Abstract
We present detailed maps of local-scale 3D deformation preceding the 2018 phreatic eruption at Iwo-yama volcano (south of Kyushu Island, Japan), using a combination of airborne and spaceborne Interferometric Synthetic Aperture Radar (InSAR) data. The 3D and 2.5D deformation maps obtained at different periods allow us to successfully track their spatiotemporal evolution and to infer the transition of subsurface conditions responsible for the precursory deformation observed from 2014 to 2018. From 2014 to 2016, ground inflation depicted an axisymmetric pattern with the maximum displacement at the center of the deformed area. However, from 2016 to 2018, an inflation peak moved to the southern edge of the area deformed during 2014–2016 and became more localized, which was close to the newly generated vents in the 2018 eruption. Modeling of the inflations suggests that pressurization within a crack at a depth of 150 m beneath the Iwo-yama geothermal area caused the 2014–2016 deformation and had continued until the 2018 eruption. Modeling results highlight the persistence of the local ground inflation pattern just above the southern edge of the crack, which suggests the presence of a shallower inflation source contributing to the local inflation. Consequently, we interpret the sequence of these deformations as follows: from 2014, deeper-rooted fluid started to inject into a fluid-saturated crack at 150-m depth, which caused the 2014–2016 deformation. Then, after 2016, the crack inflation continued because of the continuous fluid injection and formed another pressurized part directly above the southern tip of the crack. Additionally, the results of the time-series analysis of the satellite InSAR data revealed that the local inflation started around April 2017 for which thermal activity including a mud emission became pronounced around the location of the local inflation. As a result of an episodic increase in supply rate of magmatic fluids from a deep magma reservoir from early 2018, a phreatic eruption finally occurred in the vicinity of the most deformed point, providing a clue for predicting future eruption sites, as was also observed in the Hakone 2015 eruption.
Highlights
High spatial resolution and no requirement of groundbased instruments for satellite-based InterferometricSynthetic Aperture Radar (InSAR) make it a standard tool for studies on volcanic deformations caused by deeper-rooted magmatic activities (e.g. Wicks et al 2002; Miyagi et al 2013; Pinel et al 2014; Sigmundsson et al 2015; Lundgren et al 2017) and shallow hydrothermal systems (e.g., Hamling 2017; Doke et al 2018; Kobayashi 2018; Narita and Murakami 2018)
The selection of interferograms used for the New Small Baseline Subset (NSBAS) analysis is based on the following criteria: (i) the perpendicular baseline is less than 300 m (Fig. 2), (ii) the mean value of spatial coherence over an interferogram is larger than 0.3, and (iii) the interferograms are not affected by heavy atmospheric noise
The airborne interferograms from the SN and EW paths between 2014 and 2016 (Fig. 4a, c) have incoherent lines, likely due to fluctuations in the flight path caused by sudden changes in wind speed. These incoherent areas do not affect the 3D decomposition of the deformation around Iwo-yama because they are separated from the deformed region by ~ 500 m
Summary
High spatial resolution and no requirement of groundbased instruments for satellite-based InterferometricSynthetic Aperture Radar (InSAR) make it a standard tool for studies on volcanic deformations caused by deeper-rooted magmatic activities (e.g. Wicks et al 2002; Miyagi et al 2013; Pinel et al 2014; Sigmundsson et al 2015; Lundgren et al 2017) and shallow hydrothermal systems (e.g., Hamling 2017; Doke et al 2018; Kobayashi 2018; Narita and Murakami 2018). The Multiple Aperture Interferometry (MAI) and pixel offset methods enable us to retrieve 3D deformation (e.g., Tobita et al 2001; Bechor and Zebker 2006; Jo et al 2015), the spatial resolution and precision of these methods are limited. These deficiencies prevent us from applying these techniques for retrieval of 3D components to small deformation both in scale (< 1 km) and magnitude (< 10 cm). At Kirishima Iwo-yama volcano, in southern Japan, local-scale deformation with a maximum displacement of ~ 15 cm has been observed over an area of ~ 500 m from 2014 to 2018 (Meteorological Research Institute 2018). This study attempts to estimate high-quality 3D deformation at Iwo-yama using a couple of airborne and spaceborne InSAR datasets; repeated observations make four-dimensional analysis possible
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