Short-Term Forecasting of Aftershock Ground-Motion Hazards Within a Few Hours After a Large Earthquake in Japan

A large earthquake is typically followed by numerous aftershocks, which generate further ground shaking that can cause additional damage in disaster-stricken areas. Therefore, the early estimation of aftershock seismic hazards after the initial event is crucial for saving lives and resources. This study proposes a scheme for forecasting the short-term hazard of ground motions from aftershocks in Japan using event catalog data from the first few hours after an initial event. This scheme was validated on data from four large crustal earthquakes, including the largest foreshock and mainshock (MS) of the 2016 Kumamoto earthquake sequence. We found that the forecasts roughly reproduced the observed trends, demonstrating the usefulness of this scheme. Our findings revealed that the seismic hazard immediately after the 2016 Kumamoto largest foreshock was significantly higher than that of the MS, although the largest foreshock was approximately one order of magnitude smaller. Our study highlights the importance of hazard assessments based on actual trends in aftershock activity and provides a useful framework for rapid short-term forecasts.