We present a super-range seismic observation along the >1300-km-long profile passing through the Yinchuan basin and the Ordos block from the blasting point towards the southeast triggered by a large-(dynamite) scale coal blast in the Ningxia Hui Autonomous Region's Helan Mountain (yielded a profile). The seismic wave information from the uppermost mantle reflecting different depths was obtained by the China continental seismic survey. Pn refracted waves from the uppermost mantle were effectively traced up to 500 km and beyond. The stable dynamic and kinematic characteristics of the Pn wave-group indicate that the uppermost mantle below the Ordos block is a gently changing structure with a weak positive gradient. If Pm1, Pm2 waves are considered as the wave groups from the upper mantle, there may be distinct structural strata in the upper-mantle structures with average velocities of 7.70―7.80 km/s and 8.10―8.20 km/s, respectively. Based on the travel-time curve feature of Pm1, Pm2 waves, there may be a certain layer with a reverse velocity between the M1 and M2 interfaces. The depth range indicated by them is about 110―120 km and 200―220 km. These phenomena basically reflect the structural features of the upper-mantle lithosphere in this region. The information obtained may provide meaningful references for surveying North China craton lithospheric structures, as well as under-standing the shapes and structures of different spherical layers within the upper-mantle.
ZHAO JinRen ZHANG XianKang WANG FuYun ZHANG ChengKe ZHANG JianShi LIU BaoFeng PAN SuZhen
The authors processed the seismic refraction Pg-wave travel time data with finite difference tomography method and revealed velocity structure of the upper crust on active block boundaries and deep features of the active faults in western Sichuan Province. The following are the results of our investigation. The upper crust of Yanyuan basin and the Houlong Mountains consists of the superficial low-velocity layer and the deep uniform high-velocity layer, and between the two layers, there is a distinct, and gently west-dipping structural plane. Between model coordinates 180-240 km, P-wave velocity distribution features steeply inclined strip-like structure with strongly non-uniform high and low velocities alternately. Xichang Mesozoic basin between 240 and 300 km consists of a thick low-velocity upper layer and a high-velocity lower layer, where lateral and vertical velocity variations are very strong and the interface between the two layers fluctuates a lot. The Daliang Mountains to the east of the 300 km coordinate is a non-uniform high-velocity zone, with a superficial velocity of approximately 5 km/s. From 130 to 150 km and from 280 to 310 km, there are extremely distinct deep anomalous high-velocity bodies, which are supposed to be related with Permian magmatic activity. The Yanyuan nappe structure is composed of the superficial low-velocity nappe, the gently west-dipping detachment surface and the deep high-velocity basement, with Jinhe-Qinghe fault zone as the nappe front. Mopanshan fault is a west-dipping low-velocity zone, which extends to the top surface of the basement. Anninghe fault and Zemuhe fault are east-dipping, tabular-like, and low-velocity zones, which extend deep into the base-ment. At a great depth, Daliangshan fault separates into two segments, which are represented by drastic variation of velocity structures in a narrow strip: the west segment dips westward and the east segment dips eastward, both stretching into the basement. The east margin fault of Xichang Mesozoic basin features a strong
WANG FuYun1↑, DUAN YongHong1, YANG ZhuoXin1, ZHANG ChengKe1, ZHAO JinRen1, ZHANG JianShi1, ZHANG XianKang1, LIU QiYuan2, ZHU AiLan2, XU XiWei2 & LIU BaoFeng1 1 Geophysical Exploration Center, China Earthquake Administration, Zhengzhou 450002, China
