Physical Mechanism for a Temporal Decrease of the Gutenberg-Richter b-Value Prior to a Large Earthquake

Abstract

Observations of seismicity prior to large earthquakes show that the slope of a Gutenberg-Richter magnitude-frequency relation, referred to as a b-value, sometimes decreases with time to the mainshock. Yet, underlying physical processes associated with the temporal change of a b-value remain unclear. Here we utilize continuum models of fully dynamic earthquake cycles with fault frictional heterogeneities and aim to simulate the temporal variation of a b-value. We first identify a parameter regime in which the model gives rise to an active and accelerating foreshock behavior prior to the mainshock. We then focus on the spatio-temporal pattern of the simulated foreshocks and analyze their statistics. We find that the b-value of simulated foreshocks decreases with time prior to the mainshock. A marked decrease in the resulting b-value occurs over the duration of less than a few percent of the mainshock recurrence interval, broadly consistent with foreshock behaviors and b-value changes as observed in nature and laboratory, rock-friction experiments. In this model, increased shear stresses on creeping (or velocity-strengthening) fault patches resulting from numerous foreshocks make these creeping patches more susceptible to future coseismic slip, increasing the likelihood of large ruptures and leading to a smaller b-value with time. This mechanism differs from a widely invoked idea that the decrease of a b-value is caused by a rapid increase in shear stress that promotes micro-crack growth, and offers a new interpretation of b-value changes prior to a large earthquake.