9:30 AM - 9:45 AM
[SVC31-03] Swarm earthquakes in May 2015 on the northern coast of Lake Ashinoko, Hakone, Japan
ｰWhat caused the activity? -
Keywords:Hakone volcano, Lake Ashinoko, Swarm earthquakes, Thermal water, Diffusion
A notable swarm activity occurred on the northern coast of Ashinoko lake in May 2015. In the activity, fast diffusion of active area whose diffusion coefficient is estimated at about 10 m/sec2 or larger, was observed. The swarm activity occurred in the active period of Hakone volcano while dilatation of the edifice had been continuing, but abrupt increase of pressure source coincidental to the occurrence of the swarm activity was not observed. The swarm activity was not correlated with the seismic activity under the central cones of Hakone volcano, neither. Actuary, a spatial gap was noted between the two seismic zones. The tilt meter located at Kojiri, northern border of the swarm area, had been showing gradual inclination of the ground to the north since the end of April.
Based on these observations as well as shallow-earthquake occurrence just before the violent breakup of the swarm activity, we propose the following hypothesis as the cause of the activity which accompanied fast spreading of the swarm area: When Hakone volcano was activated in April 2015, volcanic fluids that uprose from the magma reservoir, located at the depth around 10km beneath the central cones, began to enter gradually into a fracture zone, extending to the north-west direction on the northern coast of Ashinoko. On 8 in May, at the northwestern end of the zone, the pressured fluids found chinks to rise and finally connected to the atmosphere, which broke airtight of the fluids and dissolved gasses were extracted. The increase in the gas pressure made cracks in rocks to slip easily and earthquake occurred. The influence of the pressure-decrease in the fluid which occurred at the end of the fracture zone propagated slowly through the fluid conduit, and it reached to a reservoir of high-pressured fluids that had been formed near Kojiri on 14 or 15. At midday on 15 the fluids in the reservoir finally found passes connecting to the atmosphere, and at the instance, the pressure in the reservoir decreased suddenly and a plenty of volcanic gases dissolved in the fluids were released, causing the violent seismic activity. The abrupt decrease in the pressure in the big reservoir diffused swiftly along the fluid conduit bringing about the fast spreading of the area of earthquake occurrence.
Based on these observations as well as shallow-earthquake occurrence just before the violent breakup of the swarm activity, we propose the following hypothesis as the cause of the activity which accompanied fast spreading of the swarm area: When Hakone volcano was activated in April 2015, volcanic fluids that uprose from the magma reservoir, located at the depth around 10km beneath the central cones, began to enter gradually into a fracture zone, extending to the north-west direction on the northern coast of Ashinoko. On 8 in May, at the northwestern end of the zone, the pressured fluids found chinks to rise and finally connected to the atmosphere, which broke airtight of the fluids and dissolved gasses were extracted. The increase in the gas pressure made cracks in rocks to slip easily and earthquake occurred. The influence of the pressure-decrease in the fluid which occurred at the end of the fracture zone propagated slowly through the fluid conduit, and it reached to a reservoir of high-pressured fluids that had been formed near Kojiri on 14 or 15. At midday on 15 the fluids in the reservoir finally found passes connecting to the atmosphere, and at the instance, the pressure in the reservoir decreased suddenly and a plenty of volcanic gases dissolved in the fluids were released, causing the violent seismic activity. The abrupt decrease in the pressure in the big reservoir diffused swiftly along the fluid conduit bringing about the fast spreading of the area of earthquake occurrence.