Earth Planets Space, Vol. 64 (No. 12), pp. 1259-1266, 2012
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)
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.