Temporal seismicity-rate changes on the slow slip region of Nankai trough and implications for large earthquake occurrence

The prediction of impending megathrust earthquake is high priority for seismic hazard mitigation, which has been unsuccessful despite rigorous efforts. The seismicity rate is dependent on the stress field as well as medium properties. The status of stress accumulation on interplate boundary is inferred from spatiotemporal seismicity-rate changes (b values) on the plate interface. The Nankai trough is recognized to have high seismic hazard potentals. The magnitude 8-class large earthquakes occur periodically in the Nankai trough with recurrence intervals of 100-200 years. We investigate the spatiotemporal variation of the seismicity-rate changes in the slow slip region along Nankai trough for 38 years. It is observed that the b values were generally bounded between 0.6 and 1.0. The b values are controlled by the relative composition between small and large earthquakes. The b values increase abruptly during slow slips. The b values increase concurrently at the times of slow slips. The b decay rates after slow slips are observed constant. These observations may be associated with the stress loading process on the slab interface. The consistent b-value decrease rates since 2014 may suggest approaching to large earthquakes.