Field Validation of the First Recorded Historically Major (Mw >7) Earthquake in China Based on the Age of the Landslide-Dammed Lake

The first major (Mw>7) earthquake in the Chinese historical catalog is recorded to have occurred near Qishan, in Shaanxi Province, and is entered for the year 780 B.C., but there is limited field evidence of its effects. Previous satellite images and field surveys have shown that there is a cluster of ancient seismic landslides about 50 km long in the northern margin of the Qinling Mountains, south of Qishan, which is the macroepicenter of the 780 B.C. Qishan earthquake. However, the actual age of the landslide group is debated. To constrain this age, we examined a >1 m thick lacustrine deposit above the landslide gravel of a dammed lake impounded by the largest of several landslides near its inferred macroseismic epicenter and determined these were deposited 758–486 B.C. This date is sufficiently close to the catalog age that we infer that this landslide was triggered by the 780 B.C. historical event. This lends credibility to the historical account and resolves earlier speculation based on disputed dates of surface materials on the landslide. We also re-evaluated the magnitude of the 780 B.C. Qishan earthquake and found that it could plausibly be higher than Ms 7.8 (Mw 7.5). The possible seismogenic structure belongs to the Longxian–Qishan–Mazhao fault. This work updated the damage area of the Qishan earthquake and helped us revise the seismic parameters of the historical earthquake.