Immediacy and Accuracy of the Earthquake Early Warning Method Based on the P-wave Threshold Exceedance: Application to the 2016 Kumamoto Earthquake Sequence

Earthquake Early Warning (EEW) systems are essential for ensuring the safety of trains in Japanese railways during earthquakes. The conventional warning method takes over one second to issue alerts, which is often too long for events near the fault plane. We propose an on-site EEW method using the amplitude ratios of S-waves to P-waves, reflecting local ground characteristics. This method enables real-time prediction of S-waves through simple arithmetic, which does not depend on estimating epicentral distance. When applied to the main shock of the 2016 Kumamoto Earthquake, the method issued alerts within one second and accurate S-wave amplitude predictions, with a logarithmic standard deviation of 0.308. This demonstrates its effectiveness in ensuring railway safety.