Spatial and temporal distributions of b-values related to long-term slow-slip and low-frequency earthquakes in the Bungo Channel and Hyuga-nada regions, Japan

Long-term slow-slip events (SSEs) and low-frequency earthquakes (LFEs) repeatedly occur in the area encompassing the Bungo Channel and Hyuga-nada region in Japan. SSEs are slow earthquake events with a duration of months to years, characterized by a type of slip that is considered transitional between the fast rupture of regular earthquakes and stable sliding along a plate interface, and have characteristics sensitive to stress states near the plate interface. In addition, since LFEs are also thought to occur at the plate interface, it would be useful to infer the stress state in this region. This study focuses on the stress-dependent characteristics of the b-values of regular earthquakes, SSEs, and LFEs, as well as the spatio-temporal relationships between b-values and these events in this area, using the Japan Unified high-resolution relocated Catalog for Earthquakes (JUICE). The b-value, which represents the relative proportion of large to small earthquakes, is inversely related to differential stress or effective stress. The b-values are moderate to low (b = 0.6-1.1) in regions with a high incidence of long-term SSEs. This spatial b-value anomaly is attributed to the partial release of accumulated stress in regions with a relatively high differential stress due to SSE activity. Furthermore, the b-value distributions estimated in this study provide constraints on the mechanisms that generate LFEs. On the other hand, the b-values increase and decrease before and during periods of enhanced LFE activity in the Bungo Channel, respectively. The increase and following decrease in b-values are thought to reflect episodes of low and high shear strength along the plate interface, which were caused by the presence of abundant slab-derived fluids under undrained conditions above the plate interface, and decreased pore pressure related to fluid depletion via LFE generation, respectively.