Active normal faulting and the seismogenic fault of the 1739 M ∼8.0 Pingluo earthquake in the intracontinental Yinchuan Graben, China

Abstract

The NNE-trending Yinchuan Graben is one of several intracontinental grabens that have developed in the extensional environment around the Ordos Block, northern-central China, and is bounded by active normal faults at both its eastern and western margins. In this study, we present new evidence for the Holocene activity and paleoseismicity of the active normal faults developed in the Yinchuan Graben. Interpretations of high-resolution WorldView and Google images, field investigations, trench excavations, seismic data, and radiocarbon dating results reveal the following: (i) two main active fault zones, the Helanshan Piedmont Fault Zone (HPFZ) along the western margin of the graben and the Huanghe Fault (HHF) along the eastern margin, are characterised by conspicuous fault scarps developed on both Holocene alluvial fans and terrace risers; (ii) the active faults are dominated by normal faulting; (iii) the Holocene normal slip rate is estimated to be ∼2–3mm/yr, and the recurrence interval of morphogenic earthquakes is estimated to be ∼1500–2000yr for both the HPFZ and HHF; and (iv) the HHF is the most likely seismogenic fault for the triggering of the 1739 M ∼8.0 Pingluo earthquake. Our results show that the HPFZ has the potential to produce a destructive earthquake in the near future, because no large earthquake has occurred on this fault during the past ∼1500years. This contrasts with previous findings that the fault scarps of the HPFZ were caused by the 1739 Pingluo earthquake and that therefore the likelihood of a large and destructive earthquake on that fault in the near future is relatively small because of the <300years elapsed since the earthquake. Therefore, it is necessary to reconstruct the model of faulting, tectonic activity, and paleoseismicity of the intracontinental graben and to reassess the seismic hazard of the active normal faults for the densely populated Yinchuan region.