Shallow volcanic earthquakes in the Owakudani geothermal area, Hakone volcano, Japan

Hakone volcano, located in central Japan, produced a small phreatic eruption in June 2015. Although seismic activity in the Hakone region is generally low, the area experiences episodic earthquake swarms approximately once every few years. In the volcano, many small earthquakes were observed at very shallow depths near Owakudani geothermal area from May to July 2022. In contrast to regular volcano-tectonic earthquakes, these events had no clear P- and S-wave arrivals. In this study, we sought to identify these earthquakes in continuous data and locate their hypocenters. We identified 11,016 earthquakes with similar waveforms between 2014 and 2023 using the matched-filter technique. Many earthquakes occurred in 2015, when the phreatic eruption occurred; however, the shallow seismicity was also active in 2020–2022 at a time when no other volcanic activity including volcano-tectonic earthquakes and crustal deformation occurred. The earthquakes were sometimes triggered by volcanic activity and sometimes occurred ambiently. The hypocenters of the earthquakes were located based on amplitude source location method around the Owakudani geothermal area at depths of − 1 to 0 km below sea level, close to the surface. The hypocenters are located close to the crack that opened around the time of the 2015 phreatic eruption and close to fissure consisting of older craters. Given the waveforms, locations, and timing of the earthquakes, we infer that they were caused by the movement of fluid and volcanic gas near the surface.