Paleoseismology along the range‐front fault of Helan Mountains, north central China

Abstract

The NNE trending range‐front fault of the Helan Mountains along the western margin of Yinchuan graben, northcentral China, is a 13‐km‐long high‐angle normal and right‐lateral strike‐slip fault. It cuts late Pleistocene to Holocene alluvial fans, forming a SE facing scarp, and offsets the Great Wall of the Ming Dynasty (about 400 years B.P.) right laterally for 1.45 m and vertically for 0.95 m. The offset of the Great Wall is probably a result of the M = 8 Yinchuan‐Pingluo earthquake of 1739. The 1739 earthquake formed an 88‐km‐long discontinuous surface rupture zone along the fault scarps. Both the length and height of the fault scarps are largest at Hongguozi in the north and Suyukou in the south. We have prepared large‐scale topographic and geologic maps along the Hongguozi and Suyukou scarps, surveyed 88 profiles across the scarps, and excavated 14 trenches across the fault, in order to study Holocene paleoseismicity and recurrence intervals of large earthquakes along the fault. Four levels of terraces were developed on the upthrown block of the fault along some gullies that dissected the alluvial fans. The older terraces, such as T4 and T3, are tilted to the west, whereas the younger terraces are less tilted. This indicates that the terraces may have been tilted and rotated by multiple faulting events. The height difference between the two adjacent terraces is about 1–2.7 m. The elevation differences between the lower original surface and the T3, T2, and T1 terraces were measured in 64 topographic profiles across the scarps. The results show that the elevation of T3 is highest, averaging 8.4 ± 1.9 m and 5.5 ± 1.1 m along the Suyukou and Hongguozi fault scarps, respectively. The elevations of T2 at the two sites are 5.8 ± 1.4 m and 3.4 ± 0.4 m, respectively, whereas elevations of T1 are 3.1 ± 1.2 m and 1.2 ± 0.3 m, respectively. Ages of terrace risers between T3 and T2, as well as between T2 and T1, are 3600–4600 years and 2000–2800 years, respectively, as calculated using a diffusion model of scarp development. The morphology of the fault scarps is complicated. Two or three bevels were developed on some fault scarps that were active in an earlier period, and the upper and the lower slope sections correspond to T3 and T2 alluvial terraces, respectively. A free face exists only on T1 terrace. The slope angles of the upper and lower slope sections of the scarp at Suyukou are 6–22° and 17–38°, respectively. The difference between the two in the same profiles is consistently in the range of 9–21°. In the place where the fault scarp is high, the free face, with a slope angle up to 60°, may be a result of the M = 8 earthquake of 1739. We infer that the other two bevels represent two paleoearthquake events. Eight and six trenches have been excavated across the fault scarps at Suyukou and Hongguozi, respectively. All the trenches reveal normal faults, multiple offsets, and colluvial and filled wedges along the scarps. In these trenches, we found colluvial wedges formed in four different periods indicating multiple faulting events. On the basis of this evidence, four Holocene seismic events have been identified. The timing of the events was determined through 14C dating samples collected from the trenches. On the basis of terrace offset, scarp morphology, and paleoseismic trenching, we conclude that the four large earthquakes along the fault occurred 8400, 4600–6300 (or 5700), 2600, and 256 years B.P. The latest event was the M = 8 Yinchuan‐Pingluo earthquake of 1739. The recurrence interval of these earthquakes is 2300–3000 years. We infer that the magnitudes of these events were probably ∼8, because the thickness of each colluvial wedge and the height of terrace risers produced by these events are similar to those produced by the 1739 earthquake.