A notable swarm activity occurred at the northern shore of Lake Ashi in May 2015. During the activity, a rapid diffusion of the active area was observed with a diffusion coefficient estimated at about 16 m2/sec. The swarm activity occurred during an active period of Hakone volcano while dilatation of the edifice had been continuing. The swarm activity was not correlated with seismic activity under the central cones of Hakone volcano and there was a spatial gap between the two areas of seismic activity. A tilt meter at Kojiri, located at the northern border of the swarm area, had been showing a gradual northward inclination of the ground since the end of April. The swarm activity began with shallow earthquakes at the northwestern side of the swarm area on May 8. Subsequently, the active area moved gradually eastward. Many shallow earthquakes occurred near Kojiri during the morning of May 15; then, at around midday, the active area expanded rapidly. The earthquakes at the time of the rapid diffusion were centered at depths of about 2 km. Based on these observations the following hypothesis is proposed as the cause of the 2015 Hakone swarm activity. When Hakone volcano was activated in April 2015, volcanic fluids containing a large volume of gas began to enter a fracture zone that extended northwest at a depth of about 2 km under a cap rock layer. On May 8, volcanic fluids ascended under pressure at the northwest side of the zone near the western caldera limb that was not covered by the cap rock. When the fluids came into contact with the atmosphere, dissolved gas was released and entered cracks in the rock. The increase of gas pore pressure in the cracks made them slip easily and shallow earthquakes occurred. The influence of the pressure decrease in the fluid propagated slowly through the fluid conduit. On May 14 or the morning of May 15, it reached a reservoir of high-pressure fluids that had been formed near the Kojiri tiltmeter station. Around midday on May 15 fluids in the reservoir, with the increase of hydro-pressure, ascended into the cap rock through cracks, and made contact with the atmosphere. At that time, the pressure in the reservoir decreased suddenly and a large volume of volcanic gases dissolved in the fluids was released, causing violent seismic activity. The influence of the abrupt decrease of pressure in the reservoir propagated rapidly along the fluid conduit located at a depth of around 2 km, bringing about a rapid increase in the area of earthquake occurrence.
