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.
Highlights
It has been known that strong ground motion produced by large earthquakes can cause softening of the Earth’s near surface, even leading to liquefaction in extreme cases[1,2]
We applied short-time moving-window seismic interferometry to the seismograms recorded during the Tohoku-Oki Earthquake at 12 selected Kiban-Kyoshin strong-motion observation network (KiK-net) stations in eastern Honshu to reveal the coseismic changes in near-surface seismic velocity (Fig. 2)
It was found that the mean effective confining pressure is likely to be similar across different KiK-net stations, the degree of the near-surface velocity reduction is primarily affected by the shear strain and soil type, which was empirically estimated using the peak ground acceleration, the initial unperturbed near-surface shear-wave velocity, and the plastic index of the near-surface sediments[30]
Summary
It has been known that strong ground motion produced by large earthquakes can cause softening of the Earth’s near surface, even leading to liquefaction in extreme cases[1,2]. Numerous studies have reported a sudden material softening in the upper crust caused by a large earthquake, and a subsequent healing that varies logarithmically with time by monitoring seismic velocity changes from ambient noise or earthquake records[10,11,12,13,14]. Both static strain induced by crustal deformation and dynamic strain resulting from strong ground motion are considered to be the main controlling factors[15,16,17]. We developed a basic theoretical model describing the slow recovery, and examined the model with experimental data obtained in laboratory and from the field
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