[1] 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 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 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 is estimated to be approximately 0.5-1.0 m 2 /s. 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.Citation: Yukutake, Y., H. Ito, R. Honda, M. Harada, T. Tanada, and A. Yoshida (2011), Fluid-induced swarm earthquake sequence revealed by precisely determined hypocenters and focal mechanisms in the 2009 activity at Hakone volcano, Japan,

High-resolution images of subsurface structures are necessary to understand the transport processes of crustal fluids from deep magma sources and their relationship to earthquake swarms in active volcanic regions. Based on a seismic tomography approach, we have developed a new model for the magma-hydrothermal system beneath Hakone volcano, central Japan, where shallow earthquake swarms and crustal deformation associated with inflation of an open-crack source are often observed. By applying travel-time data for local earthquakes to a tomographic inversion, we obtained highly resolved seismic velocity structures that show a region of low P-wave velocity (Vp), low S-wave velocity (Vs), and high Vp/Vs ratios at depths of 10-20 km beneath the volcano, corresponding to the location of the open-crack source. We suggest that the high Vp/Vs ratios represent a deep magma chamber with a high concentration of melt and/or fluids. Deep low-frequency earthquakes, located just beneath this high Vp/Vs zone, may indicate that magmatic fluids are supplied from below. Above the high Vp/Vs zone, a region of low Vp, low Vs, and low Vp/Vs ratios exists at depths of 3-10 km, suggesting the presence of crack-filled water or CO 2 supplied from the inferred deep magma chamber. Many earthquake swarms occur in this low Vp/Vs zone, indicating that crustal fluids play an important role in generating the swarms. Similar relationships between magma reservoirs, overlying hydrothermal systems, and swarm activity have been reported from other volcanic areas and thus may be a ubiquitous feature beneath active volcanoes.

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