The 1673 Tianzhen Earthquake and Rupture Behavior of the Yanggao-Tianzhen Fault in the Northern Shanxi Graben, China

Studying the causative structure, extent, and magnitude of historical earthquakes is essential for regional seismic hazard assessment. There has been some debate in the literature regarding the epicenter, magnitude, and seismogenic source of the 1673 Tianzhen earthquake, one of the few documented strong earthquakes that occurred in northern Shanxi Graben, China. The Yanggao-Tianzhen Fault (YTF) has long been recognized as a seismogenic fault; however, previous studies have suggested that the fault has not ruptured at the surface for several thousand years. In this work, we investigate fresh surface ruptures along the YTF based on field investigations, unmanned aerial vehicle photogrammetry, and ground-penetrating radar surveys. Scarp formation at Yanjiabu is bracketed to the last 1.4 +/- 0.6 kyr using optically stimulated luminescence dating. More importantly, we found that the Great Wall at Li'erkou (dated from 1546 CE) was likely faulted, which led us to conclude that the YTF is the causative fault of the 1673 event. Based on our measured fault parameters, the magnitude of the 1673 event was re-estimated to Mw 6.8-7.2, suggesting that previous magnitudes were underestimated. Four trenches were excavated to investigate paleoearthquakes on the fault. Together with previous paleoseismic data, we suggest a bimodal rupture model in which the fault can produce multi-segment rupturing events with large magnitudes, such as the 1673 event, and smaller partial or single-segment rupturing events. Our results shed light on the reassessment of seismic hazards in the densely populated Yangao-Tianzhen Basin and its nearby regions.