THE MS 7.5 14 APRIL 1955 ZHEDUOTANG EARTHQUAKE AND A RECORD OF HOLOCENE-LATE PLEISTOCENE PALEOSEISMICITY ALONG THE ZHEDUOTANG FAULT WITHIN THE XIANSHUIHE FAULT ZONE, SICHUAN PROVINCE, CHINA

Numerous strong earthquakes are distributed along the N-S-trending Xianshuihe strike-slip fault zone that forms part of the eastern boundary of the Qinghai-Tibet Plateau. Its characteristics include: left-lateral and left-stepping faults, linear gorges and mountain ranges, pull-apart basins, depressions and lakes. This zone has considerable geothermal activity and geological hazards mark the zone as a prominent risk to China’s population. Our study focused on the 1955 Ms7.5 earthquake that occurred near the village of Zheduotang, Kangding County, Sichuan Province in southwestern China. The seismogenic Zheduotang fault trends 325° and is a secondary fault in the Xianshuihe structural zone where the Zheduo River was progressively offset left-laterally ~160m. Approximately 0.37km from the river along the strike of the fault to the SE, a narrow, NE-trending stream valley is deflected 8.5m left-laterally before it then returns to a NE-trend and flows northeastward toward a well-developed Holocene-Late Pleistocene alluvial fan. Adjacent to the fault along the Zheduo River, we delineated 12 tectonic terraces where the NE-flank of the fault was progressively uplifted 1-2m as each terrace was preserved with the youngest terrace (12) dated by ¹⁴C at 0.5ka and terrace number 10 dated by ¹⁴C at 2ka. In 2013, we located a trench (CUG2013-1) across the graben, preserved along the SW-flank of the fault, to specifically ensure excellent preservation of the Holocene sedimentary record. Our trench was located about 10m from a previous trench excavated about 2000 (Zhou et al., 2001). Our 2013 trench contained buried soil horizons with the youngest soil (dated by ¹⁴C at 0.1ka) broken by the 1955 event and with progressively older soils dated by ¹⁴C at 3.1, 3.9, and 6.5ka down to a depth of 1.5m where the water-table precluded going deeper.