Lowered VT activity and depressurization beneath the 2015 eruption center
The 2015 eruption appears to have been triggered by inflation at depth as indicated by an increase of baseline length across the volcano (Harada et al, 2018; Kobayashi et al.2018; Mannen et al. 2018) (Fig. 12a, b). Similar deep inflation was also recognized since the earliest phase of the post-eruptive unrest episodes in 2017 and 2019 by GNSS monitoring (Fig. 3). DLFs were also detected in the earliest phase of the pre-, co-, and post-eruptive unrest. However, subsequent seismicity seems to be different for the post-eruptive unrest episodes. VT earthquakes beneath the central cone were not as prevalent during the post-eruptive unrest episodes, especially in the shallow region (Figs. 4 and 11), although significant seismic activity took place beneath the caldera rim, remote from the active fumarole (A and B in Fig. 4). Assuming pore pressure increase is the trigger for VT seismicity (Yukutake et al. 2011; Mannen et al. 2018), such a significant difference can be interpreted as lower pore pressure rise in the hydrothermal system beneath the central cone, especially in the shallower part of the system (< 4 km deep). The insufficient pressurization of the hydrothermal system beneath the central cone compared with the pre-eruptive unrest episodes can be explained by the destruction of the cap-rock of the volcano by the 2015 eruption (Fig. 12c and d).
Mannen et al (2019) concluded that the materials erupted in the 2015 eruption were derived from cap-rock (from surface to 100 m deep) based on geological and magnetotelluric analyses. The enthalpy of the maximum temperature of steam emitted from the fumarole (2805 kJ/kg, 164.3 ºC at the surface; Fig. 6) is very close to that of saturated steam coexisting with liquid water at ~ 200 ºC and ~ 1.5 MPa, which is a hydrothermal condition at the depth of 150 m assuming hydrostatic pressure. Magnetotelluric surveys and InSAR analysis indicate that a vapor-rich portion of the hydrothermal system is located approximately 150 m below the surface of the eruption center and named the portion ‘vapor pocket’ (Kobayashi et al. 2018; Doke et al. 2018; Mannen et al. 2019). This line of evidence implies that the 2015 eruption tapped vapor from the uppermost part of the hydrothermal system and some vapor routes through the cap-rock created by the eruption are still alive as indicated by surviving high temperature fumaroles (≥ 160 ºC). Such degassing routes after the eruption can inhibit pressurization of the hydrothermal system during an unrest episode. The higher resistivity of the enlarged vapor pocket after the eruption (Mannen et al. 2019) indicates an increase in the vapor phase, presumably due to depressurization caused by the breach of the cap-rock (Fig. 12c). Also, the intensive seismic and hydrothermal activities during the 2015 activity may have increased the permeability of the hydrothermal system beneath Owakudani and contributed to the inhibition of pressure increase in the region during the post-eruptive unrest (Sibson et al. 1975).
Even though the eruption breached the cap-rock beneath Owakudani, pore pressure seems to have increased in other parts of the hydrothermal system during the post-eruptive unrest episodes. The seismic swarm beneath the northern (2017; A in Fig. 4b) and western (2019; B in Fig. 4c) caldera rim during the unrest episodes could be the manifestation of fluid injection from depth to a separated part of the hydrothermal system that caused a pore pressure rise and fluid migration as observed in the previous unrest (Yukutake et al. 2011), although a detailed analysis remains yet to be done (Fig. 12d).
Doke et al. (2020) detected a contraction source in the west of Owakudani using InSAR time series analysis, which is distinct from the vapor pocket beneath the eruption center. This result implies that the hydrothermal system beneath the central cone of Hakone volcano is not a single large expanse as implied from the resistivity structure (Yoshimura et al, 2018), which is highly interconnected. We, thus, need to assume a complex of multiple hydrothermal sub-systems separated by low-permeable seals, and each sub-system hosts a localized earthquake swarm as observed in the previous unrest episodes (Fig. 12).
Increase of magmatic gas during the post-eruptive unrest
During the post-eruptive unrest episodes, magmatic gas species, such as SO2, HCl and CO2 were observed to increase compared to H2S, which is a representative hydrothermal gas (Figs. 3, 8 and 9). The net increase of HCl emission during the post-eruptive unrest periods is also implied from river water chemistry and an increase of HCl in the vapor pocket beneath Owakudani was indicated by the chemistry of AHSs generated from SPWs (Fig. 10). Since HCl tends to be degassed from shallow magma (Rouwet et al. 2017), this might suggest intrusion and degassing of shallow magma during the post-eruptive unrest. However, no indication of shallow intrusion, such as low frequency earthquakes and harmonic tremor, was observed in our geophysical monitoring. Also, our DOAS survey observed no significant gross increase in SO2 emissions (Fig. 7). The emissions of SO2 during the post-eruptive unrest episodes were less than the standard criterion to indicate magma involvement (> 100 t/d) (Symonds et al. 2001).
SO2 and HCl are magmatic, but they are highly soluble in liquid water. We, thus, attribute the increase of these gases to a change in liquid–vapor ratio in the uppermost part of the hydrothermal system (~ 150 m deep). In the environment where liquid water exists, SO2 and HCl can be scrubbed from the coexisting volcanic gas; thus, the increase of these gases can imply drying-out of the hydrothermal system (Symonds et al. 2001). Since the fumarole temperatures showed no significant increase during the post-eruptive unrest episodes (Fig. 6), significant heating of the hydrothermal system that vaporized all liquid water in the shallowest hydrothermal system is highly improbable. Instead, a slight increase of heat flux changed the liquid–vapor ratio in the uppermost part of the hydrothermal system (~ 150 m deep) without any change of temperature in the vapor–liquid coexisting system. The increase of heat flux may be attributed to the injection of hot and pressurized magmatic fluid into the hydrothermal system by the rupture of the sealing zone at depth, possibly at the brittle–ductile transition (Fig. 12c, d).
Previous model of volcanic unrest
Ohba et al. (2019) proposed another mechanism to explain unrest phenomena at Hakone volcano, which relies on steady degassing of magma rather than newly derived magmatic fluid from depth. They emphasized the increase of atmospheric gases such as Ar and N2 in the fumarole emissions a few months before the onset of the 2015 unrest and interpreted this as the development of a sealing zone between the hydrothermal and magma-peripheral systems. During the pre-unrest phase, a sealing zone developed near the brittle–ductile transition and the pressure of magmatic gas represented by CO2 beneath the sealing zone increased. Meanwhile, pressure in the hydrothermal system dropped due to the lack of magmatic gas influx from the magma-peripheral system, allowing atmospheric gas to seep into the hydrothermal system, as detected by their survey of the fumarole gas.
This is an interesting model that interprets the results of gas monitoring; however, the model neglects geophysical observations such as deep inflation and DLFs in the early phase of the volcanic unrest episodes. If the deep inflation was caused by pressurization of volatile species beneath the sealing zone, deflation should have occurred after the breach of the sealing zone. This means deep deflation should have been observed when VT earthquakes surged. Also, the DLF events in the very early phase of the unrest episodes indicate injection of magma or magmatic fluid from depth. We acknowledge that it is hard to explain the increase of atmospheric species in the fumarole during the pre-unrest phase; however, we point out that this was observed only in fumarole N, which is located near the eruption center of the 2015 eruption. This means that the injection of atmospheric species seems to have occurred only in a very limited part of the hydrothermal system, probably near the surface of the steaming area. In the steaming area, reactivation of an almost extinct steam production well (SPW39) was recognized on April 17 (Mannen et al. 2018) and inflation of the steaming area was recognized by InSAR on May 7 (Doke et al. 2018). These observations indicate that underground fracturing and a local uplift (≤ 200 m in diameter) due to very shallow inflation (~ 150 m depth; vapor pocket) could have started before May 2015 (Kobayashi et al. 2018; Doke et al. 2018). Indeed, fracturing of the surface was visually observed after mid-June (Mannen et al. 2018) and such fracturing may have introduced air into the shallowest region of the hydrothermal system near the uplift region.
Degassing of CO2 during the unrest episodes and its implication
The increase in the C/S ratio from the very early phase of the unrest episodes at Hakone volcano can be explained by a breach of the sealing zone that leads to a significant pressure contrast between the hydrothermal system and a CO2 reservoir beneath it. The sealing zone may not be necessarily be identical to the brittle–ductile transition as implied by Ohba et al. (2019). However, it is noteworthy that no significant change in SO2 and H2S emission had been recognized in the peripheral fumaroles around the eruption center (Ohba et al, 2019). Also, our DOAS survey implies no significant increase of SO2 emission during the post-eruptive unrest episodes occurred in 2017 and 2019 (Fig. 7). These observations may mean that principally, CO2 was injected into the hydrothermal system, implying magma degassing deeper than the levels of SO2 exsolution but shallower than that of CO2. The breach of the sealing zone and the migration of the hydrothermal fluid from the magma-peripheral to the hydrothermal systems might have accompanied bubbling of CO2 (Lowenstern 2001), although no corresponding geophysical signal was observed.
Degassing from newly derived magmatic fluid may have been the source of the CO2 and the reason for the increase in the C/S ratio; however, the 2017 and the 2019 unrest episodes did not show sharp increases in the C/S ratio even though these events were accompanied by DLFs (Fig. 3). Such a difference in the temporal change in the C/S ratio during the volcanic unrest episodes indicates a change in physical structure caused by the 2015 eruption. We previously proposed a breach of the shallow cap-rock. In addition to that, we propose a breach of the deep sealing zone during the 2015 eruption, with its annealing process after the eruption still incomplete. Assuming the sealing zone remained breached, volatiles steadily degassed from the magma chamber cannot be stored beneath the sealing zone in large quantity (Fig. 12c). Thus, only newly derived magmatic fluid may contribute to the CO2 rise during the volcanic unrest, and the observed CO2 increase at the surface became subtle as observed during the unrest episodes in 2017 and 2019 (Fig. 12d). Based on this model, we expect that the sequential change of the C/S ratio characterized by a strong increase just after the beginning of an unrest and a strong decrease after the climax as observed in 2013 and 2015 (Fig. 3) will resume after a restoration of the sealing zone in the future.
Model caveats
Deep inflation sources during Hakone unrest episodes have been located using inversion analysis. However, their optimum depths were not unique, ranging from 6.5 km for the 2015 unrest and eruption (Harada et al. 2018) to 10 km for the 2019 unrest (Doke et al. 2019). Mannen et al. (2018) interpreted deep inflation during Hakone unrest episodes, as indicated by GNSS data analysis, as magma replenishment. Kobayashi et al. (2018) located the source of the deep inflation during the 2015 unrest and eruption (4.8 km deep) and considered the inflation source to represent a magma chamber. However, these source depths are shallower than Region 1, which is considered to represent a magma chamber from seismic tomography, and are instead located in the hydrothermal system (Yukutake et al. 2015). We may, thus, need to assume magma intrusion or accumulation of magmatic fluid in the deeper part of the hydrothermal system. As discussed above, intrusion of magma to the hydrothermal system is possible, but relatively low emission of magmatic gases observed are seemingly at odds with the model.
Accumulation of magmatic fluid within the deeper part of the hydrothermal system may increase the likelihood of a large phreatic eruption in the future. Post-eruptive deflations, such as those observed after the Ontake eruption in 2014 (Murase et al. 2016; Narita and Murakami 2018) and Te Maari eruption in 2012 (Hamling et al. 2016), indicate accumulation of magmatic fluid before the major phreatic eruption. Such post-eruptive deflation was not observed after the 2015 eruption of Hakone and this could be an alert for a large phreatic eruption in future. Since intrusion of magmatic fluid at depth can occur without significant seismic and geodetic precursors as shown by the 2014 eruption of Ontake (Takagi and Onizawa 2016), we should be alert even to minor volcanic unrest episodes.