InSAR analysis for detecting the route of hydrothermal fluid to the surface during the 2015 phreatic eruption of Hakone Volcano, Japan

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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.

Section: Tilt-change Data Recorded By Broadband Seismometersmentioning

confidence: 99%

Section: Pressure Source Modelmentioning

confidence: 99%

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

Honda

Yukutake

Morita

et al. 2018

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“…During the 2015 Hakone eruption and unrest, InSAR data were critically important for planning mitigation measures ) and has to date resulted in three prominent literature contributions related to InSAR, two in this special issue Kuraoka et al 2018) and one in another journal (Kobayashi et al 2018). Doke et al (2018) detected the open crack that was formed by the 2015 eruption by analysis of ground deformation observed by satellite InSAR. The eruption center was formed at the northern end of the open crack.…”

Section: Insarmentioning

confidence: 99%

“…Since old craters align on the ground surface just above the open crack, the 2015 eruption was interpreted as a reactivation of a pre-existing crack that was formed by earlier eruptions. The detailed analysis of Doke et al (2018) detected a sill-like deflation source beneath the crack, which is considered the source of hydrothermal fluid that formed the open crack. The pre-eruptive pressurization of the hydrothermal system of the volcano was also detected as local uplift by the InSAR analysis.…”

Section: Insarmentioning

confidence: 99%

Special issue “Towards forecasting phreatic eruptions: examples from Hakone volcano and some global equivalents”

Mannen

Roman

Leonard

et al. 2019

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“…Our preliminary evaluation suggested a single inflation source at ~ 5.4 km depth that reasonably reconstructed the surface dislocation (root-mean-square (RMS) error = 1.2 × 10 −3 ). However, an interferometric synthetic aperture radar (InSAR) analysis implied that a crack opened at shallow depths in the volcano during the phreatic eruption (Doke et al 2018a). They also implied the existence of a deflation sill at ~ 250 m above sea level, which appears to deflate during the eruption and is assumed to be the source of the hydrothermal fluid that erupted.…”

Section: Gnss Data and Location Of Inflation Sourcesmentioning

confidence: 99%

Temporal changes in inflation sources during the 2015 unrest and eruption of Hakone volcano, Japan

Harada

Doke

Mannen

et al. 2018

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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. 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.