Real-time Application of Earthquake Early Warning Considering Effects of Near-field Terms

This research analyzes strong motion records of 24 large earthquakes to investigate the relationship between τc (the average period of the first motion) and moment magnitude. The records of large earthquakes very close to the source include large long-period near-field terms. Therefore, if we do not consider the effect of the near-field term, we may overestimate the final size of the magnitude. We processed the records which do not have strong near-field terms, and compute the τc for each event. Our analysis shows the value of τc takes between the corner frequency and the period determined by the record duration to compute τc. If the magnitude is less than 6, τc becomes close to the period corresponding to the corner frequency, whereas τc for larger earthquakes depends on the rupture process and location of area of large slip. τc for large earthquakes regulates the lower bound of the magnitude estimation, and it approaches the period corresponding to the corner frequency, if a longer record duration is used. We also propose a method to classify the records with and without large near-field terms. If the Pd3 (peak displacement of the first 3 seconds) exceeds 1cm and τc > 2 seconds, the record is more likely to contain a large near-field term. For the purpose of quick onsite warnings, stations observing large near-field terms provide valuable information. Large near-field terms can be observed only if the magnitude is large and the epicentral distance is small. Therefore, if the displacement exceeds a threshold (e.g. Pd = 0.5cm), the ground motion at the site would likely become very large. This criterion will help to issue warnings to the blind zone that is close to the source. We incorporated this algorithm into the Seismic Automatic Triggering and Recording Network (SATARN) system operated by the Research Center for Earthquake Prediction, Kyoto University. We constructed a prototype system using the data recorded by stations around Kyoto city. Event triggers are picked by using ratios of the short-term and long-term averages, and then the minimum AIC is computed to search for the onset of the P-arrival. Currently, near- field terms and values of tauc are monitored, which may provide quicker warnings for large nearby earthquakes.