INVERSION THE PALEOEARTHQUAKE EVENT OF BEILUNTAI FAULT AT SOUTH OF TIANSHAN, IN XINJIANG

Beiluntai fault is a Holocene active fault. The fault total length is 70km. It is a line of south Tianshan and piedmont alluvial fan. Beiluntai fault activity is obvious and generating a series of apparent geomorphic traces on the surface. The fault had dislocated proluvial fan and terrace form fault escarpment. The fault strike is near EW, extending length is about 2.5km in Beiluntai fault Akeaiken part. The maximum height of the fault escarpment is 18.5m and the minimum height is 5.1m. We find different level geomorphic surfaces had different vertical displacement and the formation age of geomorphic surfaces older, the cumulative displacement greater. The dispalcemengt of fault is a process that accumulates over time. Based on the field geological investigation and surveying the deformation microtopography in order to accurate analysis the characteristics of paleoearthquake. We are surveying the deformation of alluvial fan near the Beiluntai fault. Use excavator to excavate two large paleoearthquake exploratory trench at Akeaiken part and Dihe agriculture part in Beiluntai fault. We had maked photo mosaics of the trench profile and inverted the paleoearthquake event to analysis the times of paleoearthquake. The conclusion shows the paleoearthquake event is frequently occurred in Beiluntai fault. We use the theory of erosion unconformity on the hanging wall, growth strata on the footwall and and sudden change of maker strata position between the hanging wall and footwall to inverting the times of paleoearthquake event. And the result shows that not less than three times paleoearthquake evet had been happened in the Beiluntai fault Akeaiken part, the cumulation amount of vertical displacement is 4.5m, the co-seismic displacement is 1.2 to 1.5m. The fault is dislocate the second terrace in Beiluntai fault Dihe agriculture part and formed the 1.6m faultscarp. We were analysis the photo of trench profile recorded two times paleoearthquake event and the latest paleoearthquake event caused 1.5m vertical displacement. It makes the surface layer bending deflection near the fault. Beiluntai fault have never been occurred large-scale earthquake since the earliest recorded history, this indicates that energy accumulation of the fault has reached a high level, large-scale earthquake may occurred in the future.