Surface rupture geomorphology and vertical slip rates constrained by terraces along the Wulashan piedmont fault in the Hetao Basin, China

The Wulashan piedmont fault is a typical normal fault in the northern Ordos Block and has exhibited intense activity since the Late Pleistocene. It is located among the Ordos Block, Alashan Block, Yanshan Block and North China Block and between the NE expansion of the Tibetan Plateau and the subduction of the Pacific Plate. This area is important as a key location to study the remote effects of the Tibetan Plateau NE expansion. Historical M 8 earthquakes in 7 BCE and 849 CE occurred near this fault and thus the fault poses a potential earthquake threat but has received little research attention. We observed the surface ruptures, measured the heights of terraces created by fault offsets, excavated trenches and used optically stimulated luminescence (OSL) dating of samples to study the late Quaternary fault activity. This study shows that the fault is divided into three segments. Three levels of terraces were produced by movement on the fault. The T1 terrace formed at approximately 10 ka, the T2 terrace formed between 30 and 40 ka, and the T3 terrace formed between 40 and 50 ka. Based on the average terrace heights along the fault, we calculate that the vertical fault slip rates have been approximately 1.12–1.34 mm/a in the Holocene and 2.2–2.28 mm/a since 50 ka. Our analysis shows that the Daqingshan piedmont fault has the highest vertical slip rate, the Wulashan piedmont fault has an intermediate slip rate and the Sertengshan piedmont fault has the lowest vertical slip rate. The area between the Wulashan and Daqingshan piedmont faults readily accumulates seismic stress and represents a potential future earthquake site. Trenches provide visible evidence of vertical rupture and slip movement in subsurface profiles, revealing six events on the Gongmiaozi segment since 25 ka and 14 events on the Baotou segment since 120 ka. The analysis shows that the Ordos peripheral fault system formed due to the subduction of the Pacific Plate. The northward expansion of the Tibetan Plateau and the movement of the Ordos Block have caused fault activity in the northern part of the Hetao Basin since the Late Pleistocene. The modern tectonic stress field and the upwelling of asthenospheric material east of the Hetao fault zone have resulted in an increase in the slip rate on the Hetao fault zone from west to east.