Temporal Change in Site Response Caused by Earthquake Strong Motion as Revealed from Coda Spectral Ratio Measurement

Strong earthquake shock often decreases shear modulus and increases attenuation coefficient of the ground at relatively weak sites on, for example, sedimentary layers or weathered rocks. Since spectra of coda waves of a local earthquake are independent of travel distances and the focal mechanism, relative site amplification factors at spatially separated stations have been estimated from the spectral ratio of coda waves. Applying this method to coda waves registered by vertically separated seismometers at a borehole site, we propose a method to measure the site response from the coda spectral ratio of the surface data to that of the downhole data. Using data recorded by a Japanese strong motion observation network, KiK-net, we calculate the spectral ratio at two sites (TTRH02, SMNH01), which experienced strong motion of the 2000 Western Tottori Earthquake (MW6.7). We further analyze the data at a site (IBUH03) of KiK-net, which experienced that of the 2003 Tokachi-Oki Earthquake (MW8.3). Acceleration seismometers at these sites are installed on the ground surface and at the bottom of a borehole, of which the depth is from 100 m to 150 m at each site. The maximum horizontal accelerations recorded at the sites TTRH02, SMNH01, and IBUH03 were 1109, 844, and 377 gal, respectively, for the mainshock. The spectral ratios at these stations showed remarkable temporal changes of peak frequency. The reduction rate of the peak frequency for direct S-wave of the mainshock to that of earthquakes occurring before the mainshock reached 30-70% at all the sites. Since then, the peak frequency has been logarithmically recovering to the value before the strong motion for a few years at TTRH02 and SMNH01. PS well-logging data indicate that these two sites consist of weathered and solid rocks. On the other hand, IBUH03, which is located on sandy gravel, showed short-term recovery within a few tens of minutes. At the site TTRH02, we further find clear difference in the spectral ratios of EW and NS components. The peak frequency for NS component was 70% of that for EW component just after the strong motion. The difference has gradually decreased and almost disappeared a few years after the mainshock.