Amplified Ground Shaking Triggered by Subterranean Gas Expansion: Unveiling a New Geohazard Following the 2024 Noto Peninsula Earthquake

On January 1, 2024, a major fire broke out in Wajima City approximately one hour after the M7.6 Noto Peninsula earthquake. Our previous study¹ suggested that the fire may have been triggered by the spontaneous ignition of methane gas released from underground alluvial deposits. To clarify the process that led to the fire, the present analysis focuses on an unusually amplified seismic shaking observed in Wajima just before the fire, which may have initiated the gas release. Although this shaking originated from a relatively small M3.5 earthquake located 28 km southwest of the city, it exhibited anomalous amplification and atypical waveforms that were localized to Wajima. This phenomenon is interpreted as a manifestation of the time-lagged “seismic champagne effect”, wherein gas bubbles dissolved in groundwater rapidly expand and erupt explosively in response to seismic waves. Subsequent observations—including shallow seismic events, gas emissions from the seafloor, and uplifted manholes—further suggest ongoing subsurface gas activity. These findings indicate that the time-lagged champagne effect may represent a previously unrecognized geohazard capable of inducing fires, liquefaction, and infrastructure damage.