Re-Visiting the 1739 Earthquake Ruptures in the Yinchuan Graben, Northern China
Abstract:A large, normal-faulting earthquake occurred on 3rd January 1739 on the eastern flank of the Helan Mountains. These mountains border the Yinchuan Graben, a major structural feature on the north-western side of the Ordos Plateau in northern China. Surface ruptures from this event extend discontinuously for a total of 88 kilometers with a maximum vertical component of slip of 4.4 meters. Greatest damage was sustained in the cities of Yinchuan and Pingluo and an estimated 50,000 people were killed.
Fault scarps have previously been identified in late Quaternary alluvial fans at Suyukou, Xiaoshigou, Jianquan and Hongguozigou, where it is believed that the Great Wall of China was offset. The Suyukou scarps are particularly impressive, preserving a free face along most of their length. Compound scarps and a series of terrace surfaces also document a history of at least three faulting events prior to 1739. These scarps provide a unique opportunity not only to characterise a large-magnitude (approximately Mw 7.5) continental normal-faulting earthquake, but also to determine a recurrence interval for this type of event.
In this study we combine remote sensing techniques, field observations and paleoseismological work in order to tackle these questions. We have produced a nominally 1 meter resolution digital elevation model of the entire length of the rupture using Pleiades stereo imagery. This detailed topography has allowed us to map the 1739 rupture, its associated fissures, and the footwall terrace surfaces along the Suyukou scarps in unprecedented detail. We have used a series of topographic profiles across the scarps to measure vertical offsets and the shape of the rupture front. This data is then used to reconstruct the slip variation along strike in the 1739 earthquake and compared with the offsets from older events. These offset measurements are supplemented by our field samples from the Suyukou scarps. Quaternary geochronology—including radiocarbon, optically stimulated luminescence, and 10Be exposure dating—will be used, along with previous paleoseismological work, to constrain the timing of the recent earthquakes on the fault.