Complex Near-Surface Rheology Inferred From the Response of Greater Tokyo to Strong Ground Motions

Abstract

Strong ground motion can induce dynamic strains large enough for the Earth's subsurface to respond nonlinearly and to cause permanent, or plastic, damage. The 2011 Mw 9.0 Tohoku-Oki earthquake, Japan, generated exceptional and well-recorded ground motions in the greater Tokyo area. We use continuous records from 234 stations of the dense MeSO-net seismic network to monitor the temporal evolution of the material properties of the shallow subsurface (upper ∼100 m). We apply the single-station cross-correlation method to reconstruct the near-surface reflectivity response through time. We find that the strong ground motions from the mainshock caused large perturbations in the near-surface structure, with significant drops in seismic velocities up to 11%. For most sites, we observe a logarithmic and complete recovery of the seismic wave speed, suggesting a relaxation process that can be explained by a viscoelastic rheology. Some sites exhibit an instantaneous and permanent change, which suggests a plastic rheology. Finally, dense seismic measurements allow for statistical inference between seismic velocity drops, recovery time scales, and permanent perturbations and ground motion strength and site conditions. This study highlights the potential for seismic interferometry to assess near-surface rheology with dense seismic arrays.