Shallow hydrothermal reservoir inferred from post-eruptive deflation at Ontake Volcano as revealed by PALSAR-2 InSAR

Abstract

In 2014, a phreatic eruption occurred at the Ontake Volcano in Central Japan causing multiple deaths and missing persons. Interferometric Synthetic Aperture Radar data showed local-scale subsidence around the newly created eruptive vents after the eruption. Source modeling resulted in a nearly spherical deflation source emplaced at a depth of 500 m below the vents reflecting post-eruptive depressurization in a shallow hydrothermal reservoir. The cumulative deflation volume reached 7 × 10 5 m 3 3 years after the eruption. Comparison between our source model and GNSS data indicates that this shallow reservoir could have formed by 2007 and remained stable until the 2014 eruption. The absence of significant syn-eruptive subsidence indicates that the shallow reservoir was not the main water source driving the phreatic eruption. Under simple assumptions, mass balance between the shallow reservoir and the discharge plume from the vents indicates most of the water contained in the plume comes from greater depth than the shallow reservoir. To constrain the post-eruptive process, it is necessary to track not only deformation but also plume discharge for 3 years after the eruption.

Quantitative relationship between plume emission and multiple deflations after the 2014 phreatic eruption at Ontake volcano, Japan

“…The most intense part of the eruption continued for 6 h on 27 September 2014, creating a series of linearly aligned crater chains. ALOS-2 InSAR observations revealed local-scale (~ 4 km) subsidence around new craters (Narita and Murakami 2018), with simultaneous plume emission occurring (Yamaoka et al 2016) and continuing as of July 2017 (Fig. 1).…”

“…Several studies have investigated ground deformations, usually subsidence, after phreatic eruptions, and most of these eruptions did not evolve into magmatic eruptions within the time frame of several years (Jousset et al 2000;Lu et al 2002;Nakaboh et al 2003;Hamling et al 2016;Doke et al 2018;Miller et al 2018;Narita and Murakami 2018). These subsidence are often accompanied by a continuous emission of water-vapor plumes, potentially linked with depressurization due to fluid discharge from shallow aquifers or hydrothermal reservoirs (Lu et al 2002;Nakaboh et al 2003;Miller et al 2018).…”

“…The deformation was accompanied by active emitting vapor at the surface. Narita and Murakami (2018) speculated that the shallow deflation derived by InSAR might be caused by the mass discharge, but a quantitative relationship between the deflation sources and the emitting plume has remained unclear.…”

“…1). The subsidence was attributed to a deflation source at a depth of 500 m below the vents, which might be caused by fluid discharge from a shallow, hydrothermal reservoir (Narita and Murakami 2018). However, it was observed that the shallow source could not account for all of the observed change in GNSS baseline length between Tanohara and Ochiai stations ( Fig.…”

“…The purpose of this study was to investigate the quantitative relationship between the shallow and deep deflation sources and the emitting plume after the 2014 Mount Ontake eruption. We focused on the detailed analysis of the relationship between mass discharge and deformation using new plume data that were made available to us, and compared it with previously discussed source models of shallow deflation (Narita and Murakami 2018). We also examined the GNSS data that may reflect a deep deflation.…”