Understanding the underground structures of fumarole fields is essential for deciphering lethal hydrothermal eruptions and elucidating the chemical evolution of hydrothermal fluids en route to surface hot springs and steams. We implemented a controlled-source audio-frequency magnetotellurics (CSAMT) survey in Owakudani, the largest fumarole field on Hakone volcano, Japan. The hydrothermal system lies beneath a caprock structure, detected as a low-resistivity zone in two-dimensional electrical resistivity cross-sections. The caprock formed a plateau-like convexity with a diameter of approximately 500 m, centered on an inactive, forest-covered steaming area west of the 2015 hydrothermal eruption center. The caprock top subsided before the 2015 eruption, possibly because of depressurization of the hydrothermal system. Notably, local but distinctive high-resistivity zones exist within the caprock, especially in the southern part of the survey area, which contains major fumaroles and hot springs. Such zones, along with the high resistivity detected beneath the caprock, can be interpreted as vapordominated. Therefore, the fumaroles and hot springs in Owakudani overlie a two-layered vapor-dominated zone, with the upper layer located within the caprock and the lower layer beneath it. Classical works suggested that meteoric water heated by steam from the vapor-dominated zone of the hydrothermal system forms hot springs in the fumarole area. However, the chemical composition of the artificial hot spring-fromed from a steam and meteoric water-suggests simple mixing cannot explain its composition. Therefore, the two-layered vapor-dominated system beneath the fumarolic zone potentially contributes to the chemical differentiation of the hydrothermal fluid.

We investigate the rupture dynamics of the Mw 7.7 Mandalay earthquake in Myanmar on March 28, 2025. The earthquake involved a near-vertical strike-slip rupture exceeding 400 km, with shallow slip up to 6 m. A previous inversion result revealed that the rupture initially propagated at supershear velocities (∼6 km/s) near the hypocenter. A unique video recording of surface rupture, captured 124 km south of the epicenter, indicates a deceleration to subshear speeds (∼3 km/s) before reaching the camera location. This deceleration is supported by observed fault-normal acceleration patterns. Satellite imagery also indicated a local minimum in slip (2–3 m) around 50 km south of the epicenter, suggesting a region of reduced stress drop, which likely caused the temporary deceleration. Beyond this point, the rupture appears to have re-established supershear propagation.

Owakudani (Owakuzawa Valley), the largest fumarole area in Hakone volcano, Japan, is a place where volcanic disasters occur frequently, such as the mudslide disaster that killed six people in 1910 and the phreatic eruption that destroyed hot spring supply facilities in 2015. Therefore, monitoring surface displacements that may lead to slope failure is important for protecting life and property. In this study, we estimated three-dimensional displacements from interferometric synthetic aperture radar (InSAR) analysis from the ALOS-2/PALSAR-2 satellite data and detected landslide displacement around Owakudani. Moreover, the temporal variation of the landslide displacement was clarified from the InSAR time series analysis results. The landslide displacement detected in this study corresponds to a pre-existing collapsed landform whose underground structure was unknown. Additionally, the results of finite element analysis clarified the subsurface deformation concentration zone, suggesting the displacement on the pre-existing sliding surface. The landslide displacement started after the 2015 phreatic eruption. Therefore, fluid injection during the eruption might be one of the triggers of the displacement. Moreover, the annual displacement variation corresponds to changes in precipitation, suggesting that underground pore pressure changes have accelerated the displacement. These results demonstrate that the series of our approach can contribute to understanding the causes and mechanisms of landslides for which underground geological surveys are inadequate.

AbstractRecent studies have proposed the contribution of aseismic slip (AS) to earthquake swarms. We investigated the role of AS in earthquake swarms that occurred in 2009, 2015, and 2019 at the Hakone volcano, central Japan, through highly resolved hypocenter distribution analysis, geodetic observation analysis, and identification of similar earthquakes. We observed diffusion-like migration of hypocenters during these swarms. The hydraulic diffusivity varied among the swarms, indicating differing dynamics. The 2015 swarm exhibited rapid hypocenter migration and significant crustal deformation, as revealed by the temporal sequences of tiltmeters near the swarm region. Right-lateral shear dislocation on fault planes could explain the crustal deformation observed in 2015, indicating that AS released approximately 90% of the moment. However, the 2009 swarm lacked evidence of significant AS contribution, indicating that the primary mechanism was fluid pressure diffusion. The substantial contribution of AS to the 2015 swarm might be attributed to increased fluid pressure due to the intrusion of hydrothermal fluid into the shallow part beneath the volcano during volcanic unrest. Our findings imply that the temporal and spatial patterns of seismicity can provide valuable insights into the underlying mechanics of earthquake swarms. Graphical abstract

“allrfstationlist.dat” contains the list of the used seismic stations. The four columns indicate the name, latitude, longitude, and altitude (m) of each station, respectively. “allrfevent.dat” contains the list of the used teleseismic events. From left to right, the 10 columns indicate the year, month (in number), day, hour, minute, and second of the origin time (Japan Standard Time), and the latitude (from –90 to 90), longitude (from –180 to 180), depth of the hypocenter, and magnitude of each event, respectively. “RFmoho.dat” contains the depth distribution of the Moho determined by our RF analysis. The third column indicates the depth (km) of the Moho at the given latitude (the second column) and longitude (the first column) “Tomo_depth_limited.txt” contains the depth distribution of the lower boundary of a layer with a P-wave velocity of 7.5–7.7 km/s in the model by Ishise et al. (2021), which was assumed as the Moho. The third column indicates its depth (km) at the given latitude (the first column) and longitude (the second column) “crustthickness_tomorf.dat” contains the thickness distribution of the crust of the Philippine Sea Plate determined from the geometry of its upper surface estimated by Hirose et al. (2008a, b) and Nakajima et al. (2009) and the Moho depth distribution shown in RFmoho.dat and Tomo_depth_limited.txt. The third column indicates the thickness (km) of the crust at the given latitude (the second column) and longitude (the first column). “RF” and “tomo” in the fourth column indicate the corresponding thickness determined based on the RF analysis and the model by Ishise et al. (2021), respectively.

From June 29 to July 1, 2015, a phreatic eruption occurred in Owakudani, the largest fumarole area in Hakone volcano, Japan. In this study, an interferometric synthetic aperture radar (InSAR) time series analysis of the Advanced Land Observing Satellite‐2 (ALOS‐2)/Phased Array type L‐band Synthetic Aperture Radar‐2 (PALSAR‐2) data was performed to measure deformation after the eruption. The results show that the central cones of the volcano have subsided since the eruption and its deflation source is located beneath the previously estimated bell‐shaped conductor, which is considered as a sealing layer confining a pressurized hydrothermal reservoir. Therefore, the InSAR results demonstrate the deflation of the hydrothermal system beneath the volcano. One possible cause of this deflation is compaction due to a decrease in pore pressure caused by rupture and fluid migration during and after the eruption.

Since a phreatic eruption is caused by ruptures in hydrothermal systems beneath volcanoes, detecting and monitoring a hydrothermal system can play an important role in predicting such an eruption. Interferometric Synthetic Aperture Radar (InSAR), which detects ground deformations over a large area, may be a key technology for use in various fields, as shown from the exponential growth of recent studies in terms of number and quality. The present contribution reviews surface deformations caused by the hydrothermal system of Hakone volcano, as detected by InSAR before, during, and after the 2015 eruption. The opening of the NW-SE-trending crack and localized uplift in the Owakudani fumarole area were captured by InSAR analyses during the 2015 unrest at Hakone volcano. Moreover, an InSAR time series analysis showed steady subsidence on the west side of the Owakudani fumarole area. Based on models explaining these surface displacements, the shallow hydrothermal system of Hakone volcano is characterized by NWSE to WNW-ESE-trending crack-shaped fluid supply paths and pocket-shaped fluid reservoirs. During the 2015 and previous phreatic eruptions, it is probable that fluid was supplied using the same crack-like path, implying that fluid was repeatedly supplied using the same structure. Therefore, in order to predict the occurrence of phreatic eruptions at Hakone volcano, it is necessary to monitor volcanic activity by taking into account these structures. The activity of Hakone volcano, including formations of these NW-SE to WNW-ESE-trending cracks, is dominated by a regional stress field. This stress field is caused by shear deformation due to plate motion occurring in this region; that is, the subducting Philippine Sea Plate, and the colliding Izu Peninsula.

Since the beginning of the twenty-first century, volcanic unrest has occurred every 2–5 years at Hakone volcano. After the 2015 eruption, unrest activity changed significantly in terms of seismicity and geochemistry. Like the pre- and co-eruptive unrest, each post-eruptive unrest episode was detected by deep inflation below the volcano (~ 10 km) and deep low frequency events, which can be interpreted as reflecting supply of magma or magmatic fluid from depth. The seismic activity during the post-eruptive unrest episodes also increased; however, seismic activity beneath the eruption center during the unrest episodes was significantly lower, especially in the shallow region (~ 2 km), while sporadic seismic swarms were observed beneath the caldera rim, ~ 3 km away from the center. This observation and a recent InSAR analysis imply that the hydrothermal system of the volcano could be composed of multiple sub-systems, each of which can host earthquake swarms and show independent volume changes. The 2015 eruption established routes for steam from the hydrothermal sub-system beneath the eruption center (≥ 150 m deep) to the surface through the cap-rock, allowing emission of super-heated steam (~ 160 ºC). This steam showed an increase in magmatic/hydrothermal gas ratios (SO2/H2S and HCl/H2S) in the 2019 unrest episode; however, no magma supply was indicated by seismic and geodetic observations. Net SO2 emission during the post-eruptive unrest episodes, which remained within the usual range of the post-eruptive period, is also inconsistent with shallow intrusion. We consider that the post-eruptive unrest episodes were also triggered by newly derived magma or magmatic fluid from depth; however, the breached cap-rock was unable to allow subsequent pressurization and intensive seismic activity within the hydrothermal sub-system beneath the eruption center. The heat released from the newly derived magma or fluid dried the vapor-dominated portion of the hydrothermal system and inhibited scrubbing of SO2 and HCl to allow a higher magmatic/hydrothermal gas ratio. The 2015 eruption could have also breached the sealing zone near the brittle–ductile transition and the subsequent self-sealing process seems not to have completed based on the observations during the post-eruptive unrest episodes.

The authors wish to make the following corrections to this paper [...]

Abstract The Izu collision zone, which is characterized by the collision between the Izu-Bonin arc and the Honshu arc, is located in the northernmost part of the Philippine Sea Plate. Particularly in the northeastern margin of the zone, numerous large earthquakes have occurred throughout history. To clarify the convergent tectonics of this zone related to the occurrence of these large earthquakes, in this study we collected and analysed Global Navigation Satellite System (GNSS) observation data from the Izu collision zone. The result verified that a shear deformation zone exists in the northeastern part of the Izu Peninsula, which agrees with the maximum shear directions in the left-lateral slip of the active Kita-Izu Fault Zone in the study area. In the shear deformation zone, the seismic activities both in the eastern part of the Tanzawa Mountains and the northeastern part of the Izu Peninsula may be related to each other because the temporal patterns of the seismic activity in both areas are correlated. Based on the relative motion between the main part of the Izu Peninsula and the subducting forearc of the Philippine Sea Plate, the shear zone can be regarded as a transition zone affected by both plate collision and subduction.

Monitoring of surface displacement by satellite-based interferometric synthetic aperture radar (InSAR) analysis is an effective method for detecting land subsidence in areas where routes of leveling measurements are undeveloped, such as mountainous areas. In particular, InSAR-based monitoring around well-developed hot spring resorts, such as those in Japan, is useful for conserving hot spring resources. Hakone Volcano is one of the major hot spring resorts in Japan, and many hot spring wells have been developed in the Owakudani fumarole area, where a small phreatic eruption occurred in 2015. In this study, we performed an InSAR time series analysis using the small baseline subset (SBAS) method and ALOS/PALSAR scenes of the Hakone Volcano to monitor surface displacements around the volcano. The results of the SBAS-InSAR time series analysis show highly localized subsidence to the west of Owakudani from 2006–2011 when the ALOS/PALSAR satellite was operated. The area of subsidence was approximately 500 m in diameter, and the peak rate of subsidence was approximately 25 mm/year. Modeling using a point pressure source suggested that the subsidence was caused by a contraction at approximately 700 m above sea level (about 300 m below the ground surface). The rate of this contraction was estimated to be 1.04 × 104 m3/year. Hot spring water is collected from a nearby well at almost the same depth as the contraction source, and its main dissolved ion component is chloride ions, suggesting that the hydrothermal fluids are supplied from deep within the volcano. The land subsidence suggests that the fumarole activity is attenuating due to a decrease in the supply of hydrothermal fluids from deeper areas.

Earthquake swarm activity in the Hakone volcano, which is located in the western part of Kanagawa Prefecture, Japan, has been observed every few years. Based on the observation of earthquakes and crustal deformation at the volcano, it is considered that the swarm activity is caused by hydrothermal activity in the shallow part of Hakone volcano (Daita et al., 2009; Yukutake et al., 2011). In the 2015 event, abnormal fumarolic was observed at the hot spring supply facility in Owakudani in early May. A local swelling was found around the hot spring supply facility, where abnormal fumarolic was found, by InSAR analysis of the data of ALOS-2/PALSAR-2. It is thought that the local swelling was caused by the hydrothermal water or volcanic gas in the very shallow part, directly underneath Owakudani, and it is presumed that it is closely related to the fumarolic activity in Owakudani. This study aims to clarify the mechanism of fumarolic activity and steam eruption at the Hakone volcano, by detecting the surface displacement caused by hydrothermal activity in the shallow part of the volcano. In this study, an InSAR time series analysis of ALOS/PALSAR data was performed. As a result, in Owakudani, local subsidence was detected at about 400-500 m west of the area where the local swelling was observed in the 2015 activity. Additionally, to clarify the displacement around Owakudani after the 2015 steam eruption, InSAR analysis of ALOS-2 / PALSAR-2 data was performed.

Global navigation satellite system data from Hakone volcano, central Japan, together with GEONET data from the Geospatial Information Authority of Japan, were used to investigate the processes associated with the volcanic activity in 2015, which culminated in a small phreatic eruption in late June 2015. Three deep and shallow sources, namely spherical, open crack, and sill, were employed to elucidate the volcanic processes using the observed GNSS displacements, and the MaGCAP-V software was used to estimate the volumetric changes of these sources. Our detailed analysis shows that a deep inflation source at 6.5 km below sea level started to inflate in late March 2015 at a rate of ~ 9.3 × 10 4 m 3 /day until mid-June. The inflation rate then slowed to ~ 2.1 × 10 4 m 3 /day and ceased at the end of August 2015. A shallow open crack at 0.8 km above sea level started to inflate in May 2015 at a rate of 1.7 × 10 3 m 3 /day. There was no significant volumetric change in the shallow sill source during the volcanic unrest, which is evident from interferometric synthetic aperture radar analysis. The inflation of the deep source continued even after the eruption without a significant slowdown in inflation rate. The inflation stopped in August 2015, approximately 1 month after the eruption ceased. This observation implies that the transportation of magmatic fluid to a deep inflation source (6.5 km) triggered the 2015 unrest. The magmatic fluid may have then migrated from the deep source to the shallow open crack. The phreatic eruption was then caused by the formation of a crack that extended to the surface. However, steam emissions from the vent area during and after the eruption were apparently insufficient to mitigate the internal pressure of the shallow open crack.

We successfully monitored the ground deformation of an eruption center during the 2015 phreatic eruption of Hakone volcano, Japan, using ground-based interferometric synthetic aperture radar (GB-InSAR). GB-InSAR has been developed and applied over the past two decades and enables the frequent (<10min) aerial monitoring of surficial deformation of structures and slopes. We installed a GB-InSAR 4days before the eruption of Hakone volcano on June 29, 2015, and monitored the ground deformation of an area where uplift was detected by a satellite InSAR. The ground deformation observed by the GB-InSAR began suddenly on the morning of June 29 almost coincident with the intrusion of hydrothermal fluid that was inferred by other geophysical observations. The hydrothermal crack is considered to have caused the eruption, which was known by an ash fall 5h later. The GB-InSAR results indicated a significant uplifted area which is approximately 100m in diameter, and new craters and fumaroles were created by the eruption in and around the area. The displacement reached up to a total of 45mm until the evening of June 29 and continued at least until the morning of July 1. During our observation, the displacement rate decreased twice, and the timing of each decrease seemed to correspond to the formation of new conduits as implied from geophysical observations.

Although the 2015 Hakone Volcano eruption was a small-scale phreatic eruption with a discharged mass of only about 100 tons, interferometric synthetic aperture radar successfully detected surface deformations related to the eruption. Inversion model of the underground hydrothermal system based on measured ground displacements by ALOS-2/PALSAR-2 images showed that a crack opened at an elevation of about 530–830 m, probably at the time of the eruption. A geomorphological analysis detected several old NW–SE trending fissures, and the open crack was located just beneath one of the fissures. Thus, the crack that opened during the 2015 eruption could have been a preexisting crack that formed during a more voluminous hydrothermal eruption. In addition, the inversion model implies that a sill deflation occurred at an elevation of about 225 m, probably at the time of the eruption. The deflation of sill-like body represents a preexisting hydrothermal reservoir at an elevation of 100–400 m, which intruded fluid in the open crack prior to eruption. The volume changes of the open crack and the sill were calculated to be 1.14 × 10 5 m 3 (inflation) and 0.49 × 10 5 m 3 (deflation), respectively. A very local swelling (about 200 m in diameter) was also detected at the eruption center 2 months before the eruption. The local swelling, whose rate in satellite line-of-sight was 0.7–0.9 cm/day during May 2015 and declined in June, had been monitored until the time of the eruption, when its uplift halted. This was modeled as a point pressure source at an elevation of about 900 m (at a depth of about 80–90 m from the ground surface) and is considered to be a minor hydrothermal reservoir just beneath the fumarolic field. Our analysis shows that the northernmost tip of the open crack reached within 200 m of the surface. Thus, it is reasonable to assume that the hydrothermal fluid in the open crack found a way to the surface and formed the eruption. Graphical abstract XXXX