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

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. Plain Language Summary Foreshocks are earthquakes that occur before a large earthquake (i.e., a mainshock) and are related to the mainshock in both time and space, but the mechanism of foreshock generation is not well understood. Analysis of foreshock statistics for many large earthquakes showed that larger foreshocks tend to occur more frequently in the time leading to the mainshock, a phenomenon known as the temporal decrease of a Gutenberg‐Richter b‐value. For example, the temporal decrease of a b‐value in the hypocentral region of the 2011 Mw 9.0 Tohoku‐Oki earthquake was observed prior to the mainshock, indicating the potential usefulness of b‐values for earthquake forecasting. However, the physical mechanism behind such observations is still unclear. Here, we develop a physics‐based model of fault slip evolution and successfully simulate a temporal decrease in the b‐value of foreshocks. In