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Earth Planets Space, Vol. 64 (No. 12), pp. 1259-1266, 2012
doi:10.5047/eps.2012.05.012

Long-term change of site response after the Mw 9.0 Tohoku earthquake in Japan

Chunquan Wu1 and Zhigang Peng2

1Geophysics Group (EES-17), Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A.
2School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, U.S.A.

(Received December 27, 2011; Revised May 6, 2012; Accepted May 17, 2012; Online published January 28, 2013)

Abstract: The recent Mw 9.0 off the Pacific coast of Tohoku earthquake is the largest recorded earthquake in Japan's history. The Tohoku main shock and its aftershocks generated widespread strong shakings as large as ∼3000 Gal along the east coast of Japan. Wu and Peng (2011) found clear drop of resonant frequency of up to 70% during the Tohoku main shock at 6 sites and correlation of resonance (peak) frequency and peak ground acceleration (PGA) during the main shock. Here we follow that study and systematically analyze long-term changes of material properties in the shallow crust from one year before to 5 months after the Tohoku main shock, using seismic data recorded by the Japanese Strong Motion Network KiK-Net. We use sliding window spectral ratios computed from a pair of surface and borehole stations to track the temporal changes in the site response of 6 sites. Our results show two stages of logarithmic recovery after a sharp drop of resonance frequency during the Tohoku main shock. The first stage is a rapid recovery within several hundred seconds to several hours, and the second stage is a slow recovery of more than five months. We also investigate whether the damage caused by the Tohoku main shock could make the near surface layers more susceptible to further damages, but we do not observe clear changes in susceptibility to further damage before and after the Tohoku main shock.
Key words: Tohoku earthquake, earthquake ground motion, site effects, wave propagation, soil nonlinearity, KiK-Net.


Corresponding author E-mail: cwu@lanl.gov


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