Geomorphological observations of active faults in the epicentral region of the Huaxian large earthquake in 1556 in Shaanxi Province, China
The Huaxian magnitude 8 great earthquake of January 23, 1556, is the largest one recorded in the Weihe basin, Shaanxi Province, China and caused 830,000 people either injury or death. The epicenter is located in the southeastern part of the Weihe basin, around Huaxian City. Earthquakes are closely related to active faults and active faults are well developed in the epicentral area of the Huaxian large earthquake. Thus we will discuss the activity of the major faults in the epicentral area by geomorphological observations. There are three major fault sets in the study area: striking approximately east-west, northeast and northwest. These are inhomogeneous in spatial distribution, rates and manners of faulting, as shown by geomorphological observations such as faulted fluvial terraces and alluvial fans. The ages of the second and first terraces are around 20,000 and 5,000 years B.P. by thermoluminescent dating, Carbon-14 dating and archeology. The terraces were faulted by the North Huashan fault (F 1), the main boundary fault of Weihe basin and the Piedmont fault (F 2) after the second and the first terraces formed. The distribution of the displacement shows that the intersections of the North Huashan fault and the Chishui fault (F 4) striking northwest, and the western margin fault (F 5) of Tongguan loess tableland, have the largest in offsets in the area. Perhaps the Huanxian great earthquake in 1556 A.D. had a close relation to the North Huashan fault. The Weihe fault (F 3) striking east-west is also an active fault by analysis of the flood plain structure. Thus we should pay attention to the activities of the faults to take precautions against another possible large earthquake in this region.