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

Abstract

[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,

Spatial distribution of crack structure in the focal area of a volcanic earthquake swarm at the Hakone volcano, Japan

“…The four permanent stations are operated by the Hot Springs Research Institute (HSRI) of Kanagawa Prefecture, Japan. The stations KZR and T.OSS are just above the focal area of (Yukutake et al, 2011). Dotted lines show the orientation of the maximum horizontal compressional stress in this region, estimated from a stress inversion analysis .…”

“…The activity migrated from the depths, and the north, toward shallow zones and the south. This hypocenter migration also suggests a relationship between the swarm and crustal fluid (Yukutake et al, 2011).…”

“…There were major swarms in 2001, and 2009(Tanada et al, 2002Yukutake et al, 2011). Mannen (2008) put forth a model in which increased pore pressure in cracks, caused by hydrothermal injection, generated the Hakone volcano swarms.…”

“…The 2009 swarm activity was from 4 to 12 August. Yukutake et al (2011) located the hypocenters of 1156 events during this period with the Double-Difference method. Hypocenters of the 2009 swarm were distributed on two vertical planes.…”

Why do aftershocks occur? Relationship between mainshock rupture and aftershock sequence based on highly resolved hypocenter and focal mechanism distributions

“…9 indicates the relationship between the thickness of the hypocenter distribution and its length associated with the swarm activity in the geothermal region at Hakone volcano in central Japan (Yukutake et al 2011). Yukutake et al demonstrated that the hypocenters of the swarm activity were concentrated on a planar zone having a thickness comparable to the damage zone of the natural fault reported by Vermilye and Scholz (1998) and interpreted the earthquake swarms as being triggered by the migration of highly pressurized fluid within the fault damage zone, which is considered to be highly permeable and to serve as a conduit for fluid flow (e.g., Faulkner et al 2010).…”