• early warning;
  • seismic intensity;
  • ground shaking

[1] The infamous Tokai Earthquake, which by some accounts is overdue, is expected to be a magnitude ≥ 8 event that will cause unprecedented damage in regions of Japan. To mitigate hazards from large earthquakes in Japan, an earthquake early warning (EEW) system was developed that is able to determine hypocentral locations from P-arrival data within a few seconds and then transmit this information before the onset of large ground motions from the later arrival of S-waves. We introduced a new source parameter for seismic intensity magnitude that can be estimated from the real-time P-wave data during the early stages of fault rupture for most earthquakes M ≥ 6.5. The use of this parameter results in a significant improvement in the uncertainty in the estimated seismic intensity compared to estimates derived from earthquake magnitude. A pre-established relation between the P- and S-wave seismic intensity therefore enables an EEW system to issue a rapid and reasonably reliable prediction of the amount of ground shaking that may be expected from the damaging S-waves.