  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.
