Temporal Evolution of b-values in Rate-and-State Fault Models with Frictional Heterogeneities

Understanding the mechanism of foreshock generation is an important issue for earthquake forecasting as well as our fundamental understanding of earthquake physics. Observations of seismicity prior to large earthquakes show that a slope of Gutenberg-Richter magnitude-frequency relations, known as b-value, often decreases with time before the occurrence of a large earthquake. For example, Nanjo et al. (2012) reported that b-values in the hypocentral region of the 2011 Mw9.0 Tohoku-Oki earthquake had decreased prior to the mainshock. Yet, robustly interpreting observed temporal changes of b-values remains challenging. Here we use numerical simulations of earthquake cycles with frictional heterogeneities and attempt to simulate the temporal variations of b-value. We first identify a parameter regime in which the model gives rise to an accelerating foreshock behavior prior to the onset of a mainshock. We then analyze the resulting foreshock statistics. We find that the model results in foreshock activities characterized by a decrease of b-value prior to the onset of a mainshock. Our results also suggest that frictional heterogeneities could significantly affect foreshock activities. We will report our current efforts on identifying key physical parameters controlling the statistics of simulated foreshock activities.