The CE 1303 Hongdong Earthquake and the Huoshan Piedmont Fault, Shanxi Graben: Implications for Magnitude Limits of Normal Fault Earthquakes

The CE 1303 Hongdong earthquake, with ~270,000 deaths, has been suggested as the first magnitude 8 earthquake recorded in North China. We use 31 AMS‐14C ages to date earthquakes recorded in the Huoshan Piedmont Fault, which is interpreted to be the causative fault for the event. We interpret fault traces and map the Hongdong earthquake surface rupture in detail. Four events are identified with timings constrained at 1,060–590 year BP (corresponding to the Common Era 1303 event), 3,310–3,210 year BP, 5,460–5,380 year BP, and 26,380 year BP, respectively. The later three events have a recurrence interval of ~2,000–3,000 years. We find that the Hongdong earthquake had a rupture length of ~98 km, a maximum throw of 5.0 m and a best estimate for magnitude in the range Mw 7.2–7.6. We suggest that previous magnitude estimates are overestimates. Normal fault earthquakes have smaller upper limits to their magnitudes than thrusts, because their relatively steep dips produce intersections at the base of the seismogenic layer at smaller downdip widths than gently dipping thrust faults. The Hongdong event was not an exception to this pattern. The historic released seismic moment may be closer to the accumulated moment than previously calculated. Plain Language Summary The Common Era 1303 Hongdong earthquake with ~270,000 deaths suggested as the first M 8 in North China. We use 31 AMS‐14C ages to date earthquakes recorded in the Huoshan Piedmont Fault, which is interpreted to be the causative fault for the event. We interpret fault traces and map the Hongdong earthquake rupture in detail. Four events are identified with timings constrained at 1,060–590 year BP (corresponding to the Common Era 1303 event), 3,310–3,210 year BP, 5,460–5,380 year BP, and 26,380 year BP, respectively. The later three events have a recurrence interval of ~2,000–3,000 years. We find that the earthquake had a rupture length of ~98 km, and a maximum throw of 5.0 m, and a best estimate for magnitude in the range Mw 7.2–7.6. We suggest that previous magnitude estimates are overestimates. Normal fault earthquakes have smaller upper limits to their magnitudes than thrusts, because their relatively steep dips produce intersections at the base of the seismogenic layer at smaller downdip widths than gently dipping thrust faults. The Hongdong event was not an exception to this pattern. The historic released seismic moment may be closer to the accumulated moment than previously calculated. Key Points The