Seismic history of a bedrock fault scarp using quantitative morphology together with multiple dating methods: A case study of the Luoyunshan piedmont fault, southwestern Shanxi Rift, China

Abstract

The study of bedrock fault scarps is a useful approach to identify paleo-earthquakes and acquire the faulting history in bedrock areas where traditional trench techniques are not applicable. Here, we report a study on the Luoyunshan piedmont fault (LYSPF), southwestern Shanxi Rift, China. Although several studies have analyzed the LYSPF, its faulting behavior is still under debate. For example, the tectonic geomorphology fails to determine the frequency of past slips and the elapsed time. To address these problems, we investigated two bedrock fault surfaces, Luoyuncun (northern segment) and Weicun (southern segment) using quantitative morphology analyses together with multiple Quaternary dating techniques. Quantitative morphology analysis shows that both bedrock fault surfaces have the characteristics of vertical segmentation. This kind of segmentation indicates that the fault surfaces are exhumed intermittently, likely owing to periodic faulting earthquakes. Therefore, earthquake events can be identified as morphological segments, and the corresponding co-seismic displacement can be determined by the height of these segments. According to the results from the quantitative morphology and dating techniques, four seismic events, with a co-seismic vertical displacement of 0.9 m, 0.82 m, 1.64 to 2.38 m and 2.30 to 2.54 m, on the northern segment occurred during the late Pleistocene, whereas, two seismic events, with a co-seismic vertical displacement of ~1.33 m and 1 to 1.08 m are identified on the southern segment occurred during the Holocene. Combined with the results of knickpoint series and trench techniques of the LYSPF, eight paleo-earthquake events were identified since 45 ka, with an average recurrence interval of 5.8 kyr. The more complete paleo-earthquake sequence also showed the possibility that the variation of the slip rate along the LYSPF depends on the segment, and that the displacement produced by the Linfen M 7½ earthquake of 649 CE has been recorded at the WC fault surface.