Precursory crater contraction associated with the 2017 eruption of Shinmoe-dake volcano (Japan) detected by PALSAR-2 and Sentinel-1 InSAR

Precursory crater contraction associated with the 2017 eruption of Shinmoe-dake volcano (Japan) detected by PALSAR-2 and Sentinel-1 InSAR | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Precursory crater contraction associated with the 2017 eruption of Shinmoe-dake volcano (Japan) detected by PALSAR-2 and Sentinel-1 InSAR Yuji Himematsu, Taku Ozawa, Yosuke Aoki This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4572750/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 04 Nov, 2024 Read the published version in Earth, Planets and Space → Version 1 posted 5 You are reading this latest preprint version Abstract The time series of PALSAR-2 and Sentinel-1 images reveal inflation at the volcanic flank and contraction at the crater for approximately five months before the 2017 eruption of Shinmoe-dake volcano, Japan. While the observation of inflation at the volcano’s flank is ubiquitous, few studies have reported crater contraction at a crater preceding an eruption. The flank inflation stopped after the 2017 eruption, while the contraction at the crater continued until the 2018 eruption. We found that a pipe-shaped deformation source above sea level best fits the observation preceding the 2017 eruption. Suppose the flux of ejected materials constrains the conduit radius during the previous 2011 eruption. In that case, the amount of deformation of the pipe-shaped deformation source, whether open or closed at its top, is too large to be realistic. Although constraining the conduit radius from the eruption flux overestimates the pressure change of the pipe-shaped deformation source, water-saturated fractures along the volcanic conduit could extend the effective conduit radius of the pressure source. We propose one potential scenario for the mechanism of the crater contraction preceding volcanic eruptions based on the combination of compaction due to cooling by ambient groundwater and material withdrawal within the conduit. The groundwater inflows from the ambient aquifer through cracks in the porous conduit wall, which are generated by conduit expansion during the magma ascent. Decoupling from the conduit wall due to a decrease in volume of the material promotes material instability and crater contraction. The interaction between the groundwater and the magma triggers the 2017 eruption of Shinmoe-dake volcano, as previous studies have reported. Volcanic crater Eruption precursor Shinmoe-dake volcano Ground deformation InSAR Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction Elucidating precursors of volcanic eruptions helps us understand the preparatory processes of the upcoming eruption. Previous studies reported various precursors of eruptions, including increased seismicity, amount of volcanic gas emission, and temperature in geothermal areas (e.g., Stix et al. 2018). Initiation of these precursors varies from years to minutes. For example, magma accumulation at shallow magma storage can precede an eruption by more than ten years, increasing seismicity often precedes an eruption by a few weeks, and tilt changes often precede an eruption by only minutes (e.g., Bell et al. 2021 ; Kato et al. 2015 ). A decrease in deformation rate or seismicity preceding volcanic eruptions has also been reported (Sigmundsson et al. 2022 ). Some eruptions lack any precursory signals because of their small amplitudes, the lack of detection ability, or the inexistence of the precursors (Lesage et al. 2018 ; Maeda et al. 2015 ). Variation in the deformation rate is one of the indicators of an imminent eruption: an increase in the deformation rate implies an increase in the vertical or horizontal influx of volcanic materials (Kato et al. 2015 ; de Moor et al. 2016 ; Ardid et al. 2022 ; Smittarello et al. 2022 ). Geodetic instruments such as Global Navigation Satellite System (GNSS), tiltmeters, and Synthetic Aperture Radar (SAR) can detect the precursory overpressure of the volcanic system by observing its inflation. SAR is superior in detecting small-scale deformation signals or deformation in inaccessible areas, such as craters or geothermal areas. Several studies have succeeded in detecting local-scale ground deformations before eruptions using SAR images and suggested that the overpressure of the volcanic system commenced several months before eruptions (Hamling 2017 ; Kobayashi et al. 2018 ; Narita et al. 2020 ). The high spatial resolution data derived from SAR images helps understand the spatiotemporal feature of the ground deformation and the geometry of the deformation source. Shinmoe-dake volcano is one of the active volcanoes in southwestern Japan and a part of the Kirishima volcanic complex (Fig. 1 ). Recent major eruptions of Shinmoe-dake volcano were in 2011, 2017, and 2018 (Nakada et al. 2013 ; Yamada et al. 2019 ). The 2011 eruption started with a subplinian eruption with the ejection of altered materials. A new vent was formed at the west side of the crater during the subplinian eruption, followed by the lava ejection from the crater’s center. SAR intensity images gave the amount of extruded lava at the summit 1.5×10 7 m 3 (Kozono et al. 2013 ; Ozawa and Kozono 2013 ). The extruded lava became a dome shape within the crater. The lava dome inflated from the termination of the 2011 eruption until the middle of 2016 (Miyagi et al. 2014 ; National Research Institute for Earth Science and Disaster Resilience (NIED) 2017). Broad-scale inflations with 4 cm of line-of-sight (LOS) change and a spatial extent of 10 km located 5 km to the west-northwest from the crater preceding the 2011 eruption of Shinmoe-dake volcano (Miyagi et al. 2014 ). Following the eruption in 2011, the broad deformation signal transitioned to deflations at a similar spatial extent. This deflation implies a decrease in pressure caused by magma extrusion from the reservoir. The 2017 eruption, which started on 11 October, was mainly accompanied by ash emissions (Japan Meteorological Agency (JMA) 2018). Although a new vent was formed during the 2017 eruption at the east side of the crater, the eruption did not change the shape of the lava dome (Additional File 1: Figure S1 ). GNSS baselines crossing the Kirishima volcanic complex (> 10 km) started to extend in mid-2017, suggesting an overpressure of the magma source (Yamada et al. 2019 ). The 2017 eruption made no significant changes in the GNSS baselines, while they extended with acceleration over time toward the following eruption in 2018. The seismic background noise increased a few months before the 2017 eruption and was kept high until the eruption. This observation suggests a long-term degassing from the magma within the conduit connecting the surface and the shallow magma reservoir (Ichihara et al. 2023a , b ). The 2018 eruption, which started on 1 March, was accompanied by both ash emission and lava extrusion. The time series of GNSS baselines crossing Shinmoe-dake volcano showed a baseline shortening during the early stage of the eruption when volcanic ash was mainly emitting. In contrast, the GNSS baselines turned to extension after the early stage of the eruption (Yamada et al. 2019 ). The lava was ejected from the vent from the east side of the crater after the ash emission phase and covered the pre-existing lava dome, which was formed during the 2011 eruption (Additional File 1: Figure S1 ; Ichihara et al. 2023a ). During the observation period of this study, Iwo-yama volcano, located 5 km to the northwest of Shinmoe-dake volcano, also experienced a steam blowout from the vent in April 2017 and a phreatic eruption in April 2018 (Narita et al. 2020 ; Tajima et al. 2020 ). These eruptions are related to the overpressure of the hydrothermal system beneath Shinmoe-dake and Iwo-yama volcanoes across the Kirishima volcanic complex (Narita et al. 2020 ; Ichihara et al. 2023a ). The spatiotemporal characteristics of ground deformations at the Kirishima volcanic complex (southwest Japan) from 2006 to 2019 have been reported from satellite SAR images (Yunjun et al. 2021 ). Notwithstanding previous studies, further discussions of the spatiotemporal features of the precursory ground deformation of the Shinmoe-dake volcano eruptions are required. Ground deformations at the crater especially contain crucial information about the pressure conditions within the volcanic conduit and the shallow part of the subsurface volcanic system. This paper aims to investigate the local-scale ground deformation at the crater before the Shinmoe-dake volcano eruptions in 2017 and 2018 based on the analysis of satellite SAR images. Through estimating the pressure source geometry and engaging in discussion, we aim to update the understanding of the preparatory processes of eruptions of Shinmoe-dake volcano. SAR image processing and SAR time-series analysis To detect the ground deformation related to the eruptions of Shinmoe-dake volcano in 2017 and 2018, we processed L-band SAR images acquired from Phased Array L-band SAR-2 (PALSAR-2) onboarding Advanced Land Observation Satellite-2 (ALOS-2) satellite between February 2016 and February 2018 and C-band Sentinel-1 image acquired between January 2017 and February 2018, respectively (Fig. 1 ; Additional File 1: Table S1 ). SAR images were processed using the GAMMA software (Wegmüller and Werner 1997 ). Topography-correlated phase changes were corrected using the Digital Elevation Model (DEM) with a spatial resolution of 10 m from the Geospatial Information Authority of Japan, which updated DEM around the Kirishima volcanic complex in 2016, before the eruption in 2017. Phase unwrapping was done with the minimum cost flow algorithm (Costantini 1998 ). Pixels with coherence below 0.1 were discarded as unreliable. We generated InSAR images from Sentinel-1 images with a temporal separation of less than 36 days to avoid decorrelation problems and those from PALSAR-2 images from all possible pairs. We created two subsets of LOS change time series, one before the 2017 eruption and the other between the 2017 and 2018 eruptions, to avoid the contamination induced by decorrelation noise associated with the eruptions. We applied multi-temporal InSAR analysis to all Sentinel-1 and PALSAR-2 images acquired before the 2017 eruption (Berardino et al. 2002 ; Schmidt and Bürgmann 2003 ). The stacking approach was also applied to PALSAR-2 images acquired between the 2017 and 2018 eruptions because of limited available images. We applied the Laplacian operator for the multi-temporal InSAR analysis to temporally smooth LOS changes. The strength of the Laplacian operator was optimized using the L-curve criteria (Hansen, 1992 ; Additional File 1: Figure S2 ). Errors in DEM data were also simultaneously estimated with the spatiotemporal variation of LOS changes based on the perpendicular baselines of SAR images (Fattahi and Amelung, 2013 ). For the LOS change preceding the 2017 eruption, we applied multi-temporal InSAR analysis to PALSAR-2 images acquired since the beginning of 2016 to improve the measurement accuracy for SAR time series analysis by increasing the number of images. We then isolated the time series of PALSAR-2 LOS changes between the last data acquired in 2016 (Path 23: November 14, 2016; Path 130: December 15, 2016; Path 131: December 6, 2016) and the last data before the onset of the eruption in 2017 (October 11) from the entire LOS change time series. Result of SAR time-series analysis Deformation preceding the 2017 eruption The cumulative LOS change preceding the 2017 eruption shows the LOS shortening dominantly at the near-range side of the volcanic flank in each orbit and the LOS extending at the center of the summit crater of Shinmoe-dake volcano in images both from ascending and descending orbits (Fig. 2 ). The LOS shortening at the flank in ascending and descending interferograms indicates an uplift, while the LOS extension at the crater in ascending and descending interferograms indicates subsidence. The spatial extent of LOS shortening is ~ 3 km, comparable to the spatial extent of the volcanic cone of Shinmoe-dake volcano. The asymmetric pattern of LOS shortening at the volcanic flank is due to the SAR observation geometry and the deformation direction. The smaller magnitude of LOS shortening at the far-range side of the volcanic flank implies that the direction of the deformation is nearly perpendicular to the LOS vector. Low coherence induced by temporal variations in backscatter in vegetated areas at the edifice of Shinmoe-dake volcano masks most of the Sentinel-1 images. However, the Sentinel-1 images also show similar deformation characteristics to the extent of sparse vegetation on the summit of Shinmoe-dake volcano. The east-west cross-section of LOS changes crossing the crater shows the peak of LOS extension at the crater’s center. The spatial wavelength of the LOS extension is approximately 200 m, 1/4 of the diameter of the crater’s outer rim (Fig. 3 ). The peak of the LOS extension is about 230 m from the vent formed by the 2017 eruption. We also identify another local peak of LOS shortening (inflation) at the east side of the crater, where the eruptive vent was formed in 2017, especially in the ascending interferograms (Fig. 3 ; Additional File 1: Figure S3). The PALSAR-2 Path 23 images detected 3 cm of LOS shortening on the west flank near the summit, even though the west flank is the far-range side of the observation geometry in the descending orbit (Figs. 3 c and 3 f). The local LOS shortening at the west flank suggests a local deformation at the fumarole on the west flank, where anomalies of ground temperature and intermittent emission of volcanic gas or steam have been reported since November 2015 (JMA 2017). The LOS changed at the west and east flank near the summit, and the crater’s center accelerated toward the eruption in October 2017 (Fig. 4 ). The Sentinel-1 images also indicate the onset of LOS shortening on the volcanic flank in May 2017, five months before the eruption. At the western rim, the LOS change from all paths increased, indicating major inflation at the flank and minor inflation at the fumarole at the west flank. Figure 4 shows that the LOS shortening reached 3 cm from the descending interferograms at the eastern rim, while there was negligible LOS change from the ascending interferograms over the observation period. In contrast, we identify the LOS extension in Sentinel-1 images from both ascending and descending interferograms from May – July 2017 (Fig. 4 ). PALSAR-2 and Sentinel-1 images showed continuous LOS extension at the crater between the 2017 and 2018 eruptions, indicating a crater contraction (Fig. 4 ; Additional File 1: Figure S4). The LOS extension at the crater reached 6 cm from the Sentinel-1 Path 163 and 3.5 cm from the Sentinel-1 Path 156 over four months between the 2017 and 2018 eruptions, when the peak of PALSAR-2 LOS extension moved about 100 m to the west from that before the 2017 eruption (Fig. 3 ; Additional File 1: Figure S1 ). In contrast, the flank was subjected to LOS extension by 1–2 cm over four months of the inter-eruptive period, implying a contraction. Decorrelation noise at the crater and the flank near the summit contaminates PALSAR-2 InSAR, spanning the eruptions in 2017 and 2018 (Additional File 1: Figure S5). Intensity images in 2017 and 2018 also show variations of back-scatter characteristics at the crater. The deposition of tephra and the formation of a vent at the east side of the crater due to the 2017 eruption decreased the back-scatter intensity. Likewise, the extruded lava emplaced above the pre-existing lava dome during the 2018 eruption also decreased the back-scatter intensity (Additional File 1: Figure S2 ). Therefore, we cannot extract the co-eruptive deformation in 2017 and 2018 at the crater from InSAR images. Modeling for the precursory deformation of the 2017 eruption We tried to infer the pressure source geometry to explain the observed surface deformations and to elucidate how the volcano works preceding eruptions. We assume an elastic, homogeneous, and isotropic half-space because analytical solutions of the surface deformation are available in response to (de)pressurization of some deformation sources. We employed the varying depth model to account for the first-order approximation of the topographic effect (Williams & Wadge, 1998 ). The spatial characteristics of the observed deformation preceding the 2017 eruption are consistent with deformations caused by outward displacements of an open pipe (Bonaccorso and Davis, 1999 ; Segall, 2010 ). Inflation of an open pipe induces uplift on the side and subsidence above the pipe (Additional File 1: Figure S6). Overpressure of a closed pipe also generates inflation on the side. Unlike an open pipe, pressurization of a closed pipe uplifts above the pipe but with a smaller magnitude than the side (Bonaccorso and Davis, 1999 ; Segall, 2010 ). Here, we assume an open and a closed pipe as a deformation source to model the detected precursory deformation. The best-fit parameters for each model were estimated by fitting the cumulative PALSAR-2 LOS changes in Path 23 and 131 in 2017 based on the particle swarm optimization algorithm, one of the swarm intelligence algorithms (Kennedy and Eberhart 1995 ). The algorithm can infer the best-fit model with uncertainties as a form of marginal posterior distribution, which can be derived from the evolution of each particle position over iterations. The window size for further searching the best positions in the next step is adjusted based on the distances between the present position of each particle at step t ( t = 1,2…) and the global best position (minimum RMSE within the swarm) or the personal best positions (minimum RMSE for each particle). The initial particle positions were randomly assigned within the designated range of parameters. The computation was iterated 400 times by searching with 30 particles. Table 1 shows the parameter ranges and the best-fit parameters. The data fitness was evaluated by root-mean-square errors (RMSE) between the observation and the calculation. To reduce computational costs and optimize data weighting, the input data were subsampled by circular grids centered on the crater. Data weighting between two LOS changes was based on the data variance. The first 20% of particle position histories were discarded to compute the posterior marginal distribution. The reflection boundary was set so the parameters would not exceed the designated search range. Table 1 Parameter search range and best-fit parameters. Search range Open pipe Close pipe UTM Easting [km] [677, 679] 677.98 (0.19) 678.03 (0.25) UTM Northing [km] [3531, 3533] 3532.19 (0.17) 3532.10 (0.22) s*a [m*m] [0, 50] 33.1 (5.7) Log10(a 2 ΔP) [m 2 *Pa] [5, 14] 11.7 (1.3) Top depth [m (b.s.l.)] [-1350, 0] -1152.7 (148.1) -577.7 (196.7) Pipe length [m] [0, 2500] 535.4 (247.5) 438.9 (388.9) RMSE [cm] 1.28 2.13 Values in round brackets show standard deviations of particle positions through the inversion. Our result suggests that the synthetic deformation caused by outward displacements of an open pipe fits better to reconstruct the observation than the overpressure of a closed pipe (Figs. 5 and 6 ). The RMSE with the open pipe is 1.3 cm, smaller than that with the closed pipe (2.1 cm). The best-fit open pipe reproduces both LOS extension (contraction) at the crater and LOS shortening (inflation) at the flank. In contrast, the closed pipe reproduces only LOS shortening with smaller magnitudes above the pipe (Additional File 1: Figure S6). Both models find residuals of LOS shortenings from the ascending images at the east side of the crater, where minor inflation occurred during the 2017 eruption. The best-fit open pipe source is 620–1150 m above sea level, approximately 200–730 m below the surface. The inferred extent of the pipe-like deformation source suggests that most of the pressurization preceding the 2017 eruption occurred above sea level. The pressurized pipe is less than 10% of the conduit, connecting the shallow magma chamber 5 km below sea level and the summit crater (Aizawa et al. 2014). Our results infer that the optimum value of the product of the conduit radius and the outward displacements on the volcanic conduit is 33.1 [m*m] (Table 1 , Fig. 7 ). Because the pipe radius and the amount of deformation are coupled in the modeling with an open pipe, the radius of the volcanic conduit must be given as a priori to infer the amount of displacement. The flux of tephra associated with the 2011 eruption constrained the conduit radius of Shinmoe-dake volcano as between 4.5 and 6.0 m (Sato et al. 2013 ). Given these values, we derive 5.5–7.4 m of the outward displacement of an open pipe, which exceeds twice the conduit radius. The optimum value of log 10 (a 2 ΔP) (a: conduit radius [m], ΔP: Pressure change [Pa]) for a close pipe was 11.7 under the assumption of rigidity of 5 GPa (Table 1 ). The given conduit radius of the close pipe requires more than 10 10 Pa of excess pressure change to reconstruct the observed deformation. Even with the rigidity of 0.1 GPa, the pressure change still needs to be on the order of 10 8 Pa. These values are implausible because they exceed the rock's tensile strength at the crust's shallow depth (~ 10 MPa; Heap et al. 2021 ). Therefore, we need to discuss further the implications of the derived outward displacement of the open pipe. Discussion Implication of the modeling result Volcano inflation is a typical indicator of pre-eruptive deformations by overpressure within a subsurface volcanic system. However, some studies reported that the continuous release of magmatic fluid from a conduit system potentially induces non-magmatic eruptions (Girona et al. 2014 ; Nobile et al. 2017 ). They interpreted that the outgassing of magmatic fluid cause broader subsidence by depressurization. In contrast, the spatiotemporal characteristics of the small-scale precursory deformation, with crater contraction, flank inflation and increasing deformation magnitude toward the eruption have rarely been reported. Assuming pipe as the deformation source would be reasonable because the distribution of tremor sources implies a well-developed conduit beneath Shinmoe-dake volcano from the crater to a depth of 1 km below sea level (Ichihara et al. 2023a ). The geometry of the pipe-like deformation source has been employed to explain tilt changes and displacements associated with volcanic eruptions (Genco & Ripepe, 2010 ; Ripepe et al., 2021 ; Saballos et al., 2014 ). Pressurization of an open pipe induces radial displacements at the flank (the side of the conduit). To our knowledge, no study has employed the open-pipe model to reproduce contractions at craters and flank inflations simultaneously. The conduit radius is one of the critical parameters to interpret not only pressure changes or deformation of a conduit but also to understand physical processes based on numerical models, such as a conduit flow model (Aravena et al. 2018a ), and is usually constrained by the flux of volcanic materials within a conduit (Kazahaya et al. 1994 ; Stevenson and Blake 1998 ). Pressurization of a pipe sometimes yields physically unrealistic pressure changes if we assign the conduit radius constrained by the flux of the volcanic material (Widiwijanti et al. 2005; Green et al. 2006 ). As previously mentioned, we also proposed that an outward displacement of twice the original pipe radius is required to explain the observed deformation at the crater in the case of the deformation preceding the 2017 Shinmoe-dake eruption. This paradox comes from the gap between the conduit radius as the pathway of the material (actual radius) and the effective radius for pressure changes. The effective radius of the conduit must be larger than the actual conduit radius for the pressure changes to be realistic. The fluid-saturated fracture around the volcanic conduit increases the cause of the larger effective pressurized extents of the conduit (Widiwijanti et al. 2005). Although the shear traction along the conduit wall due to viscous flow resistance allows us to avoid unrealistic pressure changes or the conduit radius, it cannot explain the contraction at the crater in this case (Green et al. 2006 ). A precise tremor relocation associated with the 2017–2018 eruption reveals a conduit-shaped structure with a width of about 500 m, implying degassing from the magma within the conduit (Ichihara et al. 2023a , b ). If the pipe radius is 250 m, the required outward displacement of an open pipe is ~ 13 cm, which is considered plausible. Potential mechanisms for the precursory deformation Pressurization due to either mass supply from depths or an accumulation of volcanic gas exsolving from the magma is responsible for observed inflations preceding eruptions. Instead, depressurization within a volcanic conduit or a collapse of the conduit structure is responsible for crater contraction (e.g., Lipman, 1997 ). While contraction at a crater is usually observed during or after an eruption, it has not been often observed before an eruption. To interpret the crater contraction preceding the 2017 eruption of Shinmoe-dake volcano, we propose one potential scenario below. The scenario starts with ascending magma head with migrating from depths causing the conduit expansion (Fig. 8 ). The expansion of the conduit can generate cracks along the porous conduit wall, inducing an inflow of groundwater from an ambient aquifer into the conduit through the thus-generated cracks. A low-resistivity structure at depths shallower than sea level is considered an ambient aquifer beneath Shinmoe-dake and Iwo-yama volcanoes (Kagiyama et al. 1996 ). The physical contact between pre-existing materials within the conduit and the inflowing groundwater causes the decrease in the volume of the materials. The material within the conduit is likely the pre-existing magma ejected during the 2011 eruption, which is not entirely solidified because of the gradual magma ascent by mid-2016 (NIED, 2017). The cooling materials cause decoupling between the materials within the conduit and the conduit wall, inducing gradual withdrawal of the materials. The compaction of the materials within the shallower part of the conduit causes the local-scale contraction at the crater. In contrast, the ascent of magma through the conduit from deeper depths induced the broader inflation at the flank. Seismic observation suggests that the interaction between magma and groundwater triggered the 2017 eruption (Konstantinous et al. 2022). The partial failure of the conduit or the collapse of material within the conduit triggers an eruption through the interaction between groundwater and magma (Aravena et al. 2018b ; Furuya et al. 2003). Based on these backgrounds, our proposed scenario can explain the process of the crater contraction and the trigger of the 2017 eruption. After the 2017 eruption, further material instability continued, leading to further crater contraction. Simultaneously, the conduit expansion due to the magma ascent stalled, causing the cessation of the inflation at the flank. The additional material instability can induce continuous contraction preceding the 2018 eruption even if the ambient aquifer has already been depleted (Konstantinou et al. 2022 ). The formation of either the vent on top or the pathway formation for the ejected material within the conduit, caused by the 2017 eruption, potentially prevents pressure accumulation within the conduit, even as magma ascends. Similar mechanism is a drain-back of magma within the volcanic conduit (Watanabe et al. 1998 ). A gravity observation proposed that the descent of a magma head within a conduit subsides the summit area during the post-eruptive periods of Izu-Oshima volcano, Japan, in 1986–1987 (Ida et al., 1988 ). As the magma head descended, the gas evaporation from the magma decreased the magma pressure. Pressurization by accumulating exsolved volcanic fluid above the magma head pushes down the magma head. The height change of the magma head can be correlated with the vertical deformations at the crater, while the drain-back of magma has been reported only during and after, not before, the eruption. So far, we cannot identify the temporal variation of the magma head preceding the 2017 eruption because no gravity observations are available near the summit of Shinmoedake volcano. Furthermore, no significant increase in volcanic earthquakes was identified for several months before the 2017 eruption (Yamada et al. 2019 ). Ichihara et al. ( 2023a ) suggested that the increasing magnitude of the seismic background noise since early 2017 is due to the degassing from the magma within the conduit. Therefore, we have identified no robust supporting evidence for the material withdrawn preceding the 2017 eruption. To further discuss the preparatory process within the conduit, additional geodetic observations near the summit, such as gravimetry, are required to investigate how much materials within the conduit moved. Conclusion This study investigates the precursory deformation of the 2017 and 2018 Shinmoe-dake volcano eruptions. PALSAR-2 and Sentinel-1 images reveal that the inflation at the volcanic flank and the contraction at the center of the summit crater, which started five months before the 2017 eruption. The best-fit model, assuming an open pipe, well reproduces the spatial pattern of the observed deformation, contractions above the pipe, and inflation at the side of the conduit. The location of the inferred open pipe is 200–730 m below the surface, implying that most processes responsible for the precursory deformation occur above sea level. If we constrained the conduit radius by the flux of the ejection materials during the previous eruption in 2011 as a priori information, double the original conduit radius is required to explain the observed crater contraction. Instead, we can derive a reasonable outward displacement of a pipe if we assign a conduit radius, estimated by the conduit-shaped tremor distribution between and during the 2017 and 2018 eruptions. We suggested one scenario to interpret the observed crater contraction based on the combination of the compaction caused by cooling by physical contact with the ambient groundwater and the material withdrawal. However, no robust information for supporting our proposed scenario, so far. Overpressure by the supply of volcanic materials from depths or heating often induces inflation at the surface, preceding an eruption. Instead, the contraction at the crater preceding an eruption is rare. Our observations raised a new question about the physical process for inducing the crater contraction preceding an eruption. SAR data is still useful for detecting local-scale deformations, such as those at the crater, while further various observations, such as gravimetry, near the summit are required to identify the eruption’s preparatory processes of Shinmoe-dake volcano. Moreover, additional investigations are necessary to constrain the trigger of the crater contraction and to verify the plausibility of the effective pipe radius as a deformation source for sophisticating the model. Abbreviations JMA: Japan Meteorological Agency SAR: Synthetic aperture radar InSAR: Interferometric synthetic aperture radar GNSS: Global navigation satellite system ALOS-2: Advanced L-band Observation Satellite-2 PALSAR-2: Phased Array Synthetic Aperture Radar-2 DEM: Digital elevation model LOS: Line-of-sight NIED: National Research Institute for Earth Science and Disaster Resilience RMSE: Root-mean-square error Declarations Ethics approval and consent to participate Not applicable. Consent for publication Not applicable. Availability of data and materials Original Sentinel-1 images are available from the Copernicus Open Access Hub website (https://scihub.copernicus.eu/) after registration. The data in this study can be available after reasonable request. Competing interests The authors declare that they have no competing interests. Funding This study is supported by ERI JURP 2018-B-02 and 2021-B-03 in Earthquake Research Institute, the University of Tokyo. This study is conducted under the framework of Subtheme 2-1, Project B of “Integrated program for next generation volcano research and human resource development” led by the Ministry of Education, Culture, Sport, Science and Technology, Japan (MEXT). Authors' contributions Yuji Himematsu: Conceptualization, Formal analysis, Investigation, Writing – original draft. Taku Ozawa: Funding acquisition, Project administration, Conceptualization, Supervision, Writing – review & editing. Yosuke Aoki: Project administration, Conceptualization, Supervision, Writing – review & editing. Acknowledgments PALSAR-2 level 1.1 data in this study are shared among a Japan InSAR consortium PIXEL and provided by JAXA under a cooperative research contract with the PIXEL (PI No. PER2A2N187 and PER3A2N013). 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Geophys Res Lett 48(11):1–12. https://doi.org/10.1029/2021GL092879 Supplementary Files SupplementaryMaterialHimematsuetal.docx graphicalabstractv1finereso.png Cite Share Download PDF Status: Published Journal Publication published 04 Nov, 2024 Read the published version in Earth, Planets and Space → Version 1 posted Editorial decision: Major Revision 11 Aug, 2024 Reviewers agreed at journal 28 Jun, 2024 Reviewers invited by journal 28 Jun, 2024 Editor assigned by journal 27 Jun, 2024 First submitted to journal 18 Jun, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4572750","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":320085775,"identity":"c77c912f-715d-481f-8470-edaa270ae95f","order_by":0,"name":"Yuji Himematsu","email":"data:image/png;base64,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","orcid":"https://orcid.org/0000-0002-1724-8497","institution":"Geospatial Information Authority of Japan","correspondingAuthor":true,"prefix":"","firstName":"Yuji","middleName":"","lastName":"Himematsu","suffix":""},{"id":320085776,"identity":"d2955177-e719-4071-9113-32e968eff830","order_by":1,"name":"Taku Ozawa","email":"","orcid":"","institution":"National Research Institute for Earth Science and Disaster Prevention: Bosai Kagaku Gijutsu Kenkyujo","correspondingAuthor":false,"prefix":"","firstName":"Taku","middleName":"","lastName":"Ozawa","suffix":""},{"id":320085777,"identity":"f282ce3f-c376-41ec-b07a-24b0c5bd14a6","order_by":2,"name":"Yosuke Aoki","email":"","orcid":"","institution":"The University of Tokyo: Tokyo Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Yosuke","middleName":"","lastName":"Aoki","suffix":""}],"badges":[],"createdAt":"2024-06-13 00:42:43","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4572750/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4572750/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1186/s40623-024-02083-8","type":"published","date":"2024-11-04T15:57:41+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":60851561,"identity":"85dc8a6c-7379-47b4-aa84-a8538657c181","added_by":"auto","created_at":"2024-07-22 21:02:37","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":762148,"visible":true,"origin":"","legend":"\u003cp\u003eSummary of the study area and SAR images in this study. (a) Overview of the study area with SAR image footprint. (b) Enlarged view of the study area of the Kirishima volcanic complex. (c) Plot of perpendicular baseline of SAR image and network of interferogram pair for SAR time series analysis. Vertical red lines indicate the timing of Shinmoe-dake eruptions in 2017 and 2018.\u003c/p\u003e","description":"","filename":"Picture1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4572750/v1/83c9dd3f0e879a738671f3f4.jpg"},{"id":60850652,"identity":"c823a0b0-c62a-42b3-a0d6-12ebe6eda25e","added_by":"auto","created_at":"2024-07-22 20:54:37","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":504470,"visible":true,"origin":"","legend":"\u003cp\u003eCumulative LOS change from the beginning of 2017 to the last data acquisition before the 2017 eruption. (a) PALSAR-2 (P2) Path 130 (December 15, 2016 – September 17, 2017), (b) PALSAR-2 Path 131 (December 6, 2017 – October 10, 2017), (c) PALSAR-2 Path 23 (November 14, 2016 – September 18, 2017), (d) Topographic map, (e) Sentinel-1 (S1) Path 153 (January 3, 2017 – October 6, 2017), and (f) Sentinel-1 Path 168 (January 9, 2017 – September 18, 2017). Positive values (blue) indicate distance changes moving away from the satellite.\u003c/p\u003e","description":"","filename":"Picture2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4572750/v1/c01d0e2afbda5ce257c967ab.jpg"},{"id":60850660,"identity":"c828d5e4-7db1-4f1d-9eb6-0553693c304d","added_by":"auto","created_at":"2024-07-22 20:54:37","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":477305,"visible":true,"origin":"","legend":"\u003cp\u003eEast-west cross-section of cumulative LOS changes before the 2017 eruption and the inter-eruptive period. (a) PALSAR-2 Path 130, (b) PALSAR-2 Path 131, (c) PALSAR-2 Path 23, (d) Topographic map with 50 m contour intervals, (e) Sentinel-1 Path 153, and (f) Sentinel-1 Path168. Positive values (downward) indicate distance changes moving away from the satellite. Vertical dashed lines indicate the location of the inner and outer rims of the crater. Red and black lines indicate median values of extracted deformation before the 2017 eruption and during the inter-eruptive period between the eruptions in 2017 and 2018 along the cross-sections within 100 m. All extracted data are plotted as gray dots.\u003c/p\u003e","description":"","filename":"Picture3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4572750/v1/bf411d8da3192d94477ec0a0.jpg"},{"id":60850651,"identity":"c1e976f4-575c-4a68-94aa-8b978ffd2ffc","added_by":"auto","created_at":"2024-07-22 20:54:36","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":476990,"visible":true,"origin":"","legend":"\u003cp\u003eTime series of PALSAR-2 and Sentinel-1 LOS change before the 2018 eruption. Time series of LOS change at (a) the western side, (b) the eastern side, and (c) the center part of the caldera. (d) The extracted position of LOS changes time series. Error bars for PALSAR-2 LOS change indicate the standard deviation of LOS change outside the region of the Kirishima volcanic complex. Red vertical lines indicate the timing of the eruptions in 2017 and 2018. Positive LOS changes (downward in the vertical axes) indicate distance changes moving away from the satellite.\u003c/p\u003e","description":"","filename":"Picture4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4572750/v1/406bfad06524e4edf9825f58.jpg"},{"id":60851562,"identity":"6c2a1d8c-d8cf-4734-9373-308e7ac2a0ac","added_by":"auto","created_at":"2024-07-22 21:02:37","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":800787,"visible":true,"origin":"","legend":"\u003cp\u003eResults of modeling using analytical solution of surface deformations. (a–c) Path 131, (d–f) Path 23 for an open pipe. (g, h) Path 131, (i, j) Path 23 for a close pipe. Each panel shows observations, best-fit calculations, and residuals from the left column.\u003c/p\u003e","description":"","filename":"Picture5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4572750/v1/025ee5fb36f945cc8d535948.jpg"},{"id":60850655,"identity":"508ce2b2-17fa-4a52-bc24-a0bb057e9155","added_by":"auto","created_at":"2024-07-22 20:54:37","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":176128,"visible":true,"origin":"","legend":"\u003cp\u003eTrade-off curve between conduit radius and outward displacements for the pipe-like deformation source. (a) Open pipe, and (b) close pipe. The gray-scaled line coloring represents the normalized likelihood of modeling. Red solid and dashed lines show the median and standard deviation of the particle position (model parameter) during the iterative computation, respectively.\u003c/p\u003e","description":"","filename":"Picture6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4572750/v1/8c303025f0e1212f0eb1e32d.jpg"},{"id":60850662,"identity":"12752e7f-56c6-4bc5-a5cf-6f07aee03df9","added_by":"auto","created_at":"2024-07-22 20:54:37","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":226596,"visible":true,"origin":"","legend":"\u003cp\u003ePosterior marginal distribution of modeling. (a–e) Open pipe, and (f–j) closed pipe. Red solid and dashed lines indicate the median and standard deviation of the particle position (model parameter) during the iterative computation, respectively.\u003c/p\u003e","description":"","filename":"Picture7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4572750/v1/0657336b6bba52dd37d26622.jpg"},{"id":60850661,"identity":"3fdbacd2-9574-460e-8176-b71d88f4e94e","added_by":"auto","created_at":"2024-07-22 20:54:37","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":163820,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic image of the proposed scenario. Precursory processes preceding (a) the 2017 eruption and (b) the 2018 eruption.\u003c/p\u003e","description":"","filename":"Picture8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4572750/v1/783dfb99175d298cc586a5de.jpg"},{"id":68749920,"identity":"1a8a103c-7f2f-4768-bf98-866bb9c51d4a","added_by":"auto","created_at":"2024-11-11 16:07:34","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4093902,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4572750/v1/574916eb-eff4-4473-9cd5-4c2e930c1a31.pdf"},{"id":60850658,"identity":"efa4b9ef-c746-4935-839f-158153ac0b62","added_by":"auto","created_at":"2024-07-22 20:54:37","extension":"docx","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":2300569,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryMaterialHimematsuetal.docx","url":"https://assets-eu.researchsquare.com/files/rs-4572750/v1/b10037ecc93da6064e1d91ec.docx"},{"id":60850663,"identity":"7e571ccf-8412-49f4-befc-9863744ab597","added_by":"auto","created_at":"2024-07-22 20:54:37","extension":"png","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":10349781,"visible":true,"origin":"","legend":"","description":"","filename":"graphicalabstractv1finereso.png","url":"https://assets-eu.researchsquare.com/files/rs-4572750/v1/fa6530655e8e0fd7a06ed96b.png"}],"financialInterests":"","formattedTitle":"Precursory crater contraction associated with the 2017 eruption of Shinmoe-dake volcano (Japan) detected by PALSAR-2 and Sentinel-1 InSAR","fulltext":[{"header":"Introduction","content":"\u003cp\u003eElucidating precursors of volcanic eruptions helps us understand the preparatory processes of the upcoming eruption. Previous studies reported various precursors of eruptions, including increased seismicity, amount of volcanic gas emission, and temperature in geothermal areas (e.g., Stix et al. 2018). Initiation of these precursors varies from years to minutes. For example, magma accumulation at shallow magma storage can precede an eruption by more than ten years, increasing seismicity often precedes an eruption by a few weeks, and tilt changes often precede an eruption by only minutes (e.g., Bell et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Kato et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). A decrease in deformation rate or seismicity preceding volcanic eruptions has also been reported (Sigmundsson et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Some eruptions lack any precursory signals because of their small amplitudes, the lack of detection ability, or the inexistence of the precursors (Lesage et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Maeda et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eVariation in the deformation rate is one of the indicators of an imminent eruption: an increase in the deformation rate implies an increase in the vertical or horizontal influx of volcanic materials (Kato et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; de Moor et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Ardid et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Smittarello et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Geodetic instruments such as Global Navigation Satellite System (GNSS), tiltmeters, and Synthetic Aperture Radar (SAR) can detect the precursory overpressure of the volcanic system by observing its inflation. SAR is superior in detecting small-scale deformation signals or deformation in inaccessible areas, such as craters or geothermal areas. Several studies have succeeded in detecting local-scale ground deformations before eruptions using SAR images and suggested that the overpressure of the volcanic system commenced several months before eruptions (Hamling \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Kobayashi et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Narita et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The high spatial resolution data derived from SAR images helps understand the spatiotemporal feature of the ground deformation and the geometry of the deformation source.\u003c/p\u003e \u003cp\u003eShinmoe-dake volcano is one of the active volcanoes in southwestern Japan and a part of the Kirishima volcanic complex (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Recent major eruptions of Shinmoe-dake volcano were in 2011, 2017, and 2018 (Nakada et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Yamada et al. \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The 2011 eruption started with a subplinian eruption with the ejection of altered materials. A new vent was formed at the west side of the crater during the subplinian eruption, followed by the lava ejection from the crater\u0026rsquo;s center. SAR intensity images gave the amount of extruded lava at the summit 1.5\u0026times;10\u003csup\u003e7\u003c/sup\u003e m\u003csup\u003e3\u003c/sup\u003e (Kozono et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Ozawa and Kozono \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). The extruded lava became a dome shape within the crater. The lava dome inflated from the termination of the 2011 eruption until the middle of 2016 (Miyagi et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; National Research Institute for Earth Science and Disaster Resilience (NIED) 2017). Broad-scale inflations with 4 cm of line-of-sight (LOS) change and a spatial extent of 10 km located 5 km to the west-northwest from the crater preceding the 2011 eruption of Shinmoe-dake volcano (Miyagi et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Following the eruption in 2011, the broad deformation signal transitioned to deflations at a similar spatial extent. This deflation implies a decrease in pressure caused by magma extrusion from the reservoir.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe 2017 eruption, which started on 11 October, was mainly accompanied by ash emissions (Japan Meteorological Agency (JMA) 2018). Although a new vent was formed during the 2017 eruption at the east side of the crater, the eruption did not change the shape of the lava dome (Additional File 1: Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). GNSS baselines crossing the Kirishima volcanic complex (\u0026gt;\u0026thinsp;10 km) started to extend in mid-2017, suggesting an overpressure of the magma source (Yamada et al. \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The 2017 eruption made no significant changes in the GNSS baselines, while they extended with acceleration over time toward the following eruption in 2018. The seismic background noise increased a few months before the 2017 eruption and was kept high until the eruption. This observation suggests a long-term degassing from the magma within the conduit connecting the surface and the shallow magma reservoir (Ichihara et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2023a\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003eb\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe 2018 eruption, which started on 1 March, was accompanied by both ash emission and lava extrusion. The time series of GNSS baselines crossing Shinmoe-dake volcano showed a baseline shortening during the early stage of the eruption when volcanic ash was mainly emitting. In contrast, the GNSS baselines turned to extension after the early stage of the eruption (Yamada et al. \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The lava was ejected from the vent from the east side of the crater after the ash emission phase and covered the pre-existing lava dome, which was formed during the 2011 eruption (Additional File 1: Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e; Ichihara et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2023a\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eDuring the observation period of this study, Iwo-yama volcano, located 5 km to the northwest of Shinmoe-dake volcano, also experienced a steam blowout from the vent in April 2017 and a phreatic eruption in April 2018 (Narita et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Tajima et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). These eruptions are related to the overpressure of the hydrothermal system beneath Shinmoe-dake and Iwo-yama volcanoes across the Kirishima volcanic complex (Narita et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Ichihara et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2023a\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe spatiotemporal characteristics of ground deformations at the Kirishima volcanic complex (southwest Japan) from 2006 to 2019 have been reported from satellite SAR images (Yunjun et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Notwithstanding previous studies, further discussions of the spatiotemporal features of the precursory ground deformation of the Shinmoe-dake volcano eruptions are required. Ground deformations at the crater especially contain crucial information about the pressure conditions within the volcanic conduit and the shallow part of the subsurface volcanic system. This paper aims to investigate the local-scale ground deformation at the crater before the Shinmoe-dake volcano eruptions in 2017 and 2018 based on the analysis of satellite SAR images. Through estimating the pressure source geometry and engaging in discussion, we aim to update the understanding of the preparatory processes of eruptions of Shinmoe-dake volcano.\u003c/p\u003e\n\u003ch3\u003eSAR image processing and SAR time-series analysis\u003c/h3\u003e\n\u003cp\u003eTo detect the ground deformation related to the eruptions of Shinmoe-dake volcano in 2017 and 2018, we processed L-band SAR images acquired from Phased Array L-band SAR-2 (PALSAR-2) onboarding Advanced Land Observation Satellite-2 (ALOS-2) satellite between February 2016 and February 2018 and C-band Sentinel-1 image acquired between January 2017 and February 2018, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e; Additional File 1: Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). SAR images were processed using the GAMMA software (Wegm\u0026uuml;ller and Werner \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e1997\u003c/span\u003e). Topography-correlated phase changes were corrected using the Digital Elevation Model (DEM) with a spatial resolution of 10 m from the Geospatial Information Authority of Japan, which updated DEM around the Kirishima volcanic complex in 2016, before the eruption in 2017. Phase unwrapping was done with the minimum cost flow algorithm (Costantini \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1998\u003c/span\u003e). Pixels with coherence below 0.1 were discarded as unreliable. We generated InSAR images from Sentinel-1 images with a temporal separation of less than 36 days to avoid decorrelation problems and those from PALSAR-2 images from all possible pairs.\u003c/p\u003e \u003cp\u003eWe created two subsets of LOS change time series, one before the 2017 eruption and the other between the 2017 and 2018 eruptions, to avoid the contamination induced by decorrelation noise associated with the eruptions. We applied multi-temporal InSAR analysis to all Sentinel-1 and PALSAR-2 images acquired before the 2017 eruption (Berardino et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Schmidt and B\u0026uuml;rgmann \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). The stacking approach was also applied to PALSAR-2 images acquired between the 2017 and 2018 eruptions because of limited available images. We applied the Laplacian operator for the multi-temporal InSAR analysis to temporally smooth LOS changes. The strength of the Laplacian operator was optimized using the L-curve criteria (Hansen, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Additional File 1: Figure \u003cspan refid=\"MOESM2\" class=\"InternalRef\"\u003eS2\u003c/span\u003e). Errors in DEM data were also simultaneously estimated with the spatiotemporal variation of LOS changes based on the perpendicular baselines of SAR images (Fattahi and Amelung, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). For the LOS change preceding the 2017 eruption, we applied multi-temporal InSAR analysis to PALSAR-2 images acquired since the beginning of 2016 to improve the measurement accuracy for SAR time series analysis by increasing the number of images. We then isolated the time series of PALSAR-2 LOS changes between the last data acquired in 2016 (Path 23: November 14, 2016; Path 130: December 15, 2016; Path 131: December 6, 2016) and the last data before the onset of the eruption in 2017 (October 11) from the entire LOS change time series.\u003c/p\u003e"},{"header":"Result of SAR time-series analysis","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eDeformation preceding the 2017 eruption\u003c/h2\u003e \u003cp\u003eThe cumulative LOS change preceding the 2017 eruption shows the LOS shortening dominantly at the near-range side of the volcanic flank in each orbit and the LOS extending at the center of the summit crater of Shinmoe-dake volcano in images both from ascending and descending orbits (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The LOS shortening at the flank in ascending and descending interferograms indicates an uplift, while the LOS extension at the crater in ascending and descending interferograms indicates subsidence. The spatial extent of LOS shortening is ~\u0026thinsp;3 km, comparable to the spatial extent of the volcanic cone of Shinmoe-dake volcano. The asymmetric pattern of LOS shortening at the volcanic flank is due to the SAR observation geometry and the deformation direction. The smaller magnitude of LOS shortening at the far-range side of the volcanic flank implies that the direction of the deformation is nearly perpendicular to the LOS vector. Low coherence induced by temporal variations in backscatter in vegetated areas at the edifice of Shinmoe-dake volcano masks most of the Sentinel-1 images. However, the Sentinel-1 images also show similar deformation characteristics to the extent of sparse vegetation on the summit of Shinmoe-dake volcano.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe east-west cross-section of LOS changes crossing the crater shows the peak of LOS extension at the crater\u0026rsquo;s center. The spatial wavelength of the LOS extension is approximately 200 m, 1/4 of the diameter of the crater\u0026rsquo;s outer rim (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The peak of the LOS extension is about 230 m from the vent formed by the 2017 eruption. We also identify another local peak of LOS shortening (inflation) at the east side of the crater, where the eruptive vent was formed in 2017, especially in the ascending interferograms (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e; Additional File 1: Figure S3).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe PALSAR-2 Path 23 images detected 3 cm of LOS shortening on the west flank near the summit, even though the west flank is the far-range side of the observation geometry in the descending orbit (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef). The local LOS shortening at the west flank suggests a local deformation at the fumarole on the west flank, where anomalies of ground temperature and intermittent emission of volcanic gas or steam have been reported since November 2015 (JMA 2017).\u003c/p\u003e \u003cp\u003eThe LOS changed at the west and east flank near the summit, and the crater\u0026rsquo;s center accelerated toward the eruption in October 2017 (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). The Sentinel-1 images also indicate the onset of LOS shortening on the volcanic flank in May 2017, five months before the eruption. At the western rim, the LOS change from all paths increased, indicating major inflation at the flank and minor inflation at the fumarole at the west flank. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows that the LOS shortening reached 3 cm from the descending interferograms at the eastern rim, while there was negligible LOS change from the ascending interferograms over the observation period. In contrast, we identify the LOS extension in Sentinel-1 images from both ascending and descending interferograms from May \u0026ndash; July 2017 (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003ePALSAR-2 and Sentinel-1 images showed continuous LOS extension at the crater between the 2017 and 2018 eruptions, indicating a crater contraction (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e; Additional File 1: Figure S4). The LOS extension at the crater reached 6 cm from the Sentinel-1 Path 163 and 3.5 cm from the Sentinel-1 Path 156 over four months between the 2017 and 2018 eruptions, when the peak of PALSAR-2 LOS extension moved about 100 m to the west from that before the 2017 eruption (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e; Additional File 1: Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). In contrast, the flank was subjected to LOS extension by 1\u0026ndash;2 cm over four months of the inter-eruptive period, implying a contraction.\u003c/p\u003e \u003cp\u003eDecorrelation noise at the crater and the flank near the summit contaminates PALSAR-2 InSAR, spanning the eruptions in 2017 and 2018 (Additional File 1: Figure S5). Intensity images in 2017 and 2018 also show variations of back-scatter characteristics at the crater. The deposition of tephra and the formation of a vent at the east side of the crater due to the 2017 eruption decreased the back-scatter intensity. Likewise, the extruded lava emplaced above the pre-existing lava dome during the 2018 eruption also decreased the back-scatter intensity (Additional File 1: Figure \u003cspan refid=\"MOESM2\" class=\"InternalRef\"\u003eS2\u003c/span\u003e). Therefore, we cannot extract the co-eruptive deformation in 2017 and 2018 at the crater from InSAR images.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eModeling for the precursory deformation of the 2017 eruption\u003c/h2\u003e \u003cp\u003eWe tried to infer the pressure source geometry to explain the observed surface deformations and to elucidate how the volcano works preceding eruptions. We assume an elastic, homogeneous, and isotropic half-space because analytical solutions of the surface deformation are available in response to (de)pressurization of some deformation sources. We employed the varying depth model to account for the first-order approximation of the topographic effect (Williams \u0026amp; Wadge, \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e1998\u003c/span\u003e). The spatial characteristics of the observed deformation preceding the 2017 eruption are consistent with deformations caused by outward displacements of an open pipe (Bonaccorso and Davis, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Segall, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). Inflation of an open pipe induces uplift on the side and subsidence above the pipe (Additional File 1: Figure S6). Overpressure of a closed pipe also generates inflation on the side. Unlike an open pipe, pressurization of a closed pipe uplifts above the pipe but with a smaller magnitude than the side (Bonaccorso and Davis, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Segall, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). Here, we assume an open and a closed pipe as a deformation source to model the detected precursory deformation. The best-fit parameters for each model were estimated by fitting the cumulative PALSAR-2 LOS changes in Path 23 and 131 in 2017 based on the particle swarm optimization algorithm, one of the swarm intelligence algorithms (Kennedy and Eberhart \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e1995\u003c/span\u003e). The algorithm can infer the best-fit model with uncertainties as a form of marginal posterior distribution, which can be derived from the evolution of each particle position over iterations.\u003c/p\u003e \u003cp\u003eThe window size for further searching the best positions in the next step is adjusted based on the distances between the present position of each particle at step \u003cem\u003et\u003c/em\u003e (\u003cem\u003et\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1,2\u0026hellip;) and the global best position (minimum RMSE within the swarm) or the personal best positions (minimum RMSE for each particle). The initial particle positions were randomly assigned within the designated range of parameters. The computation was iterated 400 times by searching with 30 particles. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the parameter ranges and the best-fit parameters. The data fitness was evaluated by root-mean-square errors (RMSE) between the observation and the calculation. To reduce computational costs and optimize data weighting, the input data were subsampled by circular grids centered on the crater. Data weighting between two LOS changes was based on the data variance. The first 20% of particle position histories were discarded to compute the posterior marginal distribution. The reflection boundary was set so the parameters would not exceed the designated search range.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eParameter search range and best-fit parameters.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSearch range\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eOpen pipe\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eClose pipe\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eUTM Easting [km]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e[677, 679]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e677.98 (0.19)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e678.03 (0.25)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eUTM Northing [km]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e[3531, 3533]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3532.19 (0.17)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3532.10 (0.22)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003es*a [m*m]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e[0, 50]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e33.1 (5.7)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLog10(a\u003csup\u003e2\u003c/sup\u003eΔP) [m\u003csup\u003e2\u003c/sup\u003e*Pa]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e[5, 14]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e11.7 (1.3)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTop depth [m (b.s.l.)]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e[-1350, 0]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e-1152.7 (148.1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e-577.7 (196.7)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePipe length [m]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e[0, 2500]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e535.4 (247.5)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e438.9 (388.9)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRMSE [cm]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.13\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003eValues in round brackets show standard deviations of particle positions through the inversion.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eOur result suggests that the synthetic deformation caused by outward displacements of an open pipe fits better to reconstruct the observation than the overpressure of a closed pipe (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). The RMSE with the open pipe is 1.3 cm, smaller than that with the closed pipe (2.1 cm). The best-fit open pipe reproduces both LOS extension (contraction) at the crater and LOS shortening (inflation) at the flank. In contrast, the closed pipe reproduces only LOS shortening with smaller magnitudes above the pipe (Additional File 1: Figure S6). Both models find residuals of LOS shortenings from the ascending images at the east side of the crater, where minor inflation occurred during the 2017 eruption.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe best-fit open pipe source is 620\u0026ndash;1150 m above sea level, approximately 200\u0026ndash;730 m below the surface. The inferred extent of the pipe-like deformation source suggests that most of the pressurization preceding the 2017 eruption occurred above sea level. The pressurized pipe is less than 10% of the conduit, connecting the shallow magma chamber 5 km below sea level and the summit crater (Aizawa et al. 2014).\u003c/p\u003e \u003cp\u003eOur results infer that the optimum value of the product of the conduit radius and the outward displacements on the volcanic conduit is 33.1 [m*m] (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). Because the pipe radius and the amount of deformation are coupled in the modeling with an open pipe, the radius of the volcanic conduit must be given as \u003cem\u003ea priori\u003c/em\u003e to infer the amount of displacement. The flux of tephra associated with the 2011 eruption constrained the conduit radius of Shinmoe-dake volcano as between 4.5 and 6.0 m (Sato et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Given these values, we derive 5.5\u0026ndash;7.4 m of the outward displacement of an open pipe, which exceeds twice the conduit radius. The optimum value of log\u003csub\u003e10\u003c/sub\u003e(a\u003csup\u003e2\u003c/sup\u003eΔP) (a: conduit radius [m], ΔP: Pressure change [Pa]) for a close pipe was 11.7 under the assumption of rigidity of 5 GPa (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The given conduit radius of the close pipe requires more than 10\u003csup\u003e10\u003c/sup\u003e Pa of excess pressure change to reconstruct the observed deformation. Even with the rigidity of 0.1 GPa, the pressure change still needs to be on the order of 10\u003csup\u003e8\u003c/sup\u003e Pa. These values are implausible because they exceed the rock's tensile strength at the crust's shallow depth (~\u0026thinsp;10 MPa; Heap et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Therefore, we need to discuss further the implications of the derived outward displacement of the open pipe.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eImplication of the modeling result\u003c/h2\u003e \u003cp\u003eVolcano inflation is a typical indicator of pre-eruptive deformations by overpressure within a subsurface volcanic system. However, some studies reported that the continuous release of magmatic fluid from a conduit system potentially induces non-magmatic eruptions (Girona et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Nobile et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). They interpreted that the outgassing of magmatic fluid cause broader subsidence by depressurization. In contrast, the spatiotemporal characteristics of the small-scale precursory deformation, with crater contraction, flank inflation and increasing deformation magnitude toward the eruption have rarely been reported.\u003c/p\u003e \u003cp\u003eAssuming pipe as the deformation source would be reasonable because the distribution of tremor sources implies a well-developed conduit beneath Shinmoe-dake volcano from the crater to a depth of 1 km below sea level (Ichihara et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2023a\u003c/span\u003e). The geometry of the pipe-like deformation source has been employed to explain tilt changes and displacements associated with volcanic eruptions (Genco \u0026amp; Ripepe, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Ripepe et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Saballos et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Pressurization of an open pipe induces radial displacements at the flank (the side of the conduit). To our knowledge, no study has employed the open-pipe model to reproduce contractions at craters and flank inflations simultaneously.\u003c/p\u003e \u003cp\u003eThe conduit radius is one of the critical parameters to interpret not only pressure changes or deformation of a conduit but also to understand physical processes based on numerical models, such as a conduit flow model (Aravena et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2018a\u003c/span\u003e), and is usually constrained by the flux of volcanic materials within a conduit (Kazahaya et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Stevenson and Blake \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e1998\u003c/span\u003e).\u003c/p\u003e \u003cp\u003ePressurization of a pipe sometimes yields physically unrealistic pressure changes if we assign the conduit radius constrained by the flux of the volcanic material (Widiwijanti et al. 2005; Green et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). As previously mentioned, we also proposed that an outward displacement of twice the original pipe radius is required to explain the observed deformation at the crater in the case of the deformation preceding the 2017 Shinmoe-dake eruption. This paradox comes from the gap between the conduit radius as the pathway of the material (actual radius) and the effective radius for pressure changes. The effective radius of the conduit must be larger than the actual conduit radius for the pressure changes to be realistic. The fluid-saturated fracture around the volcanic conduit increases the cause of the larger effective pressurized extents of the conduit (Widiwijanti et al. 2005). Although the shear traction along the conduit wall due to viscous flow resistance allows us to avoid unrealistic pressure changes or the conduit radius, it cannot explain the contraction at the crater in this case (Green et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). A precise tremor relocation associated with the 2017\u0026ndash;2018 eruption reveals a conduit-shaped structure with a width of about 500 m, implying degassing from the magma within the conduit (Ichihara et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2023a\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003eb\u003c/span\u003e). If the pipe radius is 250 m, the required outward displacement of an open pipe is ~\u0026thinsp;13 cm, which is considered plausible.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003ePotential mechanisms for the precursory deformation\u003c/h2\u003e \u003cp\u003ePressurization due to either mass supply from depths or an accumulation of volcanic gas exsolving from the magma is responsible for observed inflations preceding eruptions. Instead, depressurization within a volcanic conduit or a collapse of the conduit structure is responsible for crater contraction (e.g., Lipman, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e1997\u003c/span\u003e). While contraction at a crater is usually observed during or after an eruption, it has not been often observed before an eruption. To interpret the crater contraction preceding the 2017 eruption of Shinmoe-dake volcano, we propose one potential scenario below.\u003c/p\u003e \u003cp\u003eThe scenario starts with ascending magma head with migrating from depths causing the conduit expansion (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). The expansion of the conduit can generate cracks along the porous conduit wall, inducing an inflow of groundwater from an ambient aquifer into the conduit through the thus-generated cracks. A low-resistivity structure at depths shallower than sea level is considered an ambient aquifer beneath Shinmoe-dake and Iwo-yama volcanoes (Kagiyama et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1996\u003c/span\u003e). The physical contact between pre-existing materials within the conduit and the inflowing groundwater causes the decrease in the volume of the materials. The material within the conduit is likely the pre-existing magma ejected during the 2011 eruption, which is not entirely solidified because of the gradual magma ascent by mid-2016 (NIED, 2017). The cooling materials cause decoupling between the materials within the conduit and the conduit wall, inducing gradual withdrawal of the materials. The compaction of the materials within the shallower part of the conduit causes the local-scale contraction at the crater. In contrast, the ascent of magma through the conduit from deeper depths induced the broader inflation at the flank.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSeismic observation suggests that the interaction between magma and groundwater triggered the 2017 eruption (Konstantinous et al. 2022). The partial failure of the conduit or the collapse of material within the conduit triggers an eruption through the interaction between groundwater and magma (Aravena et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2018b\u003c/span\u003e; Furuya et al. 2003). Based on these backgrounds, our proposed scenario can explain the process of the crater contraction and the trigger of the 2017 eruption.\u003c/p\u003e \u003cp\u003eAfter the 2017 eruption, further material instability continued, leading to further crater contraction. Simultaneously, the conduit expansion due to the magma ascent stalled, causing the cessation of the inflation at the flank. The additional material instability can induce continuous contraction preceding the 2018 eruption even if the ambient aquifer has already been depleted (Konstantinou et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). The formation of either the vent on top or the pathway formation for the ejected material within the conduit, caused by the 2017 eruption, potentially prevents pressure accumulation within the conduit, even as magma ascends.\u003c/p\u003e \u003cp\u003eSimilar mechanism is a drain-back of magma within the volcanic conduit (Watanabe et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e1998\u003c/span\u003e). A gravity observation proposed that the descent of a magma head within a conduit subsides the summit area during the post-eruptive periods of Izu-Oshima volcano, Japan, in 1986\u0026ndash;1987 (Ida et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e1988\u003c/span\u003e). As the magma head descended, the gas evaporation from the magma decreased the magma pressure. Pressurization by accumulating exsolved volcanic fluid above the magma head pushes down the magma head. The height change of the magma head can be correlated with the vertical deformations at the crater, while the drain-back of magma has been reported only during and after, not before, the eruption.\u003c/p\u003e \u003cp\u003eSo far, we cannot identify the temporal variation of the magma head preceding the 2017 eruption because no gravity observations are available near the summit of Shinmoedake volcano. Furthermore, no significant increase in volcanic earthquakes was identified for several months before the 2017 eruption (Yamada et al. \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Ichihara et al. (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2023a\u003c/span\u003e) suggested that the increasing magnitude of the seismic background noise since early 2017 is due to the degassing from the magma within the conduit. Therefore, we have identified no robust supporting evidence for the material withdrawn preceding the 2017 eruption. To further discuss the preparatory process within the conduit, additional geodetic observations near the summit, such as gravimetry, are required to investigate how much materials within the conduit moved.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThis study investigates the precursory deformation of the 2017 and 2018 Shinmoe-dake volcano eruptions. PALSAR-2 and Sentinel-1 images reveal that the inflation at the volcanic flank and the contraction at the center of the summit crater, which started five months before the 2017 eruption. The best-fit model, assuming an open pipe, well reproduces the spatial pattern of the observed deformation, contractions above the pipe, and inflation at the side of the conduit. The location of the inferred open pipe is 200\u0026ndash;730 m below the surface, implying that most processes responsible for the precursory deformation occur above sea level. If we constrained the conduit radius by the flux of the ejection materials during the previous eruption in 2011 as a priori information, double the original conduit radius is required to explain the observed crater contraction. Instead, we can derive a reasonable outward displacement of a pipe if we assign a conduit radius, estimated by the conduit-shaped tremor distribution between and during the 2017 and 2018 eruptions. We suggested one scenario to interpret the observed crater contraction based on the combination of the compaction caused by cooling by physical contact with the ambient groundwater and the material withdrawal. However, no robust information for supporting our proposed scenario, so far.\u003c/p\u003e \u003cp\u003eOverpressure by the supply of volcanic materials from depths or heating often induces inflation at the surface, preceding an eruption. Instead, the contraction at the crater preceding an eruption is rare. Our observations raised a new question about the physical process for inducing the crater contraction preceding an eruption. SAR data is still useful for detecting local-scale deformations, such as those at the crater, while further various observations, such as gravimetry, near the summit are required to identify the eruption\u0026rsquo;s preparatory processes of Shinmoe-dake volcano. Moreover, additional investigations are necessary to constrain the trigger of the crater contraction and to verify the plausibility of the effective pipe radius as a deformation source for sophisticating the model.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eJMA: Japan Meteorological Agency\u003c/p\u003e\n\u003cp\u003eSAR: Synthetic aperture radar\u003c/p\u003e\n\u003cp\u003eInSAR: Interferometric synthetic aperture radar\u003c/p\u003e\n\u003cp\u003eGNSS: Global navigation satellite system\u003c/p\u003e\n\u003cp\u003eALOS-2: Advanced L-band Observation Satellite-2\u003c/p\u003e\n\u003cp\u003ePALSAR-2: Phased Array Synthetic Aperture Radar-2\u003c/p\u003e\n\u003cp\u003eDEM: Digital elevation model\u003c/p\u003e\n\u003cp\u003eLOS: Line-of-sight\u003c/p\u003e\n\u003cp\u003eNIED: National Research Institute for Earth Science and Disaster Resilience\u003c/p\u003e\n\u003cp\u003eRMSE: Root-mean-square error\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eOriginal Sentinel-1 images are available from the Copernicus Open Access Hub website (https://scihub.copernicus.eu/) after registration. The data in this study can be available after reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study is supported by ERI JURP 2018-B-02 and 2021-B-03 in Earthquake Research Institute, the University of Tokyo. This study is conducted under the framework of Subtheme 2-1, Project B of \u0026ldquo;Integrated program for next generation volcano research and human resource development\u0026rdquo; led by the Ministry of Education, Culture, Sport, Science and Technology, Japan (MEXT).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026apos; contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eYuji Himematsu: Conceptualization, Formal analysis, Investigation, Writing \u0026ndash; original draft. Taku Ozawa: Funding acquisition, Project administration, Conceptualization, Supervision, Writing \u0026ndash; review \u0026amp; editing. Yosuke Aoki: Project administration, Conceptualization, Supervision, Writing \u0026ndash; review \u0026amp; editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePALSAR-2 level 1.1 data in this study are shared among a Japan InSAR consortium PIXEL and provided by JAXA under a cooperative research contract with the PIXEL (PI No. PER2A2N187 and PER3A2N013). The ownership of PALSAR-2 image belongs to JAXA.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAravena \u0026Aacute;, Cioni R, De\u0026rsquo;michieli Vitturi M, Neri A (2018a) Conduit stability effects on intensity and steadiness of explosive eruptions. 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Geophys Res Lett 48(11):1\u0026ndash;12. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1029/2021GL092879\u003c/span\u003e\u003cspan address=\"10.1029/2021GL092879\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"earth-planets-and-space","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"epsp","sideBox":"Learn more about [Earth, Planets and Space](http://earth-planets-space.springeropen.com)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/epsp/default.aspx","title":"Earth, Planets and Space","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Volcanic crater, Eruption precursor, Shinmoe-dake volcano, Ground deformation, InSAR","lastPublishedDoi":"10.21203/rs.3.rs-4572750/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4572750/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe time series of PALSAR-2 and Sentinel-1 images reveal inflation at the volcanic flank and contraction at the crater for approximately five months before the 2017 eruption of Shinmoe-dake volcano, Japan. While the observation of inflation at the volcano\u0026rsquo;s flank is ubiquitous, few studies have reported crater contraction at a crater preceding an eruption. The flank inflation stopped after the 2017 eruption, while the contraction at the crater continued until the 2018 eruption. We found that a pipe-shaped deformation source above sea level best fits the observation preceding the 2017 eruption. Suppose the flux of ejected materials constrains the conduit radius during the previous 2011 eruption. In that case, the amount of deformation of the pipe-shaped deformation source, whether open or closed at its top, is too large to be realistic. Although constraining the conduit radius from the eruption flux overestimates the pressure change of the pipe-shaped deformation source, water-saturated fractures along the volcanic conduit could extend the effective conduit radius of the pressure source. We propose one potential scenario for the mechanism of the crater contraction preceding volcanic eruptions based on the combination of compaction due to cooling by ambient groundwater and material withdrawal within the conduit. The groundwater inflows from the ambient aquifer through cracks in the porous conduit wall, which are generated by conduit expansion during the magma ascent. Decoupling from the conduit wall due to a decrease in volume of the material promotes material instability and crater contraction. The interaction between the groundwater and the magma triggers the 2017 eruption of Shinmoe-dake volcano, as previous studies have reported.\u003c/p\u003e","manuscriptTitle":"Precursory crater contraction associated with the 2017 eruption of Shinmoe-dake volcano (Japan) detected by PALSAR-2 and Sentinel-1 InSAR","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-22 20:54:32","doi":"10.21203/rs.3.rs-4572750/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major Revision","date":"2024-08-11T04:43:24+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2024-06-28T06:59:17+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-06-28T06:45:17+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-06-28T02:46:28+00:00","index":"","fulltext":""},{"type":"submitted","content":"Earth, Planets and Space","date":"2024-06-18T09:42:07+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"earth-planets-and-space","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"epsp","sideBox":"Learn more about [Earth, Planets and Space](http://earth-planets-space.springeropen.com)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/epsp/default.aspx","title":"Earth, Planets and Space","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"652d7d1a-b583-4d36-acd9-9c862ccfc428","owner":[],"postedDate":"July 22nd, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-11-11T16:01:25+00:00","versionOfRecord":{"articleIdentity":"rs-4572750","link":"https://doi.org/10.1186/s40623-024-02083-8","journal":{"identity":"earth-planets-and-space","isVorOnly":false,"title":"Earth, Planets and Space"},"publishedOn":"2024-11-04 15:57:41","publishedOnDateReadable":"November 4th, 2024"},"versionCreatedAt":"2024-07-22 20:54:32","video":"","vorDoi":"10.1186/s40623-024-02083-8","vorDoiUrl":"https://doi.org/10.1186/s40623-024-02083-8","workflowStages":[]},"version":"v1","identity":"rs-4572750","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4572750","identity":"rs-4572750","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}