Estimation of the frequency-dependent site amplification factors of Japan based on the coda normalization method

We estimated the frequency dependent properties of the site amplification factors at each station of the K-NET and KiK-net strong motion network across Japan based on the coda normalization method. Using a large number of waveform data of 3,004 acceleration record from 48 earthquakes and waveform record from 1800 strong motion stations, we estimated the site amplification factors at each frequency band from f=0.5-1 Hz, 1-2 Hz, 2-4 Hz, to 4-8 Hz. In this study we assumed a rock-site station (Tashiro at Kyushu) as a reference of the site amplification estimates. The estimated site amplification factors show strong variations in the low frequency band (0.5-1 Hz) as compare with that of the high frequency band (4-8 Hz). Large site amplification factor in the low-frequency band is found in major populated cities such as Tokyo, Osaka, and Nagoya with varying amplification factors between 10 to 20 dB relative to the reference rock-site station. Such distribution of the site amplification factors in the low-frequency (0.5-1 Hz) band is in good corresponding to the depth distribution of a basement rocks with S-wave speed greater than 2.9 km/s. On the other hand the site-amplification factors in high-frequency (4-8 Hz) band are rather small (5 to 10 dB) as compared with that of the low-frequency band and spatial distribution is almost randomly distributed irrespective to surface geology. In order to examine the reliability of our estimates of the site amplification factors based on the coda-normalization method and to examine how the site amplification modify the distribution of seismic intensity distributions for large earthquakes, we reproduced intensity distributions of recent Japanese M7 earthquakes such as the 2004 Niigata Chuetsu (M6.8), the 2005 Western Fukuoka (M7.0), and the 2008 Coast of Iwate (M6.8) earthquakes by removing the site amplification factors of each frequency band from the acceleration waveform record at each K-NET and the KiK-net station. The site corrected seismic intensity distribution shows almost isoseismal intensity contours with decreasing intensity as hypocentral distance increase. It is confirmed large deviation of intensity from the average at each hypocentral distance is minimized significantly after the correction of site amplification factors. This result promising the effectiveness of our site effects estimates such as for predicting strong ground motions at each station expecting for future earthquake. However, the site corrected intensity pattern of the 2008 Coast of Iwate earthquake, which was a deep (H=108 km) inslab event, still shows distorted pattern of intensity with large deviation from empirical attenuation function. This is due to the anomalous wave propagation in northern Japan from deep earthquake with efficient high-frequency wave propagation in the Pacific Ocean side of northern Japan where high-Q Pacific plate subducts beneath continental plate and strong attenuation of seismic wave in the Japan Sea side.