Research On Paleoearthquake Events Recorded By The Quaternary Sediments In The Jinsha River Section Of The Chenghai Fault Zone

Most recently,large-scale earthquakes occur frequently,which come with significant casualties and property losses,in the margin of the Qinghai-Tibet Plateau.The Chenghai fault zone is one of the most important seismogenic faults in the southeastern margin of the Qinghai-Tibet Plateau.It's located in the southwestern of the Sichuan-Yunnan rhombus block which is surrounded by the Honghe fault zone,the Xianshuihe-Xiaojiang fault zone,and the Jinsha River fault zone.Historical earthquake records show that a total of 16 earthquakes with magnitude greater than 5 occurred in the Chenghai fault zone in the past 500 years.Four of them were located in the Jinsha River section of the Chenghai fault zone.The Zhoucheng-Qingshui fault zone,which intersects with the Chenghai fault zone,is also active in geological history.It has an important impact on the seismic activity of the Jinsha River section.Predecessors have done much research on earthquake events in the area in recent years or historical periods.However,the Chenghai fault zone is a mid-continental active fault zone with a long recurrence interval,usually as long as thousands to tens of thousands of years.At present,paleoearthquake research in the historical period of geology is relatively weak.Geological records are needed to reveal earlier paleoearthquake events.The identification of these ancient earthquake events is an important supplement to historical earthquake records in earthquake risk assessment,and it is also of great significance to reveal the patterns and mechanisms of earthquake formation and development.In this research,first,the detailed field geological survey of the paleoearthquake traces preserved in Quaternary sediments was carried out in the Jinshajiang section of the Chenghai fault zone.Many paleoearthquake traces left directly or indirectly by paleoseismic activities were discovered through investigations,including earthquake fissions,earthquake dammed lake sediments,earthquake-generated rock falls,seismic faults and sand liquefaction.In addition,through the comparative analysis of the cutting relationship of earthquake fissions and/or the strata and geomorphic sites where the earthquake traces are located in the field,it is found that these traces are the products of multiple earthquakes,suggesting a series of ancient earthquake events occurred in the area during geological history.Based on field investigations,a large number of samples were collected for chronological analysis.The formation age of earthquake fission is limited by the covered sediments and earthquake fission fillings.The formation age of earthquake dammed lakes is limited by the dammed lake sediments and the underlying loose sediments.The formation age of earthquake-generated rock fall is limited by the calcareous coats on the surface of earthquake-generated rock fall.The formation age of seismic fault is limited by the calcareous coats on the surface of seismic fault.The chronological samples were tested by the methods of optically stimulated luminescence(OSL),electron-spin-resonance(ESR)and uranium-series(U).A total of 68 valid chronological data were obtained.Finally,a comprehensive analysis based on the combination of chronological test results and field surveys revealed that there were 10 large-scale paleoearthquake events that occurred in the research area since 500 ka.The ages of them are about 450 ka,400 ka,345 ka,300 ka,250 ka,190 ka,155 ka,105 ka,75 ka,and 25 ka,respectively,representing ten activce tectonic periods.The large earthquakes may have a 50,000-year cycle in the research area.This research identifies paleoearthquake events from longer scales,larger spaces,and more types of traces,which provides new perspectives and new ideas for paleoearthquake research.