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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.

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The shear-wave velocity (VS) in soil is an important parameter to characterize dynamic soil properties. The Delaney Park downhole array was deployed in 2003 without measuring the shear- and compression-wave velocity (VS and VP) profiles. Thornley et al. (2019) measured the VS and VP profiles using the downhole method after the sensor was removed from the 61 m borehole with casing in the array. However, the waves propagating along the casing wall may have a great influence on the recognition of the first arrival of waves propagating in the soil. Using horizontal and vertical components of weak-motion data of eight local earthquakes recorded by the array, in situ VS and VP profiles were assessed by the seismic interferometry based on deconvolution, respectively. The results are as follows. The VS and VP profiles computed by this study and measured by Thornley et al. (2019) are in relatively good agreement at a depth of 10–45 m and at a depth of 30–45 m, respectively, and in very poor agreement at other depths. The average VS profiles computed by this study are more consistent with the derived VS from the standard penetration test data at the site with slower near-surface velocities relative to the downhole logging analysis. There are strong anisotropy in the strata below 45 m and weak anisotropy with various degrees at various depths in the strata above 45 m.

The local site effect of vibration propagation during earthquakes is usually considered by one-dimensional site response analysis, while the ground motion of the site is the result of the combined effect of the source, the propagation path and the local site conditions. In the study, the influence of ground motion input, soil layer properties and analysis methods need to be considered. Based on a large number of related literatures, this paper briefly summarizes the research status of seismic response of one-dimensional soil layer sites through the determination of bedrock ground motions, soil dynamic constitutive models, and analysis methods. In the future, the development of seismic response of soil sites is prospected and summarized.