Study on the Late Quaternary Activity of Chenghai Fault Zone

Abstract eng:
The Chenghai fault zone with north-east trending is located on the west-south boundaries of the rhombus block in Sichuan and Yunnan. We study the late Quaternary activity of the Chenghai fault zone by the image data, field investigation, trenching and radiocarbon dating of charcoal samples. The results achieved by the authors show that the Chenghai fault zone can be divided into 3 segments. The first segment is located on the north end of the Chenghai fault zone, from JingGuan Basin to Northern Qina Basin, its length is about 50km. The Yongsheng earthquake with Ms 7.75 took place on this segment in 1515. There are many ancient seismic phenomena in the Yongsheng earthquake area, including surface ruptures, steep fault ridges, ground fissures, landslide wedges, ground collapses and landslips, etc. The surface rupture located on the right bank of Lingyuan River is about 600m long and 8m wide. Its vertical displacement is about 1.5m. The Lingyuan River was left-lateral strike-slip dislocated about 450m. The fault scarp located on the east bank of the Chenghai Lake is about 100m long and 3m high formed before 4000 years. Its vertical displacement is about 1.5m. The trench at Songguan village revealed that the first terrace of Qina River was dislocated 1.5m by the fault and it is dated to be 2330±30cal.a BP. The second segment is from southern Qina Basin to northern Binchun Basin, its length is about 60km. On October 27,2001, a earthquake with Ms6.0 occurred on this segment. The Jingsha River was left-lateral strike-slip dislocated about 3km. The third segment is located on the south end of the Chenghai fault zone, from the southern Binchun Basin to Midu Basin, ending at the Red River fault zone, its length is about 60km. On this segment, the Midu earthquake with Ms 6.25 took place in 1623 and the Bingchuan earthquake with Ms 6.25 took place in 1803. The trench at Housuo village revealed that the second terrace of Naxi River was dislocated about 2.0m by fault and it is dated to be 15000±600a. Two ravines located at the south of Yongfushao village were all left-lateral strike-slip dislocated about 200-280m. The geological profile at Maolipo village revealed that the upper clay was dislocated about 2.0m by fault and it is dated to be 4390±30cal.a BP.. All these data suggest that the Chenghai fault zone are active since the Holocene, it has both normal and left-lateral strike-slip features and the active intensity was gradually weakened from north to the South.

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