Modeling crustal deformation and rupture processes related to upwelling of deep CO₂‐rich fluids during the 1965–1967 Matsushiro earthquake swarm in Japan

<jats:p>In Matsushiro, central Japan, a series of more than 700,000 earthquakes occurred over a 2‐year period (1965–1967) associated with a strike‐slip faulting sequence. This swarm of earthquakes resulted in ground surface deformations, cracking of the topsoil, and enhanced spring outflows with changes in chemical compositions, as well as carbon dioxide (CO<jats:sub>2</jats:sub>) degassing. Previous investigations of the Matsushiro earthquake swarm have suggested that migration of underground water and/or magma may have had a strong influence on the swarm activity. In this study, employing coupled multiphase flow and geomechanical modeling, we show that observed crustal deformations and seismicity could have been driven by upwelling of deep CO<jats:sub>2</jats:sub>‐rich fluids around the intersection of two fault zones: the regional east Nagano earthquake fault and the conjugate Matsushiro fault. We show that the observed spatial evolution of seismicity along the two faults and magnitudes surface uplift are convincingly explained by a few megapascals of pressurization from the upwelling fluid within the critically stressed crust, a crust under a strike‐slip stress regime near the frictional strength limit. Our analysis indicates that the most important cause for triggering of seismicity during the Matsushiro swarm was the fluid pressurization with the associated reduction in effective stress and strength in fault segments that were initially near critically stressed for shear failure. Moreover, our analysis indicates that a 2‐order‐of‐magnitude permeability enhancement in ruptured fault segments may be necessary to match the observed time evolution of surface uplift. We conclude that our hydromechanical modeling study of the Matsushiro earthquake swarm shows a clear connection between earthquake rupture, deformation, stress, and permeability changes, as well as large‐scale fluid flow related to degassing of CO<jats:sub>2</jats:sub> in the shallow seismogenic crust. Thus our study provides further evidence of the important role of deep fluid sources in earthquake fault dynamics and surface deformations.</jats:p>