17:15 〜 19:15
[SCG45-P45] A temporal decrease in Gutenberg-Richter b-values on heterogeneous velocity-weakening faults and its physical mechanism
Understanding the mechanisms behind foreshock generation is crucial for improving earthquake forecasting and advancing our knowledge of earthquake physics. Observations of seismic activity preceding large earthquakes sometimes reveal a decrease in the slope of the Gutenberg-Richter magnitude-frequency relationship, known as the b-value. For example, Nanjo et al. (2012) reported a reduction in b-values in the hypocentral region before the 2011 Mw 9.0 Tohoku-Oki earthquake. However, interpreting temporal changes in b-values remains challenging. In this study, we use fully dynamic simulations of earthquake cycles on heterogeneous, velocity-weakening faults to investigate potential b-value changes. We first identify a parameter regime that induces accelerating foreshock activity before a mainshock. Our analysis shows that models incorporating heterogeneities in the characteristic slip-weakening distance (Dc) on a velocity-weakening fault produce foreshock sequences characterized by a decreasing b-value. This behavior arises because increased shear stresses on slowly slipping, large-Dc fault patches make these patches more susceptible to coseismic slip, increasing the likelihood of larger ruptures. This mechanism is qualitatively consistent with the numerical results of Ito and Kaneko (2023), which are based on a fault model consisting of velocity-weakening and velocity-strengthening fault patches. Additionally, we identify a range of normal stress heterogeneities on a velocity-weakening fault where foreshocks also exhibit a temporal reduction in b-values leading up to a mainshock. Our results suggest that temporal b-value changes can emerge on heterogeneous velocity-weakening faults even in the absence of velocity-strengthening patches.