Seismological Evidences of the Crustal Structures and Tectonics in the Yinchuan Downfaulted Basin

A high resolution deep seismic reflection profile of 68.9 km long across the Yinchuan faulted basin has been accomplished which for the first time yields the fine crustal structures, characteristics of deep fault system (Yellow River fault, Yinchuan fault and Eastern piedmont fault of Helanshan) of faulted basin in graben style, and the relationship between shallow and deep structures in Yinchuan basin. The results show that the upper crust is the region above a reflector with 8 s two‐way traveling time (about 20 km deep), there are many strata in the upper part of the upper crust where the continuity of different segment of stratum is good, and there is no obvious layered feature in the lower part of upper crust, where the geological structure is simple. The reflection energy is weak in the lower crust (8~13 s), where reflection events are not obvious. The crust‐mantle transitional zone (around 13 s) below the lower crust consists of a group of reflection sequences that have stronger energy and longer duration (1.5 s), and the thickness is about 4.5 km. Luhuatai fault and Yinchuan fault merge into the Eastern piedmont fault of Helanshan in the depth of 12~12.5 km and 18~19 km, respectively, the Eastern piedmont fault of Helanshan merges into Yellow River fault in the depth of 28~29 km, and Yellow River fault is a deep fault cutting the Moho. Yinchuan graben is a negative flower structure that is assembled mainly by Yellow River fault and secondarily by other faults. Based on the relationship between the Eastern piedmont fault of Helanshan and Yinchuan fault, it is thought that the Eastern piedmont fault of Helanshan played a controlling role in 1739 M=8 Pingluo‐Yinchuan earthquake.