International Workshop on the mechanism of Phreatic Eruption

Earthquake swarms, subsurface structure and deep low frequency earthquakes
beneath Hakone volcano, and its relation to magma-hydrothermal system
Yohei Yukutake1
1 Hot Springs Research Institute of Kanagawa Prefecture
E-mail: yukutake(at)onken.odawara.kanagawa.jp

Beneath the Hakone volcano, central Japan, remarkable earthquake swarms have beenfrequently observed in the shallower region than a depth of 6km, sometimes accompanied
by crustal expansion whose inflation source was estimated around a depth of 7 km. The deep
low-frequency earthquakes (DLFEs) have been also observed at the depth of 20 – 30 km
beneath the volcano. A phreatic eruption in 2015 was occurred at the northern slop of central
cone (Owakudani geothermal region), following the remarkable earthquake swarms andcrustal expansion. Our recent studies reveal the detailed characteristics for the earthquake
swarms, subsurface structure and DLFEs, using the dense seismic observation, and foundthat the elevation of pressure in magmatic fluid plays an essential role to generate thephenomena, including the phreatic eruption.
During the 2009 swarm activity, we found that the hypocenters of earthquake swarms wereconcentrated on a vertical plane and showed a diffusion-like migration, suggesting thatpressurized fluid in the fault damage zone strongly influence the occurrence of swarms(Yukutake et al., 2011). By the seismic tomography method, a low S wave velocity zone withhigh Vp/Vs ratio was estimated at a depth of 10 km (Yukutake et al., 2015). According to theporoelastic theory for solid-liquid composition (e.g. Takei, 2002 JGR), this high Vp/Vs zone
can be explained by a crystallizing magma body. On the other hand, at the depth of 6 km, alow velocity zone with low Vp/Vs ratio was estimated. This zone can be interpreted by theexistence of water/gas rich region, suggesting that the dehydrated magmatic fluid/gas
developed above the magma body and just beneath the lower limit of earthquake swarms.
We also tried to detect the DLFE beneath Hakone volcano during past two decade by using
the Matched filter method. The DLFEs were activated before the crustal expansions and theintense swarm activities (Yukutake et al., 2019). In the 2015 activity, the phreatic eruption
occurred more than 4 months after the activation of DLFEs. Given these observations, we
propose the model to explain the relation between the magma-hydrothermal system andvolcanic activity in Hakone (Fig. 1). Magmatic fluid was firstly supplied from a deep area tothe source region of the DLFEs, accelerating the occurrence of DLFEs and causing anincrease in the fluid pressure at a particular depth. This pressure increase from the magmaticInternational Workshop on the mechanism of Phreatic Eruption

Fig. 1 Schematic of the time sequence of the 2015 activity (Yukutake et al., 2019)
fluid may have migrated to a shallow depth, accelerating the feeding of magmatic fluid into
the magma body at the depth of 10 km. The process stimulated dehydration and degassing
into the fluid/gas rich region, causing the inflation of pressure source. This inflation may haveincreased the flux of highly pressurized fluid into the shallow brittle zone. Consequently, thepore‐pressures in the fracture system increased within the brittle zone, triggering theearthquake swarms by the fluids migrating through the shallow fracture zone. In the case of2015, the incremental fluid pressure affected up to the shallow geothermal system, andleaded to the phreatic eruption.
Main References
Yukutake, Y., Abe, Y., & Doke, R. (2019). Deep Low-Frequency Earthquakes Beneath theHakone Volcano, Central Japan, and their Relation to Volcanic Activity. GRL.
Yukutake, Y., et al. (2015). A magma-hydrothermal system beneath Hakone volcano, centralJapan, revealed by highly resolved velocity structures. JGR.
Yukutake, Y., et al. (2011). Fluid-induced swarm earthquake sequence revealed by precisely
determined hypocenters and focal mechanisms in the 2009 activity at Hakone volcano,Japan. JGR.