Temporal changes of site response during the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2011
Received: 15 April 2011
Accepted: 8 June 2011
Published: 27 September 2011
The recent Mw 9.0 off the Pacific coast of Tohoku Earthquake and its aftershocks generated widespread strong shakings as large as ~3000 Gal along the east coast of Japan. Here we systematically analyze temporal changes of material properties and nonlinear site response in the shallow crust associated with the Tohoku main shock, using seismic data recorded by the Japanese Strong Motion Network KiK-Net. We compute the spectral ratios of windowed records from a pair of surface and borehole stations, and then use the sliding-window spectral ratios to track the temporal changes in the site response of various sites at different levels of peak ground acceleration (PGA). Our preliminary results show clear drop of resonant frequency of up to 70% during the Tohoku main shock at 6 sites with PGA from 600 to 1300 Gal. In the site MYGH04 where two distinct groups of strong ground motions were recorded, the resonant frequency briefly recovers in between, and then followed by an apparent logarithmic recovery. We investigate the percentage drop of peak frequency and peak spectral ratio during the Tohoku main shock at different PGA levels, and find that at most sites they are correlated.
Key wordsTohoku Earthquake earthquake ground motion site effects wave propagation soil nonlinearity KiK-Net
It is well known that local site conditions have significant effects on the strong ground motions generated during large earthquakes (e.g., Joyner et al., 1976; Chin and Aki, 1991; Yu et al., 1992). When the amplitude of ground motion exceeds a certain threshold, the sediment response deviates from the linear Hooke’s law, resulting in nonlinear site effects (e.g., Beresnev and Wen, 1996a). The recent Mw 9.0 off the Pacific coast of Tohoku Earthquake on 03/11/2011 is the largest earthquake in Japan’s long earthquake recording history. This great earthquake is recorded by ~1200 K-NET/KiK-NET strong motion seismic stations with peak ground acceleration (PGA) at as high as ~3000 Gal (Aoi et al., 2011). In addition, the long-duration (~200 s) of the Tohoku Earthquake (Hayes et al., 2011) generate a wide range of ground motions between less than 10 Gal to more than 1000 Gal from the direct waves, followed by long coda waves. This provides an unprecedented dataset to quantify the degrees of nonlinear site response and the temporal changes over a wide range of PGA levels.
Previous measurements on nonlinear site response were mostly from PGAs in the range of a few tens of Gal to a few hundred Gal (e.g., Sawazaki et al, 2006; Wu et al, 2009a; Rubinstein, 2011). Their results are generally characterized by sharp reductions of the peak frequencies and peak spectral ratios during the large PGAs, followed by logarithmic recovery. In this short note we present preliminary results from temporal changes of site response associated with a wide range of PGAs during the Tohoku main shock.
2. Data and Analysis Procedure
2.1 Seismic data
The analysis employs strong motion data recorded by the Japanese Digital Strong-Motion Seismograph Network KiK-Net operated by National Research Institute for Earth Science and Disaster Prevention (Aoi et al., 2000). The network consists of 659 stations with an uphole/downhole pair of strong-motion seismometers. Each KiK-Net unit consists of three-component accelerometers and a data logger having a 24 bit analog-to-digital converter with a sampling frequency of 100 Hz. Additional details on the network and site conditions can be found at the KiK-Net website (http://www.kik.bosai.go.jp).
In this short note we analyze data recorded by six stations (FKSH10, IBRH12, IBRH15, IBRH16, MYGH04, TCGH16). The six stations generally feature strong velocity contrast between the surface soil layers and the underlying bedrocks. The soil layers at the top several tens of meters generally consist of clay, sandy clay, filling, and gravel with very low S-wave velocities of ~100–200 m/s. The bedrocks are typically conglomerate, argillite, and shale with S-wave velocities of ~700–3000 m/s. These stations are chosen mainly because the PGAs recorded are well above the common soil nonlinearity threshold of ~200 Gal indicated by previous studies (Chin and Aki, 1991; Beresnev and Wen, 1996b), due to their relatively close distances to the epicenter of the Tohoku Earthquake. We also find that the observed temporal changes in peak frequencies and peak spectral ratios (maxima of the spectral ratios) at these stations are much clearer than those at other stations, allowing us to better quantify temporal changes associated with the strong ground motion caused by the To-hoku main shock.
2.2 Analysis procedure
Next, we remove the mean value of the traces and apply a 5 per cent Hanning taper to both ends. We add the power spectra of the two horizontal components and take the square root of the sum to get the amplitude of the vector sum of the two horizontal spectra. The obtained spectra are smoothed by applying the mean smoothing algorithm from the subroutine “smooth” in the Seismic Analysis Code (Goldstein et al., 2003), with half width of five points. The spectral ratio is obtained by taking the ratio of the horizontal spectra for surface and borehole stations. The amplitude of the spectra for both the surface and borehole recordings and the resulting spectral ratio at station IBRH15 are shown in Fig. 2(b) and (c).
Previous studies have found clear evidences of soil non-linearity after previous large earthquakes in Japan, including the 1995 Mw 6.8 Kobe earthquake, 2000 Mw 6.8 Western Tottori earthquake, 2003 Mw 8.3 Tokachi-Oki earthquake, 2003 Mw 7.0 Miyaki-Oki earthquake, and 2004 Mw 6.6 Niigata earthquake (e.g., Pavlenko and Irikura 2002; Sawazaki et al., 2006; Rubinstein et al., 2007; Assimaki et al., 2008; Wu et al., 2009a, 2010). The sharp reductions of the peak frequency and peak spectral ratio followed by gradual recovery observed in this study (Fig. 3) are similar to the previous observations of nonlinear site response using spectral ratio approaches (e.g., Sawazaki et al., 2006; Wu et al., 2009a). Hence, we attribute the temporal changes of the spectral ratios to nonlinear site response during the main shock.
The peak frequency and peak spectral ratio generally recover to at least 90% of the reference value in time periods of several tens to several hundred seconds after the strong shaking (e.g., Fig. 3). At station MYGH04, the peak frequency drop to ~45% of the reference value during the first phase, followed by a recovery to ~80% of the reference value within ~30 seconds. However, the recovery process is interrupted by a second phase, which is observed by many stations close the coast of Fukushima-Ibaraki (Aoi et al., 2011). The PGA of the secondary phase at MYGH04 is comparable to the first phase, and cause similar type of drop and recovery of the peak frequency and peak spectral ratio, with a recovery time scale of ~150 seconds to the value before the second phase. We note that the peak frequency still do not recovery to the reference value at the end of the main shock record (~250 s after the origin time). Whether this is due to permanent damage or longer time recovery remains to be investigated further with aftershock data. Due to the length of the available data at the time of writing, we are not able to look at the longer-period lower-amplitude temporal changes of the material properties as suggested by some previous studies (Lyakhovsky et al., 2009; Sawazaki et al., 2009).
As measured before, the percentage drop of peak frequency and peak spectral ratio measured during the Tohoku main shock correlate (with correlation coefficient larger than 0.65) with the PGA value at most sites (Fig. 4). The only exception is the station TCGH16, where there are two close peaks between 2 Hz and 6 Hz in the spectral ratio, which make the measurements of the peak frequency and peak spectral ratio more difficult and less accurate. The results in this study are generally consistent with the observation of Wu et al. (2009a) in the PGA range of 0 to ~500 Gal, but extend the correlation between the soil nonlinearity and the input PGA to a much higher PGA range of 500 Gal to more than 1000 Gal. The slope between the input PGA and the degrees of nonlinearity varies at different sites, which is likely caused by different site conditions. We have checked the site profiles for the six stations from the KiK-Net website (http://www.kik.bosai.go.jp), including average S-wave velocity (VS30) in the upper 30 m of the site (NEHRP 2003), soil types at the top layers of each site, and the S-wave velocity contrast. However we do not find clear correlation between the site conditions and observed degrees of nonlinearity.
In this short note we only focused on the clear temporal changes of site responses during the Tohoku main shock. Additional results on the long-term recovery and variable nonlinear behaviors will be reported in a follow-up work.
We thank National Research Institute for Earth Science and Disaster Prevention (NIED) for providing the KiK-net strong motion records of the 2011 Tohoku Earthquake sequence. We thank Joan Gomberg and an anonymous reviewer for their critical comments. This work is partially supported by National Science Foundation (EAR-0909310) and Southern California Earthquake Center (SCEC). SCEC is funded by NSF Cooperative Agreement EAR-0106924 and USGS Cooperative Agreement 02HQAG0008.
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