新疆天山南部北轮台断裂古地震事件反演

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

北轮台断裂是一条全新世活动断裂,全长约70km,构成了南天山南麓与山前洪积扇的界线。该断裂晚第四纪期间活动性较强,为准确分析北轮台断裂的古地震特征,在野外地质调查的基础上,对变形的微地貌进行测量,在阿克艾肯和帝禾农业两段开挖两个大型古地震探槽,对北轮台断裂古地震进行了反演模拟推演。结果显示:北轮台断裂古地震活动频繁,阿克艾肯段探槽剖面记录到3次古地震事件,垂直累计位移量4.5m,根据逆断层变形特征,建立古地震模型反演分析,多次古地震事件属于原地同震复发,最新一次古地震事件造成的地表垂直位移量为1.5m左右;帝禾农业段活动性稍弱,对探槽剖面影像解译分析,该段记录古地震事件两次,最新一次事件垂直位错量达到1.1m。北轮台断裂记录的古地震事件与上盘存在侵蚀不整合面、下盘存在生长地层的规律一致。

Beiluntai fault is a Holocene active fault. The fault total length is 70 km. 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 strataposition 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.