How precisely can we anticipate seismic intensities? A study of uncertainty of anticipated seismic intensities for the Earthquake Early Warning method in Japan

The precise calculation of anticipated seismic intensity is an important component of Earthquake Early Warning (EEW) procedures. The EEW method adopted by the Japan Meteorological Agency (JMA) uses event magnitude, hypocentral distance, and site amplification factor for this calculation, in which the site amplification factor is represented by a single scalar without consideration of spectrum contents. Even when two earthquakes occur at the same location with the same magnitude, their observed distributions of seismic intensity are not always the same. And even at adjacent measurement stations, the interstation difference in seismic intensity of one earthquake is not always the same as that of another earthquake. To evaluate these expected uncertainties in the current JMA EEW method, we analyzed the distribution of recorded seismic intensities from adjacent earthquakes and also compared the intensities at adjacent observation sites. The uncertainties are 0.29 JMA intensity units when the JMA magnitude is used as an index of source factor and 0.22 when the average of the observed seismic intensities is used. The uncertainties are 0.21 when site amplification factor is represented by single scalar value. These results may indicate the intrinsic precision limits of anticipated seismic intensities in the current JMA EEW method.