Five Short Historical Earthquake Surface Ruptures near the Silk Road, Gansu Province, China

Five historical earthquakes (M >= 7) have occurred along the Hexi Corridor at the northeastern margin of the Tibetan Plateau since A.D. 180. These are the A.D. 180 Gaotai earthquake (M 7.5), the 1609 Hongyazi earthquake (M 7.25), the 1927 Gulang earthquake (M 8.0), the 1932 Changma earthquake (M 7.6), and the 1954 Shandan earthquake (M 7.3). They are predominantly reverse slip, except for the 1932 Changma and the 1954 Shandan earthquakes. The Changma earthquake is characterized by left-lateral faulting with a reverse component, while the Shandan earthquake by right-lateral faulting with a normal component. Field investigations indicate that the length of the surface-rupture zone is only 28 km for the A.D. 180 earthquake, 11 km for the 1609 earthquake, 23 km for the 1927 earthquake, and 5.1 km for the 1954 earthquake, much shorter than that predicted using an empirical equation between moment magnitude (M(w)), surface-rupture length (SRL), coseismic displacement (D), maximum displacement (MD), average displacement (AD), and mode value combined displacement statistic (MVCDS): M(w) = 5.91 + 0.609 log(SRL x D) for strike- slip faults, M(w) = 5.81 + 0.653 log(SRL x D) for reverse faults, and M(w) = 6.93 + 0.82 log(AD x MVCDS) for all faults. This may be common for reverse faulting earthquakes worldwide and suggests that trying to estimate magnitudes of past earthquakes from one type of even the most recent surface-rupture data is unreliable and the empirical equations between magnitude, coseismic slip, and surface-rupture length should be applied with caution to seismic hazard assessment on active thrust faults.