Investigation of On-Site Earthquake Early Warning for the Threshold of P-Wave, using an Amplitude Ratio of S-Wave to P-Wave

In a railway field of Japan, seismic stations have been installed at intervals of 10km to dozens of kilometers along railways/coastline and on mountains. As earthquake early warning (EEW) for railways, the two types of EEW which are threshold of earthquake ground motions and prediction of earthquake ground motions using initial P-waves, are used. The on-site earthquake early warning for the threshold of earthquake ground motions are generally applied to S-wave dominated in seismic waves; therefore, the timing of issuing EEW is not enough rapid to stop traveling trains during earthquakes.

To issue the earthquake early warning for the threshold of earthquake ground motions earlier, in this study, we investigated the applicability of earthquake early warning for the threshold of P-waves in seismic stations, using amplitude ratios of S-waves to P-waves at a sediment site and a rock site. At first, we theoretically led that the amplitude ratios of S-waves to P-waves were affected by the site effects, the pass effects and source effects. Especially, the site effects due to both subsurface P-wave and S-wave velocity structures affected largely than the pass effects and source effects. As the results of PGA/PGV and frequency contents, S-waves predicted by P-waves using the amplitude ratios of S-waves to P-waves at each seismic station were in good agreements with observations. Also, we could stably extract the amplitude ratios of S-waves to P-waves, using seismic data more than 20. Finally, we concluded that this simple method can potentially issue the earthquake early warning earlier than the previous method by the time of Ts-p.