摘要：We analyzed the acceleration time histories recorded by the K-NET and KiK-net stations of the M w 9 Tohoku Earthquake in order to investigate site response issues related to near-source effects.Time-frequency analysis of K-NET stations in the Miyagi prefecture, closest to the rupture area, show that sites having a V s 30 < 400 m/s present a combination of deamplification at frequencies higher than 5 to 10 Hz and cyclic mobility (high acceleration peaks riding over a low frequency carrier).This suggests strong nonlinear site response at these stations.Furthermore, using KiK-net data we are able to compute borehole transfer functions from the mainshock and events having small PGA values from the local dataset.The ratio between weak-motion and strong-motion borehole transfer functions constitutes an indicator of nonlinear site response.This ratio reveals strong dependence on V s 30 and shows that widespread nonlinear behavior took place during this large earthquake. Key words Site response cyclic mobility Tohoku Earthquake K-NET KiK-net.
