick; in most parts of the region,μ value is 0.25 in the upper and middle crust, 0.27 in the lower crust and 0.28 on top of the upper mantle, indicating that the upper and middle crust is characterized by brittleness, and the lower crust and top of upper mantle is characterized by ductility; between Yanqing-Zhuolu, the lowμ values of the upper crust suggest that cracks (or pores) grow in the rock masses which present a dry (or gasfilled) state, while the highμ values in the middle crust may reflect a fluid-filled (or partially fusion) state of the rock masses. On the basis of the characteristics of phases and lateral variation features ofμ values, several deepseated faults in this region are ascertained. Most earthquakes occurred near these faults whereμ values vary significantly, and concentrate in the brittle rock masses with lowμ values.