Evaluation of the real-time earthquake information system in Japan

The real‐time earthquake information system (REIS) of the Japanese seismic network is developed for automatically determining earthquake parameters within a few seconds after the P‐waves arrive at the closest stations using both the P‐wave arrival times and the timing data that P‐waves have not yet arrived at other stations. REIS results play a fundamental role in the real‐time information for earthquake early warning in Japan. We show the rapidity and accuracy of REIS from the analysis of 4,050 earthquakes in three years since 2005; 44 percent of the first reports are issued within 5 seconds after the first P‐wave arrival and 80 percent of the events have a difference in epicenter distance less than 20 km relative to manually determined locations. We compared the formal catalog to the estimated magnitude from the real‐time analysis and found that 94 percent of the events had a magnitude difference of ±1.0 unit.