Postglacial left slip rate and past occurrence of M ≥ 8 earthquakes on the western Haiyuan fault, Gansu, China

High-resolution (HR) air photographs and a 1-m horizontal and 2-m vertical resolution digital elevation model derived from them by stereophotogrammetry provide new constraints on the behavior of the western stretch of the active Haiyuan fault, in Gansu province, China. The photographs cover three swaths along the fault, each about 2-km-long and at least 500-m-wide, near the village of Songshan, at 103.5 degrees E. This high-resolution data set is used to map and measure cumulative horizontal offsets of alluvial terraces and risers that range between 115 and 135 m, and 70 and 90 m, at two sites. Dating these terraces with C-14 yields minimum and maximum ages of 8400 and 7600, and 14,200 years B.P., respectively. This leads to a: postglacial slip rate of 12 +/- 4 mm/yr, with a most likely minimum value of 11.6 +/- 1.1 mm/yr. The smallest stream offsets observed on the HR photographs range between 8 and 16 m and are interpreted as coseismic displacements of the last few earthquakes with M greater than or equal to 8 that ruptured the 220-km-long Tianzhu gap of the fault, west; of the Yellow River. Earthquakes of that size within this gap, which has been quiescent for at least 800 years, would recur at intervals of 1050 +/- 450 years.