Near-surface softening and healing in eastern Honshu associated with the 2011 magnitude-9 Tohoku-Oki Earthquake

Abstract

The near-surface part of the crust, also called the skin of the earth, is the arena of human activity of which the stiffness is of great concern to engineers in infrastructure construction. The stiffness reduction of near-surface geomaterials also plays a vital role in geohazards triggering. However, the physical mechanism behind the material softening is still not fully understood. Here, we report a coseismic shear-wave velocity reduction in the near surface by up to a few tens of percent during the strongest shaking from the 11 March 2011 Tohoku-Oki Earthquake and a subsequent two-stage healing process including a rapid recovery within a few minutes and a slow recovery over many years. We also present a theoretical contact model between mineral grains in geomaterials containing multiple metastable contacts at small separations due to the oscillatory hydration interaction, which can explain the emergence of different stages in the healing process.

A numerical framework for underground structures in layered ground under inclined P-SV waves using stiffness matrix and domain reduction methods

“…It is widely known that the maximum shear modulus of granular soil is dependent on void ratios, confining pressures, and other parameters [23,24]. Assuming that the ground is uniform, its maximum shear modulus increases gradually with respect to depth owing to the increasing confining pressure values.…”

Reproducing ground response using in‐situ soil dynamic parameters

“…In this study, the high-resolution seismic interferometry based on deconvolution [46][47][48] is used to extract the in-situ shear wave velocity from strong ground-motion records, as shown in Equation 5.…”

“…In this study, the high‐resolution seismic interferometry based on deconvolution 46–48 is used to extract the in‐situ shear wave velocity from strong ground‐motion records, as shown in Equation . $$\begin{equation}{D_{n - 0}}(f) = \frac{{{S_0}(f){{\tilde{S}}_n}(f)}}{{{{\left| {{S_n}(f)} \right|}^2} + \varepsilon }}\end{equation}$$where f is the frequency; D n ‐0 ( f ) represents the deconvolution function between two accelerometers; S n ( f ) and S 0 ( f ) are the Fourier spectra of the ground motion at the n th downhole and surface sensors in the frequency domain; the symbol ∼ denotes the complex conjugate calculation; ε is the stable factor to avoid the denominator from approaching zero, and is set as 1% of average power spectra of the downhole ground motion in this study.…”

“…Following Wang et al., 48 a short‐time moving‐window seismic interferometry is applied to the seismograms to obtain the temporal change of shear wave velocity during the earthquake. As shown in Figure 2A, the length of the time window is selected to be 10.24 s to balance the resolution and stability of the variations of shear wave velocity.…”

Within-site variability in earthquake site response

“…Also, we cannot rule out the potential impact of soil non-linearity even though we only used ground motion with PGA of less than 0.1 g. This is because various studies (e.g. Wang et al 2021) show that sites that undergone non-linearity during strong shakings need as long as years for their near-surface deposits to 'bounce back'. These temporal changes in surficial soil properties affect the site response at high frequencies, as shown by Roumelioti et al (2019).…”