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.

Rain and small earthquakes maintain a slow-moving landslide in a persistent critical state

“…This slow healing was also observed at laboratory-scale, where it is referred to as slow dynamics 53 . Previous observations of the dv/ v recovery after an earthquake show that it often evolves logarithmically over time 47,48,51 . A similar pattern was also observed here, with recovery of the pre-earthquake soil state around one and a half months after the Ml 5.5 earthquake in August 2016 (Fig.…”

“…The co-seismic drop in velocity can therefore be attributed predominantly or even completely to damage to the soil 45,[47][48][49] , due to the opening of new or pre-existing cracks 47,48,[50][51][52] .…”

“…3f). This healing phase can be interpreted as the recompaction of the soil as fractures close, and the grains cement together, reflecting a viscoelastic response of the soil 47,48,[50][51][52] . This slow healing was also observed at laboratory-scale, where it is referred to as slow dynamics 53 .…”

“…Single station cross-correlation was performed on the three possible pairs of components 51 , i.e., North-East, vertical-East, and Vertical-North. To do so, hourlong records were spectrally whitened between 0.2 and 35 Hz to equalize the frequency content of the ambient noise 43 .…”

Noise-based ballistic wave passive seismic monitoring – Part 2: surface waves

“…The normal stress variations can be induced by atmospheric pressure variations, while the pore pressure variations can be induced by rainfalls. Large variations of shear velocity of few percent have already been observed after strong rain events (e.g., Sens-Schönfelder & Wegler 2006;Miao et al 2018;Viens et al 2018;James et al 2019) even though the amount of decrease depends on the initial state of the soil. During the monitoring period a rainfall event happened during days 51 to 54 with 40 mm of water (16 mm alone on day 54) following a 2 weeks period without rain.…”