Effects of Source Directivity and Nonlinear Soil Behavior During the January, 1 2024 Noto Earthquake (Mw = 7.5)

Abstract

The earthquake of January 1, 2024 with the epicenter at Noto Peninsula of Ishikawa Prefecture, Japan, and the moment magnitude Mw = 7.5 obviously represents an intermediate case between weaker earthquakes with relatively small sources, like the 1995 Kobe and 2000 Tottori earthquakes (Mw ~ 6.7-6.8), showing nonlinear soil response and soil softening (reduction of shear moduli) and stronger earthquakes, like the 2003 Tokachi-Oki and Tohoku earthquakes (Mw ~ 8.3-9.0) with extended sources and source directivity effects, accompanied by soil hardening and generation of high peak ground accelerations (PGA) > 1 g. In this research, based on KiK-net vertical array records (11 sites), models of soil behavior in the near-fault zones of the 2024 Noto earthquake are constructed, i.e. vertical distributions of stresses and strains in soil layers changing with time during strong motion, which showed nonlinear soil response and reduction of shear moduli in the near-fault zones. At the same time, the waveforms of acceleration time histories indicate the effects of source directivity, when seismic waves, radiated by the crack tip propagated along a~rather long section of the fault plane, arrived to remote sites almost simultaneously, overlap, harden subsurface soils and generate high accelerations on the surface, PGA ~ 2828 Gal at remote ISK006 station.