Late Quaternary Paleoearthquake History Of The Leibo Fault Zone

The Yingjing-Mabian-Yanjin tectonic zone（YMYTZ）is an important boundary structural belt between the southeast margin of the Tibet Plateau and the Sichuan Basin.Although the seismic of the area near the YMYTZ has been active since the Late Quaternary,the activity of its faults is still unclear,which brings great uncertainty to the understanding and evaluation of the current crustal deformation model and seismic potential in the region.Especially for the Leibo fault zone（LFZ）in the southern segment of the YMYTZ.Although there have been multiple the 1216 Mahu magnitude7 earthquake and strong earthquakes（M≥6）along the area near the LFZ,the key evidence of the latest activity age of the LFZ has not been obtained due to its low research level,limiting the development of regional seismic risk evaluation.To address this issue,this paper takes the LFZ as the research object.Based on the high-resolution remote sensing image interpretation and field geological and geomorphological investigations,paleoseismic trenching studies were carried out on the North,Middle and South branches of the LFZ respectively.The following conclusions were obtained:（1）Based on the joint comparison of paleoseismic event identification markers and trenching sets,three,three and five paleoseismic events were revealed on the three branch faults,respectively,and the dating results of the relevant radiocarbon samples qualified the onset times of the three paleoseismic events on the North Branch fault as21190-20590 BC（EP1）,20550-12120 BC（EP2）and 12090 BC after（EP3）,three strong seismic events on the Middle Branch Fault at 7400-6320 BC（EY1）、5690-2620BC（EY2）and 2220 BC-170 AD（EY3）,and five surface rupture-type seismic events on the South Branch Fault at 14660-9300 BC（ES1）,9270-7560 BC（ES2）,600-640AD（ES3）,740-1440 AD（ES4）,and 1650-1900 AD（ES5）.The results of paleoseismic studies show that all the branch ruptures of the LFZ are Holocene active ruptures.（2）By comparing the onset times of paleoseismic events on each branch rupture,it was determined that at least 10 paleoseismic events ruptured the surface of the LFZ since the Late Pleistocene,with corresponding age ranges of 21190-20590 BC（E1）,14600-9300 BC（E2）,12090-11820 BC（E3）,9270-7560 BC（E4）,7400-6320 BC（E5）,5690-2620 BC（E6）,2220 BC-170 AD（E7）,600-640 AD（E8）,740-1440 AD（E9）,and1650-1900 AD（E10）.The paleoseismic activity history of the Leibo Fracture Zone shows that the strong seismic activity of the three branch ruptures exhibits obvious spatial independence and temporal clustering,indicating that the branch ruptures of the Leibo Fracture Zone are independent seismogenic structures.（3）Based on the historical earthquake records and the results of paleoseismic studies,this paper concludes that the seismogenic structure of the 1216 Mahu magnitude 7 earthquake was the South Branch Fault of the Leibo Fault Zone,and in addition,the Leibo Fault Zone was probably involved in the rupture activity of the1935-1936 Mabian 6³/₄ earthquake swarm.（4）By collecting and analyzing the magnitudes of slip-type seismic events that have produced surface rupture in western China since 1920,we conclude that the lower limit of magnitude of the paleoseismic events uncovered on the Leibo fault zone is 6.5,and estimate that the fault zone has the ability to generate earthquakes of magnitude 7or higher based on empirical formulas using the length of the rupture.