Characteristic wave-propagation direction of long-period ground motions in the Kanto Plain during the 2024 Noto Peninsula earthquake (Mw 7.5)

Abstract The 2024 Noto Peninsula earthquake ( M w 7.5) in Japan generated strong long-period ground motions that were observed across the Kanto Plain, which includes the Tokyo metropolitan area. Advances in high-performance computing now allow rapid 3D simulations of long-period ground motions in sedimentary basins that can be used to provide both site-specific estimates and basin-wide reconstructions of the wavefield. We simulated long-period ground motions in the Kanto Plain using a 3D subsurface structure model and a high-degree-of-freedom seismic source process model released shortly after the earthquake, and then compared the simulated waveforms with observations. Despite some phase mismatches, the simulations reproduced the observed waveform amplitudes, durations, and pseudo-velocity response spectra. We then applied wave gradiometry analysis to the simulated long-period wavefield in the 6–10 s band and treated the north–south (NS) and east–west (EW) components separately. The estimated propagation direction of the NS component largely followed the epicentral azimuth, whereas that of the EW component progressively rotated southward along the Kanto Mountains toward central Tokyo. This component-dependent behavior indicates that the long-period ground motions in the Kanto Plain are best interpreted as the superposition of multiple packets of surface waves traveling along different paths rather than as a single plane wave. Graphical abstract