Fault zone scarp area defines earthquake magnitude

Earthquake magnitude is critical for both seismic and tsunami hazard assessments, as well as early warning. However, because modern seismological observations are limited in duration, the magnitudes of many past events were not recorded or are poorly constrained over hundreds and/or thousands of years of the earthquake cycle. This gap limits our understanding of long-term seismicity and hampers effective hazard mitigation. Here, we propose a novel approach to estimate earthquake magnitude using fault zone scarp area. Theoretical considerations suggest that coseismic scarp area scales with moment magnitude (Mw) according to a power-law relationship for both near-surface and blind faults. For near-surface faults, this relationship is calibrated using a global dataset of 16 modern earthquakes and six paleoearthquakes, including events on high-angle normal faults and low-angle thrusts, and provides a good fit to the global dataset. For blind fault structures, the relationship is validated by comparing pre- and post-earthquake surface deformation associated with the 2011 Tohoku-oki earthquake along the Japan Trench offshore northeast Japan. Applying this theoretical scaling law to a blind fault structure in the coastal zone of south China, we reconstruct five prehistoric strong-large earthquakes (Mw 5.7−7.8) on the Littoral Fault Zone using high-resolution shallow seismic-reflection data collected offshore the Greater Bay Area, China. Our approach could be applied wherever fault scarp area can be measured, both on land and offshore, enabling the extension of earthquake catalogs beyond the limits of modern records. By filling gaps in seismic records, this method improves seismic and tsunami hazard assessments, refines recurrence rates and worst-case scenarios, and supports more effective preparedness and mitigation.