Discovery and Neotectonic Significance of Fault Scarps on Alluvial Fans in the Middle of Northern Piedmont of the Zhongtiao Mountains

A fault scarp striking NW290° is identified through aerial photo examine and field survey. Differential GPS measure and DEM analysis show that the height of this scarp ranges from 3 to 13 m. This height difference is mainly related to different ages of geomorphology surfaces: the higher scarps can be found in the older alluvial fans and lower scarps in the younger ones. In addition, the fan-shaped fluvial deposition in the hanging wall also influences the scarp height on the alluvial fan of the same period, and the depositional effect goes weaker when farther away from the streams, hence higher the scarp is. Trench investigation suggests that at least two paleoseismic events occurred along this fault scarp in the southwest of Yuxiang County since the end of Late Pleistocene. The first event happened between 16720±70 and 7700±40 aBP, and close to 16720±70 aBP; the second event is between 7700±40 and 7020±40 aBP, with an over 2.2 m vertical displacement, suggesting that this is a Holocene fault scarp. The vertical rate of the fault activity is about 0.9 mm/a through comparative studies.