Studying On The Active Characteristics And Paleoearthquake Of The Eastern Piedmont Fault Of Helan Mountain In The Late Quaternary

Active tectonics refers to all structures which have been active since the late Pleistocene, 100-120 ka, are still active recently, and will be active in a certain time period in the future. Active fault is the most direct object on studying active tectonics. The basic parameters being used to study the active fault include the length of fault and its subsection, the cumulative displacement, cosiesmal displacement, sliding speed, paleoearthquake et al.The eastern piedmont fault of Helan Mountain is located in the northern part of the North-South Seismic Belt, which is the west boundary of Yinchuan graben. Meanwhile it is the main seismogenic structure of Yinchuan graben and has had strong activities since Cenozoic.This paper obtains some quantitative parameters of the eastern piedmont fault of Helan Mountain based on the geological mapping along active fault ribbon, through measuring surface topography and excavating trench of Yushougou-Taomengou, Zihuagou- Chunshukou ,Chaqikou-Liutiaogou of the fault.1. Through map plotting the detailed location and geometric shape in workspace, the spreading shape of the fault had been ascertained,which lies in the south of Zihuagou and the north of Biangou. As a studying result, the fault that the rupture range lieing from the south of Touguan to the north of Liutiaogou located in the border of Ningxia province and Inner Mongolia province is divided into five small segments, including Touguan–Taomengou, Zihuagou-Chaqikou, Chaqikou -Dawukou, Dawukou-Wangquangou, Wangquangou-Liutiaogou. The latest activity era of the fault is 1739. Besides, the section of Touguan–Taomengou of the fault that its latest activity took place in 3ka.2.The proluvial fan in the front of eastern piedmont of Helanshan mountains are divided into six periods since late Pleistocene, by map plotting and measuring, in the section of Chaqikou– Liutiaogou of the fault. And the former five proluvial fans dislocated in vertical form five grades platforms in the footwall of the fault. The height of the fault scarp in the front of each platform respectively is 95.8m、25.5m、16.7m、5.8m、2.1m.3. Based on the results that the predecessors obtained and the conclusion I had offered when I studied in the project of Exploring Active Fault in Yinchuan City, the seismogenic structure of the 8.0 magnitude earthquake at Pingluo in 1739 was discussed and fracture types that the fault had acted in 1739 was also summarized. Mapp plotting results show that the rupture range have the length of 118km including two discrete periods,Xiaogangou-Rujigou and Dawukou-Liutiaogou.4. Through ditching the trench of Hongguozigou,Zhengguangou, the south of Dashitougou and the north of Dashitougou, the characters of paleoearthquake taken place in the fault in Holocene are disclosed.