Paleoseismology of the Anqiu–Juxian Fault, Tanlu Fault Zone, Reveals Evidence That It Is the Causative Fault of the 70 B.C. Anqiu Earthquake

Abstract The timing and surface rupture length of large earthquakes are key parameters for seismic hazard studies in the Tanlu fault zone (TLFZ). Existing studies suggest that the M 81/2 Tancheng earthquake in A.D. 1668 may have been generated by the cascading rupture of the Juxian–Tancheng fault (JTF) and the Anqiu–Juxian fault (AJF) in the TLFZ. The Anqiu earthquake in 70 B.C. near the AJF also shook eastern China; however, the latest surface rupturing event along the AJF has not been studied, thus the potential earthquake hazard has large uncertainties. In this study, we excavated three trenches along the AJF to determine the most recent surface rupturing events to address these problems. As evidenced by paleoseismic and chronological investigations, the latest seismic events occurred along the northern segment (S1) and middle segment (S2) at approximately A.D. 63–225 and 2148–48 B.C., respectively. Combined with previous studies and historical records, our study inferred that the Anqiu earthquake in 70 B.C. was generated by S1 and S2, with an estimated magnitude of Mw 7.5 ± 0.2. The most recent event on S3 occurred just before 10297 ± 53 yr B.P. Our study also revealed that the Tancheng M 81/2 earthquake was generated by the JTF alone without surface rupture of the AJF. Thus, the elapsed time of the latest large earthquake along the AJF is more than 2000 yr.