Fluid-induced swarm earthquake sequence revealed by precisely determined hypocenters and focal mechanisms in the 2009 activity at Hakone volcano, Japan

A swarm earthquake sequence is often assumed to be triggered by fluid flow within a brittle fault damage zone, which is assumed to be highly permeable. However, there is little seismological evidence of the relation between the fluid flow within the fault damage zone and the occurrence of swarm earthquakes. Here, we precisely determine the hypocenters and focal mechanisms of swarm earthquakes that occurred in the caldera of Hakone volcano, central Japan, using data from a dense seismic network. We relocate 1,156 events that occurred during the period from August 4 through August 13, 2009 with the double-difference (DD) method [Waldhauser and Ellsworth, 2000]. For the relocation of the hypocenters, we use the differential arrival time obtained by both manual picking and wave form cross-correlation analysis. We determine focal mechanisms from the absolute P- and SH-wave amplitudes by adding the P-wave polarities. We demonstrate that the swarm earthquakes are concentrated on four thin plane-like zones, each of which has a thickness of approximately 100 m. One of the nodal planes of the focal mechanisms agrees with the planar hypocenter distribution. The thickness of the plane-like zones is considered to be statically significant, considering the location error of the hypocenters, and is consistent with the width of fault damage zone with 1 km length fault estimated by other studies [e.g., Vemilye and Scholz, 1998]. The swarm earthquakes that occurred during the initial stage of the activity exhibited a migration of hypocenters that appears to be represented by the diffusion equation. Based on the spatiotemporal distribution of the earthquakes, the hydraulic diffusivity (D) is estimated to be approximately 0.5 to 1.0 m2/s. The values of D are comparable to those estimated in other studies based on the reservoir-induced seismicity, the water injection-induced seismicity, and the spatiotemporal distribution of swarm activity. The observations imply that swarm earthquakes were triggered by the diffusion of highly pressured fluid within the fault damage zone. A burst-like occurrence of the swarm earthquakes is also observed in the later stage. These swarm earthquakes are thought to have been triggered primarily by local stress changes caused by the preceding activity. The complicated spatiotemporal pattern is thought to have been caused by the effect of the fluid flow within the high-permeability damage zones, as well as the stress perturbations generated by the swarm earthquakes themselves.