A Transtensional Fault Zone on the Northeastern Margin of the Tibetan Plateau: Millennial Earthquake Recurrence and High Earthquake Hazard

The seismic gaps between ruptures of large earthquakes in tectonically active deformation zones are usually considered to be dangerous areas for future earthquakes. The Western Tianzhu Basin fault (WTBF) with an oblique normal motion is located in the middle of the Tianzhu seismic gap between the M 8.5 Haiyuan and M 8.0 Gulang earthquakes, NE Tibetan Plateau. The faulting activity of the WTBF is key to understanding the seismic hazards of the seismic gap, but it remains poorly constrained. Using satellite images, field investigations, paleoseismic trenching, and radiocarbon dating, we found that the WTBF produced seven paleoearthquakes since similar to 8,000 cal. yr BP, with the latest event occurring at 1,005 +/- 584 cal. yr BP, yielding an average recurrence interval of 1,102 +/- 100 yr and a coefficient of variation of 0.38, indicating that it features a millennial quasi-periodic recurrence. Based on the comparative analysis of geometry, kinematics, and tectonic activity, we suggest that the Tianzhu seismic gap can be divided into three sections and is characterized by a segmented rupture pattern. Among them, the WTBF and Jinqianghe fault exhibit similar geometry and kinematics and together form the similar to 55 km-long Jinqianghe-Tianzhu transtensional fault zone (JTTFZ) that is capable of producing earthquakes of Mw 7.2. Given the long elapsed time since the latest event and strong seismogenic potential, it is believed that the JTTFZ poses a high seismic hazard. Our results enhance the understanding of the high seismic hazard in seismic gaps and provide new insights into the seismic rupture behavior in the NE Tibetan Plateau. The Western Tianzhu Basin fault (WTBF) features oblique normal motion and has regularly produced seven paleoearthquakes since similar to 8,000 cal. yr BP The seismic gap between the 1920 and 1927 earthquakes is characterized by a complex geometry and a segmented rupture pattern The WTBF and Jinqianghe fault together form a transtensional fault zone with a high seismic hazard