Study On The Paleoearthquake And Earthquake Risk In The Main Faults Near Shimian

The Xianshuihe-Xiaojiang Fault System(XXFS)which constitute the northeasteastern boundary of the Sichuan-Yunnan block is an intensively active seismotectonic belt located on the southeastern margin of the Qinghai-Tibet Plateau.Affected by the Channel flow in the middle-lower crust in the western Sichuan region,the XXFS is strongly active,and large earthquakes occur frequently.On average,there will be an earthquake of magnitude 7 or more every 34 years.However,since the 1973 Luhuo M7.6 earthquake,the earthquakes on the XXFS have been silent for nearly 50 years.The danger of strong earthquakes in the future has been concerned by scholars.The computer numerical simulation shows that most of large earthquakes occur in the end of fault zone or the region where fault trend is changed.The Shimian area is located in such a structural position where the Xianshuihe Fault(XF)—in the northern part of the XXFS—transforms to the Anninghe Fault(AF)and Daliangshan Fault(DF),and is also the intersection area of the XXFS and the Longmenshan Fault.In this area,the active tectonic style is extremely complex,and it has been regarded as a large earthquake tectonic area,where it will be at a risk of strong earthquakes in the future.For the risk assessment of this area,previous studies mainly discussed the stress changes and seismic gap distribution through short-term data recorded by instruments.However,these studies lack knowledge in a long time scale.Especially in Shimian County,some active tectonic basic data,such as the surface rupture of the latest activity,activity time and earthquake recurrence behavior of the faults,which is directly related to future seismic hazard assessment,are still very scarce.Also,there is still no relatively reliable trenching profile evidence for several faults,especially lacking of systematic research on the fracture relationship and whether there is interaction between faults.Paleoseismological studies often reveal traces of surface ruptures of large earthquakes in the geological record.The study makes it possible to recognize the longterm behavior of faults in several seismic recurrence periods and to estimate the risk of future large earthquakes.By studying the paleoearthquakes at a single point on the fault,we can obtain the paleoearthquake sequence of the fault and then we can analyze the earthquake recurrence behavior of the fault.By comparing the paleoearthquake sequences of different segments and different positions of the fault,we can analyze whether the fault ruptures in whole segment or in multiple segments,and then understand the fracture style of the fault;By comparing with the paleoearthquake sequences of the peripheral faults,the time-space variation regularity of large earthquakes in the region can be recognized.In the Shimian area,some trenching work has been carried out,but mainly concentrated on the northern section of the AF and the Moxi section of the XF.Along the northern segment of the DF,trenching work is very scarce.In the early research,the DF was not classified as a component of the XXFS.Recent years,some studies have found that the activity of the DF is not weaker than that of the Anninghe-Zemuhe fault,and found some evidents of strong earthquake activity,which shows that the fault may occur earthquake with magnitude more than 7.However,these studies are mainly concentrated in the middle and southern segments of the DF.In the northern part of the DF,there are also sporadic activity data and individual trenching profiles.But the direct evidence of Holocene activity and paleoearthquake data are very few,and there are great differences in understanding.In order to discuss the fracture relationship and future seismic risk along the intersection of several fault zones,we should obtain more reliable fracture histories of the proximal ends of fault zones around the Shimian area at first.On the basis of the previous work,this paper plans to excavate several additional paleoseismological troughs at the location of Lianhecun in the northern part of DF,Zheduotang in SF and Yalaxiang in SF.Furthermore,we will analyze and study the paleoseismological events revealed by these troughs.The following understandings and conclusions are obtained:(1)Since the end of the Late Pleistocene,the Lianhecun Trough Group has revealed that there have been at least four rupture events in the Shimian fault,which are as follows: Event E1,20925-16850BC;Event E2,15265BC-1785BC;Event E3,360AD-1475AD;Event E4,1655AD-1815 AD.Nowadays,the activity of the Shimian fault tends to intensify.The recurrence interval of the latest two fracture events is about 800 years,and there may be a risk of an earthquake with magnitude 7 or so in the future.(2)Two trenches were excavated in the southern segment of the XF.Two events revealed by the Yalaxiang trench,SL1,1272-1720AD;SL2,1703-1929 AD.The age of SL1 is closer to 1720 AD,suggesting that there were two surface rupture earthquakes in this area in the past 500 years.Three events revealed in Zheduotang trench: ZD1,1215BC-315AD;ZD2,830BC-705AD;ZD3,1955 AD.The sedimentary sequence of Zheduotang in the last 3000 years is complete.The recurrence interval of three earthquakes is 1200-1900 yr,and the average recurrence period is 1500 years.(3)By comparing the results of previous paleoearthquake researches,9 paleoearthquake events since Holocene have been obtained in the Moxi segment.The events in the past 2000 years have a high credibility.The Moxi segment of the XF can be divided into two sub-active segments.The Moxi-Shimian segment has an independent rupture history with a recurrence interval of about 400 years,but it can also break through the step and rupture the whole segment with a recurrence interval of about 800 years.Comparing with the paleoearthquake records of the southern segment of XF,the three secondary faults of Kangding-Huiyuansi segment are relatively independent rupture segments,and the possibility of multiple earthquake ruptures is small.However,the same fracture behavior may exist in the Seraha segment and the Moxi segment.(4)Three events revealed by at Baichunshu trench on the northern segment of the AF: BE1,before 1003AD;BE2,1000-1182AD;BE 3,after 1535 AD.Comprehensive comparison of previous studies,the northern segment of the AF has occurred five paleoearthquake events since the Holocene.The north segment of the AF can be divided into two rupture segments,and the recurrence period of the northernmost segment is about 600 years.(5)The Moxi earthquake in 1786 is a large earthquake of swarm type,which may lead to rupture of multiple faults,the northwest end of the surface rupture extends to the area of Chongcaoping,and the southeast end may trigger Shimian Fault.(6)The occurrence of large earthquakes is often characterized by clustering.The recurrence interval of cluster period in Shimian area is about 310 yr.After the Moxi earthquake in 1786,the region entered a quiet period.The recurrence interval of the Yajidong-Zimaguan segment of the AF is 600 years,with a lapse time of 500 yr.There is a stronger seismic risk in the medium and long term scale..Kangding area has entered a quiet period of large earthquakes,and the probability of large earthquakes in Kangding area is small.