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

Honda, Ryou; Yukutake, Yohei; Yoshïda, Akio; Harada, Masatake; Miyaoka, Kazuki; Satomura, Mikio

doi:10.1002/2014jb010978

Cited by 14 publications

(10 citation statements)

References 19 publications

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“…The orientation of a dyke associated with a magma intrusion is typically controlled by the axis of the maximum compressional stress (σ H ) in the region (e.g., Ukawa and Tsukahara 1996;Hayashi and Morita 2003). In this study, the estimated strike direction of the pressure source model (N316°E) was close to the microcrack orientation obtained by S-wave splitting analysis, which reflects σ H of the regional stress field (Honda et al 2014). Using focal mechanism data, Yukutake et al (2006) also estimated a consistent stress field around the Hakone volcano.…”

Section: What Does the Crack Opening Imply?supporting

confidence: 57%

“…They interpreted the low-Vp/Vs zone to indicate the presence of fluids and/or gas, while the high-Vp/Vs zone was thought to correspond to a midcrustal magma body. Honda et al (2014) reported that the anisotropic intensities from S-wave splitting in the shallow part of Hakone volcano are comparable with those observed near active faults. Collectively, these results suggest that the shallow part of the crust below Hakone volcano is highly fractured and may contain hydrothermal water.…”

Section: What Does the Crack Opening Imply?mentioning

confidence: 66%

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Precursory tilt changes associated with a phreatic eruption of the Hakone volcano and the corresponding source model

Honda

Yukutake

Morita

et al. 2018

Earth Planets Space

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The 2015 unrest of the Hakone volcano in Japan, which began on April 26, generated earthquake swarms accompanied by long-term deformation. The earthquake swarm activity reached its maximum in mid-May and gradually calmed down; however, it increased again on the morning of June 29, 2015. Simultaneously with the earthquake increase, rapid tilt changes started 10 s before 07:33 (JST) and they lasted for approximately 2 min. The rapid tilt changes likely reflected opening of a shallow crack that was formed near the eruption center prior to the phreatic eruption on that day. In this study, we modeled the pressure source beneath the eruption center based on static tilt changes determined using both tilt meters and broadband seismometers. In the best-fit model, the source depth was 854 m above sea level, and its orientation (N316°E) agreed with the direction of maximum compression estimated based on focal mechanism and S-wave splitting data. The extent of the crack opening was estimated to be 4.6 cm, while the volume change was approximately 1.6 × 10 5 m 3 . The top of the crack reached to approximately 150 m below the eruption center. Because the crack was too thin to be penetrated by magma, the crack opening was attributed to the intrusion of hydrothermal water. This intrusion of hydrothermal water may have triggered the phreatic eruption. Reverse polarity motion with respect to that expected from crack opening was recognized in 1 Hz tilt records during the first 20 s of the intrusion of hydrothermal water. This motion, not the subsidence of volcanic edifice, was responsible for the observed displacement. which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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Section: What Does the Crack Opening Imply?supporting

confidence: 57%

Section: What Does the Crack Opening Imply?mentioning

confidence: 66%

Precursory tilt changes associated with a phreatic eruption of the Hakone volcano and the corresponding source model

Honda

Yukutake

Morita

et al. 2018

Earth Planets Space

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“…They inferred that small fault networks are developed in the caldera of Hakone volcano and that the earthquake swarms represent activity on individual small faults. Honda et al [] indicated that anisotropic intensities from S ‐wave splitting in the shallow part of Hakone volcano are highly comparable with those observed near active faults. Collectively, these results suggest that the shallow part of the crust below Hakone volcano is highly fractured.…”

Section: Discussionmentioning

confidence: 99%

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

et al. 2015

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High-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 CO 2 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.

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“…10a). Honda et al (2014) reported a noticeable decrease in anisotropic intensity at Hakone volcano based on S-wave splitting analysis during the 2001 swarm activity, and concluded that the decrease in the anisotropic intensity resulted from changes in the stress caused by the opening of the shallow cracks. The results imply that changes in the strain and stress caused by the deformation sources could be a major factor influencing the changes in the subsurface structure.…”

Section: Discussionmentioning

confidence: 99%

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

Yukutake

Ueno

Miyaoka³

2016

Prog. in Earth and Planet. Sci.

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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.

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

Cited by 14 publications

References 19 publications

Precursory tilt changes associated with a phreatic eruption of the Hakone volcano and the corresponding source model

Precursory tilt changes associated with a phreatic eruption of the Hakone volcano and the corresponding source model

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

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

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