Abstract Mud volcanoes are landforms produced by the emission of fluids or mud from below the surface. We investigated the relationship between surface deformation and subsurface structure at the Murono mud volcano in Tokamachi City, Niigata Prefecture, with pre- and post-2014 Kamishiro fault earthquake leveling data. The Murono site showed extensive uplift during the earthquake, as would be expected due to fluid entry from depth. To elucidate the cause mechanisms, a vertical concentrated load was placed as the basis function to represent the grid-based subsurface pressure field, and its intensity was estimated through inverse analysis, by reversing two periods of vertical displacement measurements in 2014: before the earthquake (Period I) and during the earthquake (Period II). The result shows that during Period I, uplift and subsidence were localized at depths shallower than approximately 30 m: uplift in the eastern, western, and northern parts of the Murono mud volcano, and subsidence in the central part. In Period II uplifts became widespread to depths of up to 40 m below the ground surface. These facts imply that the Kamishiro earthquake facilitated fluid migration from deep reservoirs, which is consistent with electromagnetic surveys that registered low-resistivity bodies beneath the area. While our elastic half-space model captured complex deformation patterns not available to typical spherical source models, it remains a simplification for fluid-saturated sediments. Poroelastic or hydro-mechanical more realistic approaches are needed. The time coincidence between seismic shaking and enhanced mud inflow also leads us to question whether earthquakes increase pore pressure directly or indirectly stimulate fractures, and this needs more multidisciplinary investigation. Comparison with other comparable seismically triggered mud volcanoes worldwide shows that Murono is a manifestation of regional earthquake–fluid interaction. Spatial gradients of computed displacements and loads display subsurface fluid movements, which show upward and downward signals representing inflow and blocked flow, respectively, and that their correlation with low-density regions proves complex fluid–structure interactions. Graphical abstract
Takahashi et al. (Tue,) studied this question.