Abstract
We aimed to perform three-dimensional imaging of the underlying geothermal system to a depth of 2 km using magnetotellurics (MT) at around the Yugama crater, the Kusatsu–Shirane Volcano, Japan, which is known to have frequent phreatic eruptions. We deployed 91 MT sites focusing around the peak area of 2 km × 2 km with typical spacings of 200 m. The full tensor impedances and the magnetic transfer functions were inverted, using an unstructured tetrahedral finite element code to include the topographic effect. The final model showed (1) low-permeability bell-shaped clay cap (C1) as the near-surface conductor, (2) brine reservoir as a deep conductor (C3) at a depth of 1.5 km from the surface, and (3) a vertical conductor (C2) connecting the deep conductor to the clay cap which implies an established fluid path. The columnar high-seismicity distribution to the east of the C2 conductor implies that the flushed vapor and magmatic gas was released from the brine reservoir by breaking the silica cap at the brittle–ductile transition. The past magnetization/demagnetization sources and the inflation source of the 2014 unrest are located just below the clay cap, consistent with the clay capped geothermal model underlain by brine reservoir. The resistivity model showed the architecture of the magmatic–hydrothermal system, which can explain the episodic volcanic unrest.
Highlights
The Kusatsu–Shirane volcano is an active andesitic– dacitic quaternary volcano in Central Japan (Fig. 1a, b)
The cause of phreatic eruptions is related to the changes in transient pressure in the subsurface hydrothermal systems, which is mainly due to the intense temperature increase of shallow aquifers (Kobayashi et al 2018)
We focus on the peak area of the Kusatsu–Shirane volcano and will present a three-dimensional resistivity distribution from a densely sampled magnetotelluric data
Summary
The Kusatsu–Shirane volcano is an active andesitic– dacitic quaternary volcano in Central Japan (Fig. 1a, b). The activity of the volcano started at 0.5–0.6 Ma with magmatic eruptions and pyroclastic flows. After a long dormancy of 0.2–0.3 Ma, magmatic activity restarted from 16 Ka and ended at 1.5 Ka (Terada 2018). The subsequent historically known activities of the Kusatsu–Shirane volcano since 1805 were phreatic eruptions around. Phreatic eruptions are local-scale volcanic phenomena that include explosions of confined volumes of gas and steam, but exclude those directly involved with magma (Stix and de Moor 2018). The cause of phreatic eruptions is related to the changes in transient pressure in the subsurface hydrothermal systems, which is mainly due to the intense temperature increase of shallow aquifers (Kobayashi et al 2018). A phreatic eruption has a comparatively small impact radius; the concentrated gases can be emitted, which spread over a wide area (e.g., Tazieff 1989), and the mechanical energy of eruptions can provoke fatal ejecta near the crater (e.g., Tseng et al Earth, Planets and Space (2020) 72:161
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