Jing Ci Neo, Yihe Huang, Dongdong Yao, Shengji Wei

The locations of aftershocks are often observed to be on the same fault plane as the mainshock and used as proxies for its rupture area. Recent developments in earthquake relocation techniques have led to great improvements in the accuracy of earthquake locations, offering an unprecedented opportunity to quantify both the aftershock distribution and mainshock rupture area. In this study, we design a consistent approach to calculate the area enclosed by aftershocks of Mw≥5.4 mainshocks in California, normalized by the mainshock rupture area derived from slip contours. We also investigate the Coulomb stress change from mainshock slip and compare it to the aftershock zone. We find that overall, the ratios of aftershock zone area to mainshock rupture area, hereinafter referred to as “aftershock ratio”, lie within a range of 0.5 to 5.4, with most values larger than 1. Using different slip inversion models for the same mainshock can have a large impact on the results, but the ratios estimated from both the relocated catalogs and Advanced National Seismic System (ANSS) catalog have similar patterns. The aftershock ratios based on relocated catalogs of Southern California fall between 0.5 and 4.3, while they exhibit a wider range from 1 to 5.4 for Northern California. Aftershock ratios for the early aftershock window (within 1 day) show a similar range but smaller values than using the entire aftershock duration, and we propose that continuing afterslip could contribute to the expanding aftershock zone area following several mainshocks. Our results show that areas with positive Coulomb stress change scale with aftershock zone areas, and spatial distribution of aftershocks represents stress release from mainshock rupture and continuing postseismic slip.