| Since the Ms 8.1 earthquake at the Kunlun mountain in 2001,there were Yutian Ms7.3 in 2008,Wenchuan Ms8 in 2008,Yushu Ms 7.1 in 2010,Lushan Ms 7 in 2013,Yu Tian Ms 7.3 in 2014 and Jiuzhaigou Ms7 in 2017 around the Bayan Kara block,which show that the Bayan Kara block is going through a new big seimic active period.The magnitude 8.1 and 7.5 strong aftershocks of Nepal on the Himalaya plate boundary zone in 2015 and the occurrence of the Jiuzhaigou Ms7 earthquake in 2017 on the northeastern boundary of the Bayan block,which also imply that the large earthquakes in the Plateau-Kara Bayan block seem to continue.The Xianshuihe fault zone is an important component of the southern boundary of the Bayan Kara block.In history,there were 8 more than magnitude 7 earthquakes and one 6.9 earthquake in less than 300 years since 1725,and the average recurrence interval is about 31 years,the most recent Ms6.9 magnitude earthquake in 1981 has been 36 years ago.The risk of large earthquake in Xianshuihe has been concerned by scholars at home and abroad,and Xianshuihe is also a key monitoring hazard area of China Earthquake Administration.In November 2014,a magnitude 6.4 earthquake occurred along the Selaha segement of the Xianshuihe fault zone.Will there be a bigger earthquake next? If so,it will occur on the fault zone which segement and what possible time? These problems have become the focus of earthquake workers.Obtaining the rupture history on the longer time scale of the fault zone is the basis for answering these questions.As for the future seismic risk of the Xianshuihe fault zone,the previous quantitative assessment has been carried out based on the activity segmentation,the quantitative parameters of the late Quaternary activity and the earthquake rupture behavior,the Kulun stress change and the modern seismicity.The Selaha segment is the middle main fault of the three branch of the Xianshuihe fault zone from the south of the Qianning basin to the south.The Selaha segement is also the longest period of the elapsed time(293 years)in the whole fault zone except the Yalahe section(the latest event is not clear),and it is the most dangerous in the future.Along the Selaha segement,a series of ancient earthquake studies carried out by the predecessors have great discrepancy.In order to assess the danger of the future strong earthquakes Selaha segement,on the basis of previous studies,further research on the excavation of slots and paleoearthquakes is needed to obtain more complete foundation material for the paleoearthquake.Over the past ten years,paleoseismic research,especially the technology of trenches and dating have made new progress.Small pull-apart basins,faulted ponds or rift ponds have developed into ideal locations for the study of paleoseismic trenches on strike slip faults in the southeastern margin of the Tibet Plateau.Based on these new techniques,the paleoseismic study of the Xianshuihe,Anninghe and the Zemuhe fault zone have obtained a series of research results.The application of these techniques to the paleoseimic study in Selaha section,and a more reliable rupture history on the basis of previous work,is the precondition for the prediction of a greater earthquake in this section.After years of exploration and experiment,the bedrock temperature monitoring is developing into a new dynamic variation observation technique,and has been applied to field monitoring after the Wenchuan earthquake.The Institute of geological research of the China Earthquake Administration has set up a network of bedrock temperature observation in the Sichuan Yunnan region on theXianshuihe,the Anninghe and the Longmenshan fault zone.Since October in 2009,the bedrock temperature changes in the fault zone have been monitored,and the Lushan Ms7 in 2013 and Kangding 6.3 in 2014 have been monitored.Further analysis of the response relationship between the bedrock temperature and the two strong earthquakes,as well as the recent regional bedrock temperature and stress changes,may obtain some information on the stress variation before the earthquake in theXianshuihe fault zone,and provide a basis for judging and predicting the development trend of the seismic activity.In order to answer the above scientific questions,the paper intends to carry out the fine study of ancient earthquakes along the Selaha segement on the basis of previous work,so as to obtain a more complete and reliable ancient earthquake sequence and rupture history,to reveal the recurrence behavior of the earthquake and to study the relationship between the bedrock temperature change and the strong earthquake response.The comprehensive analysis and prediction of the earthquake risk are carried out.The main contents of the paper are as follows:(1)the geological and geomorphological survey of the Selaha and the detailed study of the paleoearthquakes in the ZhongguVillage and jiangba village.(1)Remote sensing interpretation and geological and geomorphological investigationOn the basis of previous data collection and arrangement,remote sensing interpretation and field geological and geomorphological investigation are carried out along the Selaha segment of the Xianshuihe fault zone,with the emphasis on interpreting and investigating the clear earthquake surface rupture zone and the possible preservation of the ancient earthquake sites.(2)The excavation of trenches and the subtle study of ancient earthquakesThe 2 large trenches in Zhonggu village,3 large and 5 small auxiliary trenches in jiangba village are excavated.The subtle research of ancient earthquake is carried out,including the research on the cleaning of the trenches,the site cataloging,the layer sequence and the identification of the ancient earthquake events.(3)Sample collection and testThe AMS and OSL dating samples were collected and sent to the laboratory in batches.(4)A comprehensive study of the paleoearthquake sequence and fracture behaviorBased on field work in two locations,combining with dating results and previous works,the paleoseismic sequence and rupture behavior of the Selaha segement are comprehensively analyzed.(2)observational study of bedrock ground temperature in the salaha section and its surrounding area(1)Analysis of the relationship between the bedrock temperature change of the Selaha segement and the response of Lushan earthquake and Kangding earthquakeThe relationship between bedrock temperature and Lushan earthquake and Kangding earthquake is analyzed,comparing with the observation data of borehole strain and seismology.The relationship between bedrock temperature and regional geostress change and strong earthquake activity is discussed.(3)Analysis and discussion on the change of bedrock temperature and regional stress change after the Kangding earthquake.The change trend of bedrock temperature after the Kangding earthquake is analyzed emphatically,and the regional stress change situation is discussed.(3)prediction of the danger of strong earthquake in Selaha segementAccording to the rupture history and recurrence behavior on the time scale of the paleoearthquake,the possibility of the future surface rupture type earthquake in Selaha segement is judged,and the risk degree of the Selaha segment and its surrounding strong earthquakes is further tried by the situation of the regional bedrock temperature change(stress change)after the Kangding earthquake.The main understanding and conclusion of the paper:(1)the latest earthquake rupture of the salaha section in the Xianshuihe fault zone has ruptured to the Zhonggu Village.(2)4 events were revealed in 5 trenches in Zhonggu Valley and Jiangba village,on Selaha segement,which were in turn: before(15599-14938)BP,(11121-10041)BP,(3745-2695)BP,and Kangding Ms7 earthquake in 1725.(3)Selaha segement has a long interval and short interval cluster phenomenon of selaha section,the early recurrence interval of about4500 years,after(11121-10041)BP event,entered a longer calming period of more than 7000 years,then entered the active period of earthquake,two large earthquakes was recorded about 4000 years ago,and the recurrence interval was about 3000 years.(4)A few days before the Lushan earthquake in 2013,the stress temperature of the three bedrock temperature stations in Xianshuihe fault zonea bout 90 km away from the epicenter declined synchronously,showing that the regional stress tension enhanced.The high frequency components of the three stations also changed synchronously within 80 days before the earthquake.Before the Kangding earthquake in 2014,the stress temperature of the four bedrock temperature stations in the Xianshuihe fault zone about 20-30 km away from the epicentre rised synchronously,which showed that the regional stress extrusion enhanced.In the 200 days before the earthquake,the high frequency components of the 3 stations also changed synchronously.(5)The recurrence interval of large earthquakes during the active period of Selaha segment is about 3000 years,and the elapsed time of the latest event is less than 300 years,and the possibility of large earthquakes with surface rupture is not likely in the future.By the end of August in 2017,the stress temperature of four bedrock temperature stations in the Xianshuihe fault zone did not change synchronously,but the stress in the area,especially the Shimian area,was adjusting,and the danger of the strong earthquake in the northern segment of the Anning River fault zone(Mianning-Shimian segement)is worth paying attention to. |
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