Axial Seamount: Periodic tidal loading reveals stress dependence of the earthquake size distribution (b value)

Abstract: Earthquake size-frequency distributions commonly follow a power law, with the b value often used to quantify the relative proportion of small and large events. Laboratory experiments have found that the b value of microfractures decreases with increasing stress. Studies have inferred that this relationship also holds for earthquakes based on observations of earthquake b values varying systematically with faulting style, depth, and for subduction zone earthquakes, plate age. However, these studies are limited by small sample sizes despite aggregating events over large regions, which precludes the ability to control for other variables that might also affect earthquake b values such as rock heterogeneity and fault roughness. Our natural experiment in a unique seafloor laboratory on Axial Seamount involves analyzing the size-frequency distribution of ~60,000 microearthquakes which delineate a ring-fault system in a 25 km3 block of crust that experiences periodic tidal loading of +/-20 kPa. We find that above a threshold stress amplitude, b value is inversely correlated with tidal stress. The earthquake b value varies by ~0.09 per kPa change in Coulomb stress. Our results support the potential use of b values to estimate small stress variations in the Earth's crust.