A possibility of fluid migration due to the 2023 M6.5 Noto Peninsula earthquake suggested from precise gravity measurements

Abstract

The Noto Peninsula has experienced seismic swarms accompanied by transient crustal deformation since November 2020, followed by two major earthquakes (M6.5 on May 5, 2023, and M7.6 on Jan. 1, 2024). Previous studies have suggested that fluids are involved in a series of activities. Most evidence on fluids constrains only their existence, and quantitative information on dynamic fluid migration remains scarce. Past precise gravity measurements in volcanic areas captured changes at the μGal scale (10⁻⁸ m/s²) due to magma movement. Here, we report the gravity difference caused by the M6.5 earthquake that was obtained via a similar method of measurement. Most of the observed gravity change can be explained by a fault slip model determined from the geodetic inversion of GNSS data. However, a significant change of approximately 10 μGal remains unexplainable in the northern coastal area of the northeastern tip of the Noto Peninsula. To explain this change, we estimate environmental effects, such as groundwater and sea-level variations. These environmental effects are too small to fully explain the change unless large local groundwater changes that are not represented in the groundwater model are considered. Instead, adding a fluid-fed fault that opens above the coseismic fault could reasonably explain both the GNSS and gravity data. The inferred volume of fluids is approximately 10% of the volume to have accumulated in a deeper fault by June 2022, as estimated from GNSS data. This result suggests that fluids migrating to shallower areas may have increased the risk of the M7.6 earthquake. The relatively shallow seismic velocity anomalies inferred by seismic tomography might indicate that such an upward migration process due to large earthquakes has been repeated in the past.