川西地区安宁河断层古地震行为及其与则木河断层的比较

PALEOSEISMIC BEHAVIOR OF THE ANNINGHE FAULT AND ITS COMPARISON WITH THE ZEMUHE FAULT IN WESTERN SICHUAN

安宁河断层与则木河断层是青藏高原东南川滇地块东边界断裂带的重要组成部分,晚第四纪以来变形样式主要以左旋走滑运动为主.安宁河断层走向近SN向,以冕宁为界分为南北两段,北段古地震复发周期500～700a,全新世以来左旋平均位移速率约4mm/a.南段的古地震研究程度较低,通过在月华一带开挖探槽及大量14C测年限定其大地震平均复发间隔为600～ 800a.则木河断层北接安宁河断层,全新世以来平均左旋位移速率为2.4～ 3.6mm/a,大地震复发周期约2 300a.对比安宁河断层与则木河断层的古地震行为,发现安宁河断层大地震复发间隔相对较短,平均左旋位移速率稍大,存在着古地震行为的不协调性.安宁河断层与则木河断层走向不一致、螺髻山的快速隆升以及安宁河断层南侧SN向展布的走滑断层系统等可能是造成安宁河-则木河断层古地震行为差异的原因.

The Anninghc and Zemuhe Fault systems show characteristics of a left-lateral strike-slip movement since late Quaternary and they are located along the eastern boundary of the Sichuan-Yunnan Fault block in the southeastern region of the Tibetan plateau.The N-S striking Anninghe Fault is divided into the northern and southern segment around Mianning.The northern segment has an average recurrence interval of large earthquakes of about 500 ～ 700 years and a left-lateral slip rate of 4mm/a since Holocene.However paleoseismic behavior along the southern segment has been less focused.We excavated several trenches at Yuehua along the southern segment and used multiple radiocarbon dating to constrain the average recurrence interval of large earthquakes of this segment,which is about 600 ～ 800 years.The Zemuhe Fault has an average recurrence interval of paleoearthquakes of about 2300 years with a left-lateral slip rate of 2.4～ 3.6mm/a since Holocene.Comparing with the fault behavior between the Anninghe Fault and Zemuhe Fault,we find that the recurrence interval of the Anninghe Fault is shorter than that of the Zemuhe Fault and has a relatively larger left-lateral slip rate,indicating an inconsistent paleoseismic behavior.We suggest this inconsistence may be related to different strikes of the two faults,the uplift of the Luoji Shan and the distribution of the N-S trending strike-slip fault system on the south of the Anninghe Fault.