Fault scarps related to the 1739 earthquake and seismicity of the Yinchuan graben, Ningxia Huizu Zizhiqu, China

Surface faulting accompanying the great Yinchuan-Pingluo earthquake of 1739 in Ningxia Huizu Zizhiqu (Ningsia Hui Autonomous Region) produced two sections of fault scarps 3.5 and 16.5 km long and separated from one another by 65 km along strike. The scarps are on the west side of the Yinchuan graben along the east flank of the Helan Shan (Holan Mountains). The east side of the faults is downthrown, and surface offsets at the fault are as much as 5.3 m on the Hongguozigou (northern) section, and 4.6 m on the Suyukou (southern) section. Actual net displacement may be slightly less. Near the north end of the set of faults, the Great Wall is offset by about 2.7 m vertically and about 3 m right laterally. On scarps more than 2 m high, a free face has persisted for the 245 yr since the scarps were formed in 1739; free faces commonly are 2 to 3 m high. The 1739 fault displacement occurred, at least in part, along an older fault scarp that is estimated from profile analysis to be about 12,000 yr old. The historical record of destructive earthquakes in the Yinchuan graben since 1010 A.D. includes only one near M 8 in 1739, and only two of approximately M 6.5, one in 1143 and the other in 1477. Average recurrence intervals for major earthquakes comparable to that of 1739 in the Yinchuan graben probably are measured in thousands and possibly ten thousand or more years.