Matsushiro, Japan, Earthquake Swarm: Confirmation of the Dilatancy-Fluid Diffusion Model

The uncertainties in seismic velocities derived from earthquake data, the ambiguities of interpretation, and the lack of substantial evidence from explosion travel-time data leave the question of the reality of the dilatancy-diffusion model unresolved. To resolve this question, the more direct phenomena of crustal surface deformation and transient fluid flow induced by dilatant deformation has been theoretically analyzed. The results show excellent agreement with the main in situ features observed during the Matsushiro, Japan, earthquake swarm: (1) symmetrical upheaval and subsequent subsidence, (2) antisymmetrical horizontal deformation, (3) extensive outflow of water, and (4) initial decrease and later increase of gravity. This agreement between theoretical predictions and the directly observed in situ physical processes at a single site provides the strongest evidence to date in support of the dilatancy-fluid diffusion model.