LATE QUATERNARY PALEOSEISMIC HISTORY ON THE KUSAI LAKE SEGMENT OF EAST KUNLUN FAULT ZONE IN NORTHERN TIBET
东昆仑断裂带库赛湖段晚第四纪古地震研究*

The East Kunlun fault zone in northern Tibetan Plateau is a typical active fault zone, characterized by sinistral strike-slip faults. Field investigation indicates that there are a lot of relics along the fault traces of the Kusai Lake segment, Kunlun Fault. The relices include push-up, pull-apart basins, sag pond, and earthquake fault scarps. This implies that the Kusai Lake segment of the Kunlun Fault is a seismogenic fault on which surface-rupturing earthquakes often occur in regular or irregular intervals. To determine the chronology of Late Quaternary surface ruptures of this part of the Kunlun Fault, five trenches were excavated. All sediment samples collected from the trenches were dated by using thermoluminescence (TL) method.Paleoseismic evidence, combined with TL dating, show that the Kusai Lake segment of the Kunlun Fault has generated at least 9 paleoearthquakes since Late Quaternary, whose ages are 31900±1923aB.P. , 27990±1681aB.P. , 23635±1427aB.P. , 20345±1225aB.P. , 16865±1018aB.P. , 12935±774aB.P. , 9730±592aB.P. , 6955±425aB.P., and 3100±201aB.P. , respectively. The recurrence interval between two adjacent events is 3910±2554a,4355±2205a,3290±1881a,3480±1593a,3930±1279a,3205±975a,2775±728a,3855±470a, and 3100±201a, respectively. Its average recurrence interval is 3544±416a. It shows that recurrence of these events is uniform with time, and suggests that the Kusai Lake segment of the fault displays time-dependent seismic behavior.These results have significant implications for seismic hazard assessment in the Kusai Lake segment of the fault. On Nov.14, 2001, a great earthquake with magnitude of 8.1 occurred along the Kusai Lake segment. The surface rupture has a length of 426km with a maximum left-lateral co-seismic displacement of 6.4m. The penultimate great earthquake occurred before 3100a, coherenting with the average recurrence interval.