Hakone Volcano Research Articles

A3-14 箱根火山2015年噴火の火口近傍地域の観察(箱根火山,口頭発表)

A3-14 箱根火山2015年噴火の火口近傍地域の観察(箱根火山,口頭発表)

A34 無斑晶質火山岩による箱根火山のマグマ液組成変化の検討(マグマプロセス,口頭発表)

A34 無斑晶質火山岩による箱根火山のマグマ液組成変化の検討(マグマプロセス,口頭発表)

A3-12 箱根火山2015年噴火の推移(箱根火山,口頭発表)

A3-12 箱根火山2015年噴火の推移(箱根火山,口頭発表)

B3-10 箱根火山中央火口丘最新期安山岩質溶岩に含まれる集合斑晶の成因(マグマ・岩石学,口頭発表)

B3-10 箱根火山中央火口丘最新期安山岩質溶岩に含まれる集合斑晶の成因(マグマ・岩石学,口頭発表)

B06 フィリピン海プレート衝突付加地域での古応力場の時間的変化と火山活動の変化-箱根火山を例にして

B06 フィリピン海プレート衝突付加地域での古応力場の時間的変化と火山活動の変化-箱根火山を例にして

P90 箱根火山2015年噴火の火口と噴気孔 : 形成・崩壊・活動(ポスターセッション)

P90 箱根火山2015年噴火の火口と噴気孔 : 形成・崩壊・活動(ポスターセッション)

A45 箱根火山外輪山の形成史(火山発達史と地質,口頭発表)

A45 箱根火山外輪山の形成史(火山発達史と地質,口頭発表)

A3-17 最近の箱根火山のマグマ供給系と噴火のタイプ(箱根火山,口頭発表)

A3-17 最近の箱根火山のマグマ供給系と噴火のタイプ(箱根火山,口頭発表)

Two Types of Glass Shards within the Owakidani Tephra Group Deposit, Hakone Volcano:Constraints on Their Sources and Ages

Two Types of Glass Shards within the Owakidani Tephra Group Deposit, Hakone Volcano:Constraints on Their Sources and Ages

Determination of temporal changes in seismic velocity caused by volcanic activity in and around Hakone volcano, central Japan, using ambient seismic noise records

Autocorrelation functions (ACFs) for ambient seismic noise are considered to be useful tools for estimating temporal changes in the subsurface structure. Velocity changes at Hakone volcano in central Japan, where remarkable swarm activity has often been observed, were investigated in this study. Significant velocity changes were detected during two seismic activities in 2011 and 2013. The 2011 activity began immediately after the 2011 Tohoku-oki earthquake, suggesting remote triggering by the dynamic stress changes resulting from the earthquake. During the 2013 activity, which exhibited swarm-like features, crustal deformations were detected by Global Navigation Satellite System (GNSS) stations and tiltmeters, suggesting a pressure increment of a Mogi point source at a depth of 7 km and two shallow open cracks. Waveforms that were bandpass-filtered between 1 and 3 Hz were used to calculate ACFs using a one-bit correlation technique. Fluctuations in the velocity structure were obtained using the stretching method. A gradual decrease in the velocity structure was observed prior to the 2013 activity at the KOM station near the central cone of the caldera, which started after the onset of crustal expansion observed by the GNSS stations. Additionally, a sudden significant velocity decrease was observed at the OWD station near a fumarolic area just after the onset of the 2013 activity and the tilt changes. The changes in the stress and strain caused by the deformation sources were likely the main contributors to these decreases in velocity. The precursory velocity reduction at the KOM station likely resulted from the inflation of the deep Mogi source, whereas the sudden velocity decrease at the OWD station may reflect changes in the strain caused by the shallow open-crack source. Rapid velocity decreases were also detected at many stations in and around the volcano after the 2011 Tohoku-oki earthquake. The velocity changes may reflect the redistribution of hydrothermal fluid in response to the large stress perturbation caused by the 2011 Tohoku-oki earthquake.

Dissemination of information about the 2015 eruption of Hakone volcano in Hakone Geopark

Dissemination of information about the 2015 eruption of Hakone volcano in Hakone Geopark

Igneous petrology on the bases of rock forming minerals in the Hakone volcano.

Igneous petrology on the bases of rock forming minerals in the Hakone volcano.

P94 箱根山大涌谷周辺における全磁力繰り返し観測(ポスターセッション)

P94 箱根山大涌谷周辺における全磁力繰り返し観測(ポスターセッション)

P95 2015年箱根火山活動に伴う地殻変動とその変動源の推定(ポスターセッション)

P95 2015年箱根火山活動に伴う地殻変動とその変動源の推定(ポスターセッション)

A magma‐hydrothermal system beneath Hakone volcano, central Japan, revealed by highly resolved velocity structures

AbstractHigh‐resolution images of subsurface structures are necessary to understand the transport processes of crustal fluids from deep magma sources and their relationship to earthquake swarms in active volcanic regions. Based on a seismic tomography approach, we have developed a new model for the magma‐hydrothermal system beneath Hakone volcano, central Japan, where shallow earthquake swarms and crustal deformation associated with inflation of an open‐crack source are often observed. By applying travel‐time data for local earthquakes to a tomographic inversion, we obtained highly resolved seismic velocity structures that show a region of low P‐wave velocity (Vp), low S‐wave velocity (Vs), and high Vp/Vs ratios at depths of 10–20 km beneath the volcano, corresponding to the location of the open‐crack source. We suggest that the high Vp/Vs ratios represent a deep magma chamber with a high concentration of melt and/or fluids. Deep low‐frequency earthquakes, located just beneath this high Vp/Vs zone, may indicate that magmatic fluids are supplied from below. Above the high Vp/Vs zone, a region of low Vp, low Vs, and low Vp/Vs ratios exists at depths of 3–10 km, suggesting the presence of crack‐filled water or CO2 supplied from the inferred deep magma chamber. Many earthquake swarms occur in this low Vp/Vs zone, indicating that crustal fluids play an important role in generating the swarms. Similar relationships between magma reservoirs, overlying hydrothermal systems, and swarm activity have been reported from other volcanic areas and thus may be a ubiquitous feature beneath active volcanoes.

神奈川県箱根火山における活発な群発地震活動(2015)

神奈川県箱根火山における活発な群発地震活動(2015)

Stress‐induced spatiotemporal variations in anisotropic structures beneath Hakone volcano, Japan, detected by S wave splitting: A tool for volcanic activity monitoring

AbstractHakone volcano, located at the northern tip of the Izu‐Mariana volcanic arc, Japan, has a large caldera structure containing numerous volcanic hot springs. Earthquake swarms have occurred repeatedly within the caldera. The largest seismic swarm since the commencement of modern seismic observations (in 1968) occurred in 2001. We investigated the anisotropic structure of Hakone volcano based on S wave splitting analysis and found spatiotemporal changes in the splitting parameters accompanying the seismic swarm activity. Depth‐dependent anisotropic structures are clearly observed. A highly anisotropic layer with a thickness of ~1.5 km is located beneath the Koziri (KZR) and Kozukayama (KZY) stations. The anisotropic intensity in the region reaches a maximum of 6–7% at a depth of 1 km and decreases markedly to less than 1% at a depth of 2 km. The anisotropic intensity beneath Komagatake station (KOM) decreases gradually from a maximum of 6% at the surface to 0% at a depth of 5 km but is still greater than 2.5% at a depth of 3 km. At KZY, the anisotropic intensity along a travel path of which the back azimuth was the south decreased noticeably after the 2001 seismic swarm activity. During the swarm activity, tilt meters and GPS recorded the crustal deformation. The observed decrease in anisotropic intensity is presumed to be caused by the closing of microcracks by stress changes accompanying crustal deformation near the travel path.

Post-caldera geology of Gora Region in Hakone Volcano Group, Japan

Post-caldera geology of Gora Region in Hakone Volcano Group, Japan

Remotely triggered seismic activity in Hakone volcano during and after the passage of surface waves from the 2011 M9.0 Tohoku-Oki earthquake

Immediately after the March 11, 2011, M9.0 Tohoku-Oki earthquake, seismic activity increased remarkably beneath Hakone volcano, central Japan, at an epicentral distance of 450km. The heightened seismicity was initiated during the passage of the large-amplitude surface waves from the main shock and continued over the subsequent 2 months. We obtained hypocenters and focal mechanisms of the seismic sequence, with the aim of clarifying the physical mechanism responsible for the remotely triggered seismicity. We used data from a dense seismic network containing 56 online permanent and offline temporary stations in and around the Hakone volcano. We found that the earthquakes that occurred during the passage of the surface waves are located at the lower depth limit of ordinary seismicity in the caldera. These earthquakes have larger magnitudes than both the ordinary seismicity prior to the Tohoku-Oki earthquake and the seismicity triggered after the passage of the surface waves. The focal mechanism that we determined is a strike–slip fault type with the P-axis in the NW–SE direction, which is consistent with the focal mechanisms of earthquakes that occurred after the passage of the surface waves and the tectonic stress field in the region. We also tried to detect missing events that occurred immediately after the passage of the surface waves, by using a waveform correlation technique. The detected events are distributed near the hypocenters of the earthquakes that occurred during the passage of the surface waves. The origin times of the first four events after the arrival of surface waves are consistent with the phases of the decrease in normal stress generated by the surface waves. The results suggest that the changes in dynamic stress due to the surface waves from the 2011 Tohoku-Oki earthquake contributed significantly to the initiation of the sequence of triggered seismic activity. Assuming that normal stress changes on the faults did play an important role in the triggering of earthquakes, we propose that fluid flow induced by the oscillation of permeability on the faults is the main mechanism for the initiation of post-Tohoku-Oki earthquakes beneath the Hakone volcano.

Spatial distribution of crack structure in the focal area of a volcanic earthquake swarm at the Hakone volcano, Japan

We have performed shear wave splitting analyses for seismograms recorded at stations located just above, and outside, the focal area of the earthquake swarm at the Hakone volcano, Japan, in August 2009. Average values of the direction of faster split shear wave polarization (Φ) at two stations above the focal area correspond to each focal alignment of the earthquake swarm. In contrast, average values of Φ at three stations outside the focal area correspond to the direction of the maximum horizontal compressional stress. We found that the average values of the time lag between the two split shear waves inside the focal area are relatively high compared with those outside the focal area. These facts suggest that cracks with a high density aligned parallel to the faults of the earthquake swarm in the focal area. Crustal fluid was selectively injected into this pre-existing cracked media accompanied by effective normal stress reduction in the cracks, resulting in the earthquake swarm.