Fault segmentation and paleoseismicity along the North Wutaishan Fault of the Shanxi Graben System, China

Abstract

The Shanxi Graben System has been the location of large, devastating intraplate earthquakes in the past, making it an ideal location for the study of the temporal recurrence of intracontinental earthquakes. This study conducts a paleoseismic investigation on the North Wutaishan Fault (NWTSF) at the Northeastern end of the graben system. Located in North China between the Ordos Plateau and the Taihang Mountains, the Shanxi Graben System is composed of a series of en echelon left-stepping half graben basins oriented roughly to the North-Northeast. The NWTSF fault is an 85km long normal fault, bordering the southern edge of the Daixian basin. The fault trace was mapped using high resolution satellite imagery. Three trenches were excavated for paleoseismic and slip rate investigations on a 4.5 km long section of the fault located at the eastern end of the basin. Two of the three trenches had a depth and length of ̃8 and 16 meters respectively while the other had a depth and length of 1.5 and 3 meters. Anthropogenic modification and large amounts of erosion eliminated evidence of the most recent earthquake event at all trench sites. A paleoslope identified in one of the trenches suggests a large amount of sedimentation in addition to the large amount of erosion, possibly due to the lateral migration of the Yangyan River to the north of the study area. A significant change in stratigraphy from the west to the east wall of one of the trenches is a result of interaction from drainages to the east. A large alluvial fan deflected water from a drainage to the east of the trench, causing it to flow along the scarp, eroding part of the trench, while gravel layers were deposited from the fan, dipping southward, away from the basin. A Real Time Kinematic GPS survey of the area surrounding one of the trench sites highlighted two relict stream terraces that were faulted. These terraces show two paleoseismic events with vertical offsets of 2.9 and 2.5 meters, which equate to moment magnitudes of 7.1M and 7.0-7.1M respective