Amplified Ground Shaking from Subterranean Gas Expansion A New Geohazard in the 2024 Noto Earthquake

On Jan. 1, 2024, just over an hour after the M7.6 Noto Peninsula earthquake, a large-scale fire broke out in Kawai-machi, Wajima City, located in the central–northern region of the Japanese archipelago. In the previous work1, it was proposed that this conflagration may have been triggered by the eruption and spontaneous ignition of methane gas accumulated within the underlying alluvial deposits. Yet one crucial question remained unresolved: why did the fire ignite, originating from a parking lot with no apparent ignition source, more than an hour after the mainshock? In the present study, attention is directed to an anomalous seismic tremor recorded in Wajima immediately before the fire was first detected, and the possibility is examined that this ground shaking served as the trigger for the sudden expansion and eruption of subterranean gas. Analyses lead to the conclusion that a delayed phenomenon, which may be termed a “time lagged seismic champagne effect,” caused by earthquake-induced rapid degassing and expansion of groundwater, was responsible for a cascade of geohazards. These included the outbreak of the fire, the uplift of sealed manholes onto road surfaces, and the massive discharge of gas bubbles from beneath the coastal seabed.