Earth Planets Space, Vol. 63 (No. 7), pp. 785-789, 2011
Luis Fabián Bonilla1, Kenichi Tsuda2, Nelson Pulido3, Julie Régnier4, and Aurore Laurendeau5
1Université Paris-Est, IFSTTAR, France
2Shimizu Corporation, Tokyo, Japan
3National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan
4CETE-Méditerranée, Nice, France
5ISTERRE, Grenoble, France
(Received April 7, 2011; Revised May 25, 2011; Accepted June 8, 2011; Online published September 27, 2011)
We analyzed the acceleration time histories recorded by the K-NET and KiK-net stations of the Mw 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 Vs30 < 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 Vs30 and shows that widespread nonlinear behavior took place during this large earthquake.
Key words: Site response, cyclic mobility, Tohoku Earthquake, K-NET, KiK-net.