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

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.

Automated summary

The near-surface part of the Earth's crust, where human activities take place, is of great importance to engineers in infrastructure construction. Reduction in the stiffness of near-surface geomaterials plays a crucial role in triggering geohazards, but the physical mechanism behind material softening is not fully understood. The study reports a coseismic shear-wave velocity reduction in the near surface during a major earthquake event, followed by a two-stage healing process that includes rapid recovery within minutes and slow recovery over many years. A theoretical contact model between mineral grains in geomaterials is proposed to explain the different stages in the healing process.