Joint seismological–magnetotelluric investigation of shallow and implosive non-DC and DC earthquakes beneath the gravitationally unstable Heisei-Shinzan Lava Dome, Unzen Volcano, Japan

Hashimoto, Tasuku M.; Aizawa, Katsuo; Hayashida, Yuto; Yuasa, Yuhei; Matsushima, Takeshi; Yamamoto, Yuto; Tsukamoto, Kaori; Miyano, Kanta; Matsumoto, Satoshi; Shimizu, Hiroshi

doi:10.1016/j.jvolgeores.2020.107066

Cited by 3 publications

(2 citation statements)

References 38 publications

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“…This observation is in line with Kendrick et al (2021), who found these banded cataclastic samples were susceptible to strain localization during loading, which increased their chance of unpredictable failure. However, UNZ9a, which was also foliated but cut in such a way that the bands were parallel to the principal applied stress in both compression and tension (see et al, 2019), and has numerous shallow structural elements (e.g., Hashimoto et al, 2020). If oriented unfavorably (e.g., dips with the slope), rock fabric or heterogenous structures may make the dome more prone to failure.…”

Section: Discussionmentioning

confidence: 99%

“…Between 2007 and 2014, EDM and ground-based synthetic aperture radar measured non-uniform eastsouth movement of Lobe 11B with displacements between 25-65 mm/year, which was attributed to differences in the deformability of the materials underlying Lobe 11B (Satou et al, 2014). Lasting heat sources are also evident from ongoing fumarolic activity that has continued to the present day (Figure 1e; Hashimoto et al, 2020), causing local thermal and chemical alteration (Almberg et al, 2008) that could contribute to instability via mechanical weakening of material as primary minerals transform into weaker secondary products such as clay minerals (e.g., Ball et al, 2015). At the time of writing, the risk of collapse of Lobe 11 is considered high by the Committee of Survey and Countermeasure on Lava Dome Collapse in Mt.…”

Section: Mt Unzen Lava Dome Growth and Structural Instabilitymentioning

confidence: 99%

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Laboratory Simulation of Earthquake-Induced Damage in Lava Dome Rocks

Schaefer

Kendrick

Lavallée

et al. 2023

tekt

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Earthquakes can impart varying degrees of damage and permanent, inelastic strain on materials, potentially resulting in ruptures that may promote hazards such as landslides and other collapse events. However, the accumulation of damage in rocks under the frequency and amplitude of shaking experienced during earthquake events is rarely systematically measured due to technical limitations. Here, we characterize damage evolution during laboratory experiments on a suite of dacitic rocks from Unzen volcano, Japan, to help resolve accumulated damage and landslide susceptibility of lava domes during regional earthquake events. Damage was imparted during slow (time-dependent creep) and fast (stress-oscillation earthquake simulations) uniaxial loading in compression and tension. Damage evolution is approximated from strain during experiments; all samples accumulate strain during earthquake events, but microfracture-dominated samples tend to be more susceptible to damage than vesicle-dominated samples. The orientation of existing fabrics with respect to loading direction dictates the magnitude of strain accumulation under load oscillations. During each “earthquake” experiment of multiple dynamic stress-oscillations, samples accumulate inelastic strain. The strain imparted during each successive event is initially high and then reduces after 5-7 events, except when stressing results in failure. The strain rate during phases of intermittent stressing tends to be higher than prior to them. Understanding the accumulation of damage and the potential for brittle failure of rocks subjected to earthquakes can help define the origin and timing of certain landslides, rockfalls, lava dome collapses, and other failure events.

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Section: Discussionmentioning

confidence: 99%

Section: Mt Unzen Lava Dome Growth and Structural Instabilitymentioning

confidence: 99%

Laboratory Simulation of Earthquake-Induced Damage in Lava Dome Rocks

Schaefer

Kendrick

Lavallée

et al. 2023

tekt

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Seismic velocity structure of Unzen Volcano, Japan, and relationship to the magma ascent route during eruptions in 1990–1995

Miyano

Aizawa

Matsushima

et al. 2021

Sci Rep

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Subsurface structures may control the migration of magma beneath a volcano. We used high-resolution seismic tomography to image a low- P-wave velocity (Vp) zone beneath Unzen Volcano, Japan, at depths of 3–16 km beneath sea level. The top of this low-Vp zone is located beneath Mt. Fugendake of Unzen volcano, which emitted 0.21 km3 of dacitic magma as lava domes and pyroclastic flows during eruptions in 1990–1995. Based on hypocenter migrations prior to the 1990–1995 eruptions and modeled pressure source locations for recorded crustal deformation, we conclude that the magma for the 1990–1995 eruptions migrated obliquely upward along the top of the low-Vp zone. As tectonic earthquakes occurred above the deeper part of the low-Vp zone, the deep low-Vp zone is interpreted to be a high-temperature region (> 400 °C) overlying the brittle–ductile transition. By further considering Vs and Vp/Vs structures, we suggest that the deeper part of the low-Vp zone constitutes a highly crystalized magma-mush reservoir, and the shallower part a volatile-rich zone.

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Magma transport along structural boundaries in the upper crust: insights from broad-band magnetotelluric constraints on the structure beneath Unzen volcano, Japan

Triahadini

Aizawa

Hashimoto

et al. 2023

Geophysical Journal International

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Summary Unzen volcano, located on Shimabara Peninsula, Nagasaki, Japan, is an active volcano that has been intensively monitored since 1989, one year before the most recent eruption in 1990–1995. Previous earthquake and surface deformation studies have revealed that magma is transported obliquely from a magma reservoir beneath Tachibana Bay, to the west of Shimabara Peninsula. Here we conduct broadband magnetotelluric (MT) surveys at 99 sites around Shimabara Peninsula to investigate the crustal structure beneath Unzen volcano that is related to magma migration. A three-dimensional resistivity model that is constructed from 25 broadband MT sites and 45 telluric sites shows a broad high-resistivity zone beneath Shimabara Peninsula and low-resistivity zones to the west and east of the peninsula. An unexpected observation is the spatial alignment of the high-resistivity zone with a seismic low-velocity zone at 3–15 km depth. Quantitative analysis indicates this high-resistivity zone contains <0.7% melt under the assumption that the melt is stored in a good porosity network, while <11% melt in relatively poor pore network. We infer this high-resistivity, low-velocity zone to be a highly crystallized mush zone with low permeability. The hypocenters and pressure sources of the 1990–1995 eruption are distributed along the boundary between the high- and low-resistivity zones beneath the western part of the peninsula. We therefore conclude that the magma migrated along a structural boundary that possessed a relatively high permeability. Previous studies have suggested that eruptible magma is usually transported vertically upward through the center of the mush zone, whereas the present results reveal that magma can be transported along the upper boundary of a highly crystallized mush zone.

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Joint seismological–magnetotelluric investigation of shallow and implosive non-DC and DC earthquakes beneath the gravitationally unstable Heisei-Shinzan Lava Dome, Unzen Volcano, Japan

Cited by 3 publications

References 38 publications

Laboratory Simulation of Earthquake-Induced Damage in Lava Dome Rocks

Laboratory Simulation of Earthquake-Induced Damage in Lava Dome Rocks

Seismic velocity structure of Unzen Volcano, Japan, and relationship to the magma ascent route during eruptions in 1990–1995

Magma transport along structural boundaries in the upper crust: insights from broad-band magnetotelluric constraints on the structure beneath Unzen volcano, Japan

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