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

Abstract

The recent M w 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.

The time dependence of rock healing as a universal relaxation process, a tutorial

“…Relaxation of the seismic velocity after perturbing rocks has been observed in laboratory studies (Ten Cate & Shankland 1996;Zaitsev et al 2003;Vakhnenko et al 2004;Ten Cate 2011) and field observations (Poupinet et al 1984;Schaff & Beroza 2004;Wegler & Sens-Schönfelder 2007;Brenguier et al 2008;Wu et al 2009;Nakata & Snieder 2011Wu & Peng 2012;Brenguier et al 2014;Obermann et al 2014;Richter et al 2014), and is thus an observable property under the right conditions. These observations usually show a recovery similar to the relaxation shown in Fig.…”

“…This could be the nanoscale, or even the atomic scale. Observed post-seismic velocity variations show a logarithmic relaxation as a function of time from the earliest measurement (Wu et al 2009;Wu & Peng 2012), which indicates that the shortest relaxation mechanism operates on a time less than the smallest observation time. The minimum relaxation time τ min can, however, not be equal to zero, because the integral (6) diverges for τ min = 0.…”

“…1). Observations of this equilibration time range from about 100 s for the M W 6.6 mid-Niigata earthquake sequence (Wu et al 2009), to about 10 d for the M W Tohoku-Oki earthquake (Wu & Peng 2012), to about 2-3 yr for the M W Parkfield earthquake (Brenguier et al 2008). The maximum relaxation time τ max depends in an as yet unknown way on the initial perturbation (such as the shaking that causes slow dynamics), the ambient stress, temperature and fluid content, and the depth at which the velocity is perturbed.…”

“…Laboratory studies (Ten Cate & Shankland 1996;Zaitsev et al 2003;Vakhnenko et al 2004;Ten Cate 2011) and seismological field observations (Poupinet et al 1984;Schaff & Beroza 2004;Wegler & SensSchönfelder 2007;Brenguier et al 2008;Wu et al 2009;Nakata & Snieder 2011;Hobiger et al 2012;Nakata & Snieder 2012;Wu & Peng 2012;Brenguier et al 2014;Obermann et al 2014;Richter et al 2014;Gassenmeier et al 2016) show that the seismic velocity of rocks is often reduced during shaking, and recovers afterwards. Gassenmeier et al (2016) demonstrated that, at least in some cases, these changes are caused by the shaking of the subsurface that damages the material.…”

“…The Snieder, C. Sens-Schönfelder and R. Wu Wu et al 2009;Ten Cate 2011;Wu & Peng 2012;Gassenmeier 2015). A similar log-time dependence of relaxation has been observed for the slope of sandpiles that have been perturbed (Jaeger et al 1988;Bocquet et al 1998), numerical models for the slope of sandpiles (Duke et al 1990), the compaction of granular media (Knight et al 1995;Peng & Ohta 1998;Linz & Döhle 1999), slider-block models for earthquakes (Huisman & Fasolino 2006) and post-seismic deformation (Griggs 1939;Benioff 1951;Savage et al 2005;Avouac 2015).…”