雷波断裂带全新世活动证据

EVIDENCE FOR THE HOLOCENE ACTIVITY OF THE LEIBO FAULT ZONE

荥经-马边-盐津构造带是青藏高原东南缘与四川盆地之间的重要边界构造带,虽然晚第四纪以来地震活动频繁,但构造带内各断裂的活动性尚不明确,对认识和评价区域现今地壳变形模式和地震危险性带来了很大的不确定性。特别是构造带南段的雷波断裂带附近曾发生过多次6级以上强震,但该断裂带的研究程度很低,其最新活动时代的厘定还缺乏可靠依据。为解决这一问题,文中基于高分辨率遥感影像解译和野外地质地貌调查,在雷波断裂带的北支和南支断裂上分别开展了古地震探槽研究工作。基于古地震事件识别标志,在2条分支断裂上分别揭示了3次和7次古地震事件,相关放射性碳样品的测年结果限定了北支断裂3次古地震事件的发震时间分别为21190—20590BC(EP1)、20550—12120BC(EP2)和12090BC之后(EP3),而南支断裂最新的2次古地震事件的发震时间为9270—5040BC(EL6)和5000BC之后(EL7)。古地震探槽研究结果表明,雷波断裂带在全新世以来曾发生过多次地表破裂型强震事件,为全新世活动断裂;其北支和南支断裂揭露的事件互不相同,指示雷波断裂带的分支断裂为独立的发震构造。通过收集并分析1920年以来中国西部地区产生地表破裂的走滑型地震的震级,认为雷波断裂带上揭露的古地震事件的震级下限为6.5级,并估计断裂带具有发生7级以上地震的能力。

The Yingjing-Mabian-Yanjin tectonic zone(YMYTZ)is an important boundary structure between the southeastern margin of the Tibet Plateau and the Sichuan Basin.It consists of several small-scale secondary faults with different strikes and is generally characterized by the intersections of north-northwest oriented longitudinal faults and nearly east-west oriented transverse faults.The YMYTZ is seismically very active in the late Quaternary and hosted several moderate-strong earthquakes,including two M≥7 earthquakes since 1216AD,namely the 1216 Mahu earthquake and the 1974 Daguanbei earthquake.After the Daguanbei earthquake,several M≥6 earthquakes and hundreds of M≥5 earthquakes occurred along the YMYTZ to date,implying it is a newly generated seismotectonic belt.Even so,the activity of each fault is still unclear,bringing out great uncertainty in understanding the current crustal deformation pattern and in evaluating the regional seismic potential.Specifically,although several M≥6 earthquakes have occurred along the Leibo fault zone in the southern segment of the YMYTZ,the late Quaternary activity of the fault zone has not been well determined due to insufficient work as well as subsequent lack of solid evidence.The Leibo fault zone strikes NE-SW and spreads on the southeast flank of the Chenqiangyan-Shanzhagang anticline.It starts at the Huanglang township near the Mahu Lake,cuts through the Jingkou Mountain,Lianhuashi,and Leibo,and extends southwestwards to the vicinity of Lianlajue.The latest investigation shows that the Leibo fault zone consists of four subparallel right-lateral strike-slip faults named F 1-F_(4) from the north to the south,respectively.These fault branches together constitute a 43km-long and 10km-wide structural belt.Previous paleoseismic work along the Leibo fault zone found that the faults ruptured the late Pleistocene sedimentary layers with their upward terminations covered by the undeformed Holocene deposits,implying it was active in the late Pleistocene and has not been active since the Holocene.However,the ground surface traces of the Leibo fault zone are the most obvious among the faults in the YMYTZ,and recent seismologic studies show strong seismic activity for the Leibo fault zone,bringing out a controversy about whether it is active in the Holocene or not.To address the late Quaternary activity of the Leibo fault zone,we conducted detailed indoor deformed geomorphic feature interpretation on remote sensing imageries like 2m-resolution GF-2 imagery and high-resolution imageries on Google Earth,and further mapped the fault traces in the field using an unmanned aerial vehicle(UAV)derived digital orthographs and digital surface models(DSM).Based on the geological and geomorphological surveys,two trenches were excavated at Pengjiashan and Luohangou along the northern(F_(2))-and southern(F_(4))branches of the Leibo fault zone respectively.On the trench walls,surface-rupturing paleoearthquakes were identified for each fault according to criteria for faulting events like cut-and-cover structures,scarps,and colluvial wedges.Subsequently,we collected and dated several radiocarbon samples from the sedimentary layers imme-diately before and after the rupturing events,and finally carried out stratigraphic sequence calibration using the acquired ages with the OxCal 4.4 program to constrain the timings of the revealed paleoear-thquakes.According to the identification criteria of paleoseismic events,it was revealed 3 paleoearthquakes in the Pengjiashan trench on the northern fault branch(F_(2))-and another 7 rupturing events in the Luohangou trench along the southern fault branch(F_(4)).Radiocarbon sample dating constrain the ages of the paleoearthquakes along F_(2) to be 21190-20590BC(EP1),20550-12120BC(EP2),and after 12090BC(EP3),while the latest two paleoseismic events on F_(4) occurred 9270-5040BC(EL6)-and after 5000BC(EL7).Our paleoseismic studies show that the LFZ has experienced several surface-rupturing earthquakes in the Holocene,verifying it is a Holocene active fault zone.Moreover,the ages of the paleoseismic events revealed on two fault branches do not overlap with each other,suggesting they are different paleoearthquakes so that the fault branches in the Leibo fault zone are independent seismogenic structures.By collecting and analyzing the magnitudes of strike-slipping earthquakes that have generated surface ruptures in western China since 1920,it is believed that the minimum magnitudes of the paleoearthquakes determined on the Leibo fault zone are 6.5.Through the empirical relationships between magnitude and surface rupture length,it is estimated that the LFZ has the capability to produce an earthquake with M≥7.