DISCOVERY OF SURFACE RUPTURE ZONE ON THE SOUTH OF HELISHAN IN GAOTAI,GANSU PROVINCE

According to the new investigation in the northern Hexi corridor,remains of two surface rupture zones are discovered on the southern margin fault of Helishan.One rupture has the length of about 7km and the other about 10km.The two surface rupture zones might be produced by the nearest earthquake event.On the surface rupture zones,there are continuous scarp and free face caused by rupture.The scarp is about 1～1.5m high and on some site is up to 2m nearly.According to the OSL result,the nearest T1 terrace and higher flood plain forming 3000a BP are dislocated by the fault.All above reveal that the rupture age should be later than that of T1 terrace.But in the historical data and earthquake catalogue,we didn't find related information about the fault and surface rupture in this area.The 180 AD M 8 Biaoshi earthquake and 756 AD M 7 Zhangye-Jiuquan earthquake are documented in historical data.It is inferred by textual research that the two earthquakes are related with the northern marginal fault of Yumushan in the south of basin.Due to lack of reliable evidence,there still exist many arguments on this inferred conclusion.So we hold that the two surface rupture zones were produced by one of the two large earthquakes or another unrecorded historical event.The research on the activity and surface rupture of this fault can offer valuable information for the tectonic study and strong earthquake risk estimate of this region in the future.