New Japanese Guidelines for the Information of the Prospect of Seismic Activity After Large Earthquakes and Their Applications

The Kumamoto Prefecture suffered an earthquake of M JMA 6.5 on April 14, 2016 at 21:26 (Japan Standard Time). A seismic intensity of 7, on the Japan Meteorological Agency (JMA) seismic intensity scale, was observed, which, by definition, is the maximum possible value. After 18 hours of the earthquake, the JMA issued a prospect for aftershock activity, where the probability of aftershocks with a seismic intensity of 6 Lower (6-) or greater, was 20% within three days following 16:00 JST on April 15, 2016. Ten hours post the issuance of the prospect, at 01:25 on April 16, a larger earthquake of M JMA 7.3, with a maximum JMA seismic intensity of 7, occurred in the same region as the M JMA 6.5 event, triggering many distant earthquakes. As this seismic occurrence did not follow a mainshock-aftershock sequence, the JMA discontinued the issuance of prospective aftershock activity. With lessons learned from this occurrence sequence, the Earthquake Research Committee of Japan (ERC), including JMA, seismologists and social scientists, have formulated new guidelines for the assessment of successive seismic activity, in order to enhance the understanding of strong ground motions after large earthquakes. The five main points of the guidelines are as follows: (1) alert to a similar strong ground motion, (2) highlighting previous examples of successive large events, (3) consideration of all active source faults, (4) quantitative forecasting of aftershocks a week after the event, and (5) not using the term “aftershock” in information issued by the JMA for disaster prevention. The JMA has commenced the implementation of these new guidelines, effective August 2016.