Hakone Volcano and its activity in the 21st century
Here we review Hakone volcano and its latest activity based on Mannen et al. (2018). Hakone is a caldera volcano located at approximately 80 km SW of Tokyo (Fig. 1a). Its eruption history started at least 400 ka and after two caldera-forming stages. Andesitic effusive eruptions since 40 ka have formed a complex of lava flows and domes named the Younger Central Cones (YCC) in the center of the caldera (Fig. 1b). The latest magmatic eruption occurred near the northernmost part of YCC and formed a lava dome named Kanmurigatake, which erupted within an amphitheater that was created by a sector collapse just before the dome formation. The most active steaming area and the center of the latest phreatic eruptions named Owakudani is located at the eastern flank of Kanmurigatake.
Hakone is not very active in terms of magmatic eruptions; however, it is notable for its high seismicity, with at least 7 intensive earthquake swarms observed in the 20th century. Most of the swarms did not accompany clear intensification of steaming activity, while the volcanic unrest from 1933 to 1935 culminated with a formation of a new steam vent 1 km south of Owakudani, although the exact location of the vent is not known.
The continuous instrumental monitoring of Hakone volcano started after the volcanic unrest of 1959–60; however, the volcano monitoring network detected no major seismic swarm until 2001. The 2001 unrest accompanied an earthquake swarm, and deep and shallow inflation as observed by Global Navigation Satellite System (GNSS) and a tiltmeter network, and culminated with a blowout of a steam production well (SPW) in Owakudani (500 m deep). Since the 2001 unrest, major volcanic unrest episodes comprising earthquake swarms, deep inflation detected by a GNSS network, and deep low frequency events (DLF) were observed in 2006, 2008–2009 and 2013 (Harada et al. 2018; Yukutake et al. 2019). In terms of seismicity, these events can be intensive as historical unrest episodes before 1960. These volcanic unrest episodes were not accompanied by significant increases of steaming activity in the steaming areas of the volcano; however, in March 2015, a new volcanic unrest episode started with a deep inflation and increase of both volcano-tectonic and DLF seismicity. This volcanic unrest was followed by a blowout of SPW in early May, and eventually, on June 29, a small phreatic eruption started and lasted until the early morning of July 1. The 2015 volcanic unrest after the eruption seems to have continued until late August, which is evident from crustal inflation monitoring by GNSS (Harada et al. 2018).
Subsurface structure of Hakone volcano
Various geophysical and geochemical investigations over the last decade have modeled the subsurface structure of Hakone volcano. They are summarized in Fig. 2 and as follows. At approximately 20 km beneath the northern caldera rim of the volcano, DLFs occur sporadically. Since many of the DLF swarm events were followed by inflation of the edifice and shallow volcano-tectonic earthquake swarms, DLFs are interpreted as a signal indicating migration of magmatic fluid (Yukutake et al. 2015, 2019). A seismic tomography study revealed velocity structure beneath Hakone volcano and showed that the volcano has an active magma-hydrothermal system (Yukutake et al. 2015). Yukutake et al. (2015) identified a high-Vp/Vs and low Vs body (Region 1), which was considered to represent a magma chamber located at approximately 10 km depth. Above Region 1, a low-Vp/Vs and low-Vs body (Region 2) was identified and interpreted as a fluid-rich zone. The upper boundary of the Region 2 is shallower than 5 km, and interestingly, the boundary seems to reach near the surface just beneath Owakudani. Above Region 2 is the fracture zone, where most of the volcano tectonic earthquakes occur. Some fraction of the earthquakes in the fracture zone of Hakone volcano can be attributed to re-activation of pre-existing fractures caused by fluid migrations (Yukutake et al. 2010, 2011). Significant anisotropy in the shallow crust beneath Hakone volcano also indicates pre-existing fractures that are controlled by the regional stress field (Honda et al. 2014). The fracture zone and Region 2 overlap slightly and both are considered to form the hydrothermal system.
A magnetotelluric study in and around Hakone volcano revealed a bell-shaped conductive body beneath the volcano, the top of which reaches the surface near Owakudani (Yoshimura et al. 2018). Since the bell-shaped conductive body nests a resistive body beneath it, they are considered represent the hydrothermal system of the volcano. The bell-shaped body is interpreted as a smectite-rich zone, which was formed by a prolonged hydrothermal activity of the volcano. In Owakudani and the surrounding area (Fig. 1), a series of local high-resolution magnetotelluric surveys was conducted and revealed that the bell-shaped conductive body is exposed on the surface in the bottom of Owakudani valley (Mannen et al. 2019). A geological investigation of a borehole showed that the bell-shaped conductive body corresponds to altered volcanic sediment accompanying smectite as predicted by Yoshimura et al. (2018) (Mannen et al. 2019). Epicenters of volcano tectonic earthquakes at Hakone are located within the resistive body (i.e. hydrothermal system) (Yoshimura et al., 2018). Peculiarly, seismic signals other than volcano tectonic earthquakes, such as shallow volcanic tremor and low frequency events, are rare in this volcano. The shallow non volcano-tectonic earthquakes detected by our network are a single isolated very shallow M = − 0.3 event that occurred near Owakudani during the 2006 unrest (Tanada et al. 2007) and tremors sourced from the boiling conduit during the 2015 eruption (Yukutake et al. 2017). Since shallow tremors and low frequency earthquakes are common in volcanoes that have active hydrothermal systems, the paucity of shallow seismic signals related migration of fluids even during volcanic unrest episodes could be a significant feature of Hakone volcano.
Geochemical monitoring has provided evidence for development of a sealing zone and injection of magmatic fluid into the hydrothermal system through the zone (Ohba et al. 2019). Very shallow geological and resistivity structures (≤ 500 m deep) are summarized in Mannen et al. (2019); the very shallow inflation source of the 2015 eruption (Doke et al. 2018; Kobayashi et al. 2018), which was interpreted as a vapor pocket located 150 m deep beneath the eruption center (surface elevation is approximately 1000 m above sea level) was determined by a high-resolution magnetotelluric survey (CSAMT) as a high resistivity zone within the apex of the bell-shaped conductive body (Yoshimura et al. 2018).
The 2015 eruption and unrest
The time sequence of the 2015 unrest and eruption of Hakone volcano was already summarized in Mannen et al. (2018). Here we briefly review this event. The onset of the 2015 unrest was first recognized in early April from increases in DLFs and the baseline length across the volcano detected by GNSS, which were interpreted as inflation of magma chamber due to addition of magma or magmatic fluid (Harada et al. 2018; Mannen et al. 2018; Yukutake et al. 2019) (Fig. 3). Then an earthquake swarm, a blowout of SPW, and an increase in the CO2/H2O ratio of the fumarole gas emitted near the future eruption center followed (Mannen et al. 2018; Ohba et al. 2019). Although the seismicity and the CO2/H2O ratio began decreasing after mid-May, a small phreatic eruption occurred on the morning of June 29 and lasted until the early morning of July 1 (Yukutake et al. 2017; Mannen et al. 2018). The eruption was seemingly triggered by formation of an open crack in the morning of June 29 near the surface (830–854 m above sea level) to deeper than 530 m above sea level as indicated by satellite InSAR and analysis of records obtained by broadband seismometers and tilt meters (Doke et al. 2018; Honda et al. 2018). However, chemical and component analyses of the erupted ash and water indicated a shallow (shallower than 850 m above sea level or 150 m deep from the surface) origin (Mannen et al. 2019). Even after the eruption, shallow and deep inflation (0.8 km and − 6.5 km above sea level, respectively) continued without a significant change in the inflation rate until August (Harada et al. 2018). The seismicity began in the central part of the caldera and then propagated to the peripheral areas (Fig. 4a)
The 2017 unrest episode
The 2017 unrest of Hakone volcano was subtle to detect based on seismicity. Seismicity rates in 2017 were generally low and only 242 earthquakes were detected in the Hakone area by the routine analysis of Hot Springs Research Institute. This annual number is within the range of that in an ordinary year without volcanic unrest after 2000 (Fig. 5). However, slight increases of seismicity were observed in mid-April and early May at sea level beneath Mt. Kintoki at the northern rim of the caldera (Fig. 4b). Concurrently, in early May, the baseline length crossing Hakone volcano began to increase slowly and continued to increase until early November (Fig. 3). Daita et al. (2020) reported an increase in the CO2/H2S (C/S) ratio of fumarole gas in Kamiyu, a steaming area north of Owakudani. The increases in C/S ratio have been observed accompanying the volcanic unrest; however, this increase in C/S ratio was not sharp and did not attenuate swiftly, unlike the increases in C/S ratio accompanying the 2013 and 2015 unrest episodes (Daita et al. 2019; Ohba et al. 2019) (Fig. 3). An increase in DLF events was also observed in early April (Fig. 3).
The 2019 unrest episode
The 2019 unrest episode at Hakone volcano appears to have begun with a slight increase in seismicity in March, which lasted until the end of October. A sudden seismic swarm occurred on May 18 beneath the western caldera rim (Fig. 4c). Although the location of swarm events was remote from Owakudani (3 km west and outside of the latest eruption centers), the number of earthquakes exceeded a set criterion and the Japan Meteorological Agency (JMA), which is in charge of volcano monitoring and alerting, announced a rise in Volcano Alert Level (VAL) from 1 to 2 for the volcano in the early morning of May 19, and the VAL2 continued until October 7. The baseline length crossing Hakone volcano began to increase in mid-March and continued until the beginning of August. The C/S ratio of Kamiyu also began to increase after the end of April; however, the increase in C/S ratio was not sharp and again did not attenuate quickly, similar to the 2017 unrest episode (Fig. 3). During the volcanic unrest, ratios of magmatic gases such as SO2 and HCl relative to H2S, which is a hydrothermal gas, increased significantly in Owakudani, although a significant net increase in magmatic gas was not observed by a Differential Optical Absorption Spectroscopy (DOAS) campaign (Fig. 7) (Abe et al. 2018). A slight increase in DLF events at the beginning was also observed during the unrest; however, interestingly, a far larger number of DLFs was observed in the latest phase of the unrest in late October (Fig. 3).
Field surveys after the eruption
Since the entry of researchers around the eruption center was allowed after the 2015 eruption (beginning in March 2016), we have monitored fumarole temperature and chemical compositions of volcanic gases and hot spring waters. Here we summarize these results.
New fumaroles, which emit superheated steam (> 100 ºC) were created in the eruption center area in 2015. Most of them were formed during the eruption but some formed during the unrest phase before the eruption or even long after the eruption. Until the present, steam temperatures have been routinely measured for at least 20 fumaroles, five of which are relatively intensive and long-lived and are shown in Fig. 6 (see Fig. 1c for the locations). The maximum temperature among them (164.3 ºC) were recorded on April 10, 2018 at the 15 − 1 fumarole, which is the fumarole created in the main crater formed during the eruption (Mannen et al. 2019). As shown in Fig. 6, steam temperatures are not decreasing for all fumaroles. However, a decline of the maximum temperature in the steaming area is detected, at a rate of ~ 7.7 ºC/yr, using infrared images of the whole area taken continuous since early 2016 (Harada 2018). These observations imply a waning trend in thermal activity in the eruption center area as a whole, while several stable fumaroles constantly emit superheated steam from depth (probably ~ 150 m deep; Mannen et al. 2019). It is noteworthy that no temperature change related to the volcanic unrest in 2017 and 2019 was apparant from these observations.
SO2 emission from Owakudani steaming area
We conducted DOAS surveys to quantify emission rates of SO2 from the Owakudani steaming area (Abe et al. 2018; Fig. 7). SO2 emission from Owakudani reached more than 100 t/day just after the 2015 eruption; however, the emission rate decreased rapidly and is now estimated to be approximately 10 t/day. The DOAS measurements contain large errors (up to 2–8 t/day) and no significant increase in SO2 during volcanic unrest episodes in 2017 and 2019 was measured.
An accurate chemical analysis of volcanic gas requires meticulous sampling and complicated lab procedures (Ozawa 1968), limiting the monitoring frequency. We thus launched a long-term test of simple gas measurements using a detector tube named Passive Dosi-tube (GASTEC Co. Ltd. (GASTEC 2018)). For this study, two sets of dositubes composed of H2S, SO2 and HCl sensors (GASTEC No. 5D, 4D and 14D respectively) were installed near (2–4 m) the 15 − 2 fumarole vent (Fig. 1c). A set of dositubes is directly exposed to the air while another set is installed in a 500 ml ventilated container filled with silica-gel granules (150 g) to prevent condensation of water in and around the dositubes. The dositubes were expected to measure ratios of volcanic gas in the atmosphere near the fumarole rather than a direct measurement of steam emitting from the volcano; thus, the observed ratio may be altered by processes in the atmosphere such as gas absorption by water droplets in the steam. However, we aimed to monitor obvious sequential changes in gas ratios with high frequency measurements. Since the dositube measures the volume fraction of the target gas in the atmosphere, the gas ratio is volumetric ratio (i.e., molar ratio) assuming an ideal gas. The sequential change of SO2/H2S and HCl/H2S ratios, both of which indicate the ratio of magmatic gas to hydrothermal gas, are shown in Fig. 8. Since the start of monitoring (March in 2018), SO2/H2S ratio show a constant decrease, and HCl remained nearly undetected until March 2019. However, both SO2/H2S and HCl/H2S ratios started to increase after March 2019 and peaked around June 2019. Since then, both SO2/H2S and HCl/H2S ratios showed a gradual decline; however, both ratios are still higher than those before March 2019 at the time of writing (mid 2020).
Near the Owakudani steaming area, volcanic gas is seeping out from the soil under a building floor (Loc. 3 in Fig. 1c). We made weekly measurements of CO2 and H2S in the ventilated air from under the building floor using detector tubes since the end the eruption (Fig. 9). Since the volcanic gas emitted from soil is not affected by nearby rainfall, and the building ventilation system enables almost constant flux of air from the subfloor, we can expect stable measurements of emitted gas. The soil gas shows a constant increasing in H2S while CO2 remains almost stable. Interestingly, both H2S and CO2 show subtle increases during the 2017 and 2019 unrest episodes. The CO2/H2S ratio (hereafter C/S ratio) decreased almost constantly after the 2015 eruption; however, slight increases can be recognized during the 2017 and 2019 unrest episodes.
Artificial hot springs
In Owakudani, artificial hot springs (AHS) have been created by mixing steam from SPW and spring water pumped up from the caldera floor to supply the local hotel industry (Mannen et al. 2019). AHS is not a diluted condensation of steam from the production well because less-soluble gases such as CO2 and H2S are barely absorbed in the water; however, its chemistry can be useful to monitor the hydrothermal system beneath the steaming area. We routinely analyzed the chemistry of AHSs from SPWs, No. 52 and No. 39 (hereafter SPW52 and SPW39 respectively; see Fig. 1c for locations). Here we show temporal changes of Cl and SO4 content, which are possibly magmatic in origin, and major anions in the AHSs (Fig. 10).
SPW52, which is 500 m deep and the well that blew out during the 2001 unrest, had shown a continuous decrease in Cl of the AHS since the beginning of monitoring; however, after early April just before the onset of the 2017 unrest, Cl content spiked. The Cl content again a showed constant decrease after the end of the 2017 unrest; however, it increased significantly when the baseline length across Hakone volcano started to increase (early May in 2019). The Cl content of SPW39 (413 m deep) also showed an unrest-related increase. For both AHSs, contrary to Cl, SO4 shows no significant change even during the volcanic unrest episodes.
River water from the eruption center
The eruption center area forms the headstream of the Owakuzawa river. Thus, water from Owakuzawa is presumably affected by volcanic gas and natural hot springs within the area, and its chemical components can reflect hydrothermal activity. Indeed, just after the 2015 eruption, water from the river showed a significant increase in Cl and SO4 (Fig. 10; Mannen et al. 2018). After the eruption, the Cl and SO4 contents showed constant decline; however, they apparently rose slightly at the beginning of the 2019 unrest. The Cl and SO4 changes related to the 2017 unrest were ambiguous (Fig. 10).
Seismicity related to volcanic unrest after the 2015 eruption
We examined the depth variation of seismic events within the hydrothermal system to detect any changes related to the eruption. Figure 11 shows the cumulative ratio of earthquakes within the hydrothermal system beneath Owakudani during volcanic unrest episodes in this century. Interestingly, the seismicity depth change from before and after the eruption seems to be significant. Before the 2015 eruption, epicenters of more than 60% of earthquakes in and around the Owakudani steaming area were located shallower than 2 km depth, while such earthquakes comprise less than 40% of the total after the eruption. This observation indicates that the fraction of shallower earthquakes declined significantly after the 2015 eruption.