关键词: 大凉山断裂带, 越西断裂, 晚第四纪, 古地震

Abstract:

The Xianshuihe-Xiaojiang fault system(XXFS)is a strongly active left-lateral strike-slip fault zone on the eastern edge of the Qinghai-Tibetan plateau. It controls the eastern boundary of the Sichuan-Yunnan block, Which is one of the most active tectonic zones in the north-south seismic belts. There have been 36 destructive earthquakes since 1327AD. The historical strong earthquakes in the middle section of the XXFS fault system are mainly distributed along Anning River faults and Zemu River faults, such as M7.0 in 814AD, M7^1/2 in 1536AD, M6^3/4 in 1732AD, M7^1/2 in 1850AD and M6^3/4 earthquakes in 1952AD. However, as an important part of the middle of XXFS, the Daliangshan fault zone only recorded a magnitude of M5^1/2 in 1480AD, and there was a lack of earthquake records above a magnitude of 6 which may be due to the quiet period of earthquakes, or the location of remote mountainous areas where historical records are missing. The paleoseismic study revealed that there were surface rupture events along the Butuo and Jiaojihe faults in the southern section of the Daliangshan fault zone in 970-1510AD and 1310-1660AD respectively, with a magnitude of not less than 6.5; Along the Puxiong fault in the middle section of the Daliangshan fault zone, there was a surface rupture event in 927-1360AD, with a magnitude of not less than 7.0. However, there are no corresponding historical records of the earthquakes in these three historical periods, indicating that strong historic earthquakes in the Daliangshan fault zone may be missing.

The Yuexi fault is the only branch fault in the Daliangshan fault zone dominated by thrust slip. The fault spreads in an arc shape, with a total length of about 50km, and controls the quaternary basins such as Zhenxi, Xinmin, and Yuexi. The topographic height difference between the fault’s two sides is about 2 000m. The middle section of the fault is the eastern boundary fault of the Yuexi Basin, which cuts through the piedmont alluvial fan, forming fault scarps several meters to tens of meters high. Together with the Puxiong fault on the east side, which is dominated by left laterally slipping, a positive flower-type structure is formed in the middle section of the Daliangshan fault zone. There are previous discoveries about fault scarps of the Yuexi fault on the piedmont alluvial fans, but no paleoseismic research has been reported up to now.

On the basis of remote sensing interpretation and field geological and geomorphological survey of the Yuexi fault, a big trench was excavated across the 12m-high fault scarp on the late quaternary alluvial fan in the Yuexi Basin, which revealed four paleoseismic events since the late quaternary and the coseismic vertical slip of the last one is ~1.2m. Based on trench analysis, 14 stratigraphic units are defined from which carbon samples are acquired for geochronological analysis. Through radioactive carbon dating and correction of the dating data by the OxCal software, and OxCal model building to limit the age of paleoearthquake events, the ages of the four events were 25260-23880BC, 23930-23500BC, 20980-1400BC, and 270-1500AD. According to historical records, a destructive earthquake occurred in Yuexi County on September 13, 1480AD, which triggered landslides, 7 earthquakes on that day, and more than 20 aftershocks as of the 27th, with a tremor range of 150km. We consider that the latest event should be the Yuexi earthquake in 1480AD according to the historical records of earthquake damages. Based on the paleoearthquake research, this event very likely led to a coseismic rupture of the Yuexi and the Puxiong faults. According to the empirical scaling laws between magnitude and rupture length, the magnitude of the surface ruptured paleoearthquake is estimated to be more than M7.0. The results provide basic data for evaluating seismic activity and analyzing seismic risk in this area.

Key words: the Daliangshan fault zone, the Yuexi fault, Late Quaternary, Paleoearthquake