2017 M5.6 earthquake near Mount Ontake, central Japan: insights into fluid-driven activity based on near-field precise leveling measurements

Abstract Seismic swarms in volcanic regions are commonly attributed to fluid processes; however, the precise mechanisms linking fluid migration to earthquake generation remain poorly understood due to limited near-field geodetic observations. We investigated the deformation processes associated with the 2017 M5.6 earthquake at Mount Ontake to understand the role of fluid upwelling in earthquake generation. We conducted precise leveling surveys at multiple time periods from 2016 to 2023 and applied tensile crack and fault modeling to explain the observed deformation patterns. Significant uplift of approximately 35 mm was detected in the focal area during April 2017–April 2018, followed by subsidence of approximately 7 mm during April 2018–May 2023. A fault with a tensile crack model best explained the uplift, with a tensile crack opening of 238 mm and a fault slip of 689 mm. The tensile crack was located approximately 2 km southeast of the M5.6 earthquake, with the fault’s downdip extension adjacent to the shallow edge of the tensile crack. The results suggest that upwelling crustal fluids expanded tensile cracks and weakened fault strength through increased pore-fluid pressure, thereby triggering the M5.6 earthquake. These results demonstrate that precise leveling can provide critical insights into the fluid–earthquake relationships in volcanic seismic swarm areas. Graphical Abstract