Site amplification factors of whole Japan area estimated from spectral ratio of direct S-wave and their application to the real-time prediction of ground motion in Earthquake Early Warning

Abstract:

For applying the real-time prediction of ground motion proposed by Hoshiba (2013a, JGR) to Earthquake Early Warning, it is necessary to correct a site amplification factor in an observed waveform. In this study, we aim to estimate site amplification factors at whole area of Japan, and apply the real-time correction proposed by Hoshiba (2013b, BSSA) of site amplification factors to investigate their validity.

 To estimate site amplification factors, we used the spectral ratio of direct S-wave at two adjunct stations. We constructed a network with many pairs of stations, then solved the equations of the network in a least square sense. As a result, we successfully estimated site amplification factors almost whole of the Japan area, except a part of Hokkaido and Kyushu region, and Islands area.

 Next, we applied the real-time correction of site amplification factors in the observed waveforms of the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0). Distribution of site-corrected seismic intensity calculated in time domain (Kunugi et al., 2008) showed clear distance-dependent relation of seismic intensity, which was not found in the distribution of non-corrected seismic intensity.

 Finally, we compared the two waveforms recorded in the Ishikari Plain, Hokkaido region, Japan, with correction of site amplification factors. The features of waveform in one station was well reproduced from the waveform of other station with the correction of site amplification factor.

 Although there are some subjects, e.g. nonlinear behavior of the ground with strong ground motion and azimuth dependency of site amplification factors which are not considered in this study, estimated site amplification factors in this study is effective in real-time prediction of ground motion.