We analyzed seismicity linked to the 2014 phreatic eruption of Mount Ontake, Japan, on 27 September 2014. We first relocated shallow volcano tectonic (VT) earthquakes and long-period (LP) events from August to September 2014. By applying a matched-filter technique to continuous waveforms using these relocated earthquakes, we detected numerous additional micro-earthquakes beneath the craters. The relocated VT earthquakes aligned on a near-vertical plane oriented NNW-SSE, suggesting they occurred around a conduit related to the intrusion of magmatic-hydrothermal fluids into the craters. The frequency of VT earthquakes gradually increased from 6 September 2014 and reached a peak on 11 September 2014. After the peak, seismicity levels remained elevated until the eruption. b-values gradually increased from 1.2 to 1.7 from 11 to 16 September 2014 then declined gradually and dropped to 0.8 just before the eruption. During the 10-min period immediately preceding the phreatic eruption, VT earthquakes migrated in the up-dip direction as well as laterally along the NNW-SSE feature. The migrating seismicity coincided with an accelerated increase of pre-eruptive tremor amplitude and with an anomalous tiltmeter signal that indicated summit upheaval. Therefore, the migrating seismicity suggests that the vertical conduit was filled with pressurized fluids, which rapidly propagated to the surface during the final 10 min before the eruption.

We monitored seafloor crustal deformation at two observation stations on opposite sides of the Nankai Trough from 2013 to 2016 in order to investigate the interplate locking condition along the Central Nankai Trough. We estimated the seafloor crustal deformation with respect to the Amurian Plate based on five observations. The results for two stations, TCA and TOA, were 38±25 mm/yr toward N78°W and 57±21 mm/yr toward N69°W (1σ), respectively. The displacement rate at TOA is consistent with the motion of the Philippine Sea Plate with respect to the Amurian Plate using the Euler vector of the REVEL model. The displacement rate at TCA is similar to that at other seafloor geodetic stations, e.g., 48±10 mm/yr at KUM2 station, on the Kumano Basin. The interplate coupling rate was estimated to be roughly 70±40% (1σ) in the shallowest segment of the Nankai Trough.

The phreatic eruption of Mt. Ontake in 2014 was preceded for 25 s by a very long period (VLP) seismic event recorded at one broadband and three short-period seismic stations located within 5000 m of the summit. We performed waveform inversion of the event within a frequency band of 0.1-0.5 Hz that pointed to an ENE-WSW opening (NNW-SSE striking) subvertical tensile crack at a depth of 300-1000 m beneath the region of the eruptive vents. This crack orientation is consistent with alignments of volcano-tectonic (VT) earthquake hypocenters and eruptive vents as well as normal faulting (E-W tension) focal mechanisms of the VT earthquakes. We interpreted these results as follows: the VLP source crack was one of a group of preexisting faults that was opened immediately before the eruption because of the passage of ascending gas from depth to the surface.

Volcanic activity is often accompanied by many small earthquakes. Earthquake focal mechanisms represent the fault orientation and slip direction, which are influenced by the stress field. Focal mechanisms of volcano-tectonic earthquakes provide information on the state of volcanoes via stresses. Here we demonstrate that quantitative evaluation of temporal stress changes beneath Mt. Ontake, Japan, using the misfit angles of focal mechanism solutions to the regional stress field, is effective for eruption monitoring. The moving average of misfit angles indicates that during the precursory period the local stress field beneath Mt. Ontake was deviated from the regional stress field, presumably by stress perturbations caused by the inflation of magmatic/hydrothermal fluids, which was removed immediately after the expulsion of volcanic ejecta. The deviation of the local stress field can be an indicator of increases in volcanic activity. The proposed method may contribute to the mitigation of volcanic hazards.

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