[S-CG40] Science of slow-to-fast earthquakes

[SCG40-P50] Precise gravity observations before and after the Noto Peninsula earthquakes in 2023 and 2024

*Yoshiyuki Tanaka¹, Ryuichi Nishiyama², Akito Araya², Kohei Hotta³, Hiromu Sakaue¹, Maki Oshida¹, Taisei Takata¹, Kazuma Nakakoji¹, Takuya NISHIMURA⁴, Yoshihiro Hiramatsu⁵, Akihiro Sawada⁵ (1.Earth and Planetary Science, The University of Tokyo, 2.Earthquake Research Institute, The University of Tokyo, 3.University of Toyama, 4.DPRI, Kyoto University, 5.Kanazawa University)

Keywords:Noto Peninsula, Earthquake swarm, gravity, crustal deformation, fluid

Active earthquake swarms and corresponding crustal deformation have continued in the northern Noto Peninsula since December 2020. In addition, M6- and 7-class earthquakes occurred in May 2023 and January 2024, respectively. Geoscientific investigations indicate that abundant fluids in the subsurface are responsible for these earthquake swarms and large earthquakes. For example, transient crustal uplift observed by GNSS requires the presence of a tensile component that suggests fluid migration. The fluid is most likely water, but sufficient evidence to support this has not yet been obtained. We conducted gravity observations to constrain the density of the fluid and to obtain evidence of fluid movement. So far, six campaign observations were made in March 2022, March, May, and September 2023, and January and March 2024. An absolute gravimeter was installed at Suzu City or Kanazawa University, and relative gravimeters were used to measure the surrounding sites. At all these sites, GNSS continuous observations were conducted by Kyoto University, Kanazawa University, and the GSI. In this presentation, we mainly report the gravity changes before and after the above two major earthquakes. The observed gravity changes are generally consistent with the values estimated from the GNSS-observed uplift. However, the gravity gradient calculated using the observed vertical displacement and gravity change shows a significantly low density than the average crustal density. To clarify the cause, we are examining groundwater and near-shore effects as noise in gravity changes that are not of subsurface origin. On the other hand, GNSS data indicate that coseismic crustal deformation can be explained by shear slip without tensile opening. We calculated the subsurface density changes associated with shear slip using a half-space elasticity dislocation theory. However, this effect could not explain the discrepancy above. If the discrepancy is of surface origin, inelastic deformation involving water may be at work.