Strong Shaking Predicted in Tokyo From an Expected M7+ Itoigawa-Shizuoka Earthquake

Abstract

The Itoigawa-Shizuoka Tectonic Line (ISTL) is a major oblique left-lateral crustal fault that is expected to host M7+ events in the near future. Its proximity to the Kanto sedimentary basin poses a threat to the population of Metropolitan Tokyo. This study constructs ground motion predictions for scenario earthquakes on the ISTL using virtual earthquakes. We use the ambient seismic field to calculate the cross-correlation function that we assume proportional to the elastodynamic Green tensor between High-Sensitivity Seismograph network stations, which act as sources located above the ISTL, and the stations of the dense Metropolitan Seismic Observation network, which act as receivers in the Kanto Basin. We use the virtual earthquake approach (Denolle et al., 2013, https://doi.org/10.1029/2012JB009603; Denolle, Dunham, et al., 2014, https://doi.org/10.1126/science.1245678) to predict ground motion from a suite of 270 kinematic sources and find that predicted ground motions are strong enough that nonlinear effects, which we do not model, may become important. We find that the shape of the sedimentary basin substantially alters the shaking by amplifying long-period ground motions as seismic waves refract at the basin edge. Additionally, we quantify ground motion variability due to source uncertainty, surmise that ground motions are lognormally distributed with regard to source uncertainties, and suggest that the variability is affected (locally either enhanced or reduced) by the basin shape. Finally, we find a coupling point between source and wave paths for epicentral locations on the ISTL that generates almost twice the shaking as equivalent unilateral ruptures, despite directivity orientation that would favor southward ruptures.