Improvement in the accuracy of expected seismic intensities for earthquake early warning in Japan using empirically estimated site amplification factors

Abstract

The Japan Meteorological Agency (JMA) is able to process data for its earthquake early warning (EEW) procedure very quickly by employing a seismic intensity expectation method that uses simplified strong motion characteristics. To represent one of these characteristics, the site amplification factor at the seismic intensity station, JMA uses ARV (amplitude ratio of peak ground velocity at the ground surface relative to the engineering bedrock of average S-wave velocity 700 m/s) based on topographic data. As a potential substitute for the ARV, we investigated a station correction based on empirical site amplifications obtained from recent observed seismic intensity data. When estimating this station correction, we removed an effect of the abnormal seismic intensity distribution of deep subduction-zone earthquakes and applied a correction for the earthquake source term to the attenuation relation. Station corrections were obtained for 1258 stations, representing about 30% of all current seismic intensity stations. The correlation between our station corrections at the stations and the site amplification based on topographic data is weak (correlation coefficient = 0.30). The error of the expected seismic intensity was improved by 15% by replacing ARV with the station correction.