Multi-parameter imaging in the 2016 Mw 7.0 Kumamoto earthquake, Kyushu, Japan and its implications for fluids in seismogenesis

Fluids associated with structural heterogeneities in a seismogenic layer play a crucial role in triggering earthquakes and controlling their rupture processes, but the mechanism of fluid intrusion and the relationship with earthquake initiation are unclear. This study presents new seismic evidence of an earthquake mostly triggered by fluid intrusion associated with magmatism in Kyushu, SW Japan. Seismic imaging revealed two zones with slow seismic velocities beneath the Unzen and Aso Volcanoes with high Poisson's ratio, extending into the rupture belt of the 2016 Mw 7.0 Kumamoto earthquake. The following features, particularly the high Poisson's ratio anomalies, suggest the presence of fluids in the source area ascending from nearby magmatism. Anomalous crack density and saturation rate imply that the fractures/pores are not fully saturated with fluids in the crust of the Beppu-Shimabara Graben, possibly providing a path for fluids to penetrate into the source area, either through highly fracture-damaged faults or via incompletely saturated pores/cracks. Fluid penetrating into the hypocenter areas from deep magma increases the pore pressure and decreases the Coulomb failure function. We therefore speculate that a system such as this, with high pore pressure and fluids in the host faults might lower the threshold for fault failure, thus increasing the likelihood of failure of the mainshock fault. The results of this study suggest that the presence of fractures/pores within extensional rift zones around the world might be critical for fluid migration, and contribute to the initiation, development and migration of seismic activity.