Aira Caldera Research Articles

Voluminous magma formation for the 30-ka Aira caldera-forming eruption in SW Japan: contributions of crust-derived felsic and mafic magmas

Understanding the origin, assembly, and evolution of voluminous magma that erupts in catastrophic caldera-forming eruptions (CCFEs) is a community imperative. A CCFE of the Aira caldera at 30 ka discharged over 350 km3 of magma, which can be grouped into petrographically and geochemically distinct types: voluminous rhyolite, small amounts of rhyodacite, and andesite magmas. To further understand the magma plumbing system of the Aira CCFE, we examined the geochemical characteristics of whole rock and plagioclase from its eruptive deposits. The trace element and 87Sr/86Sr signatures recorded in the plagioclase phenocrysts of these magmas indicate that the three magmas were originally produced by partially melting an identical source rock, which was estimated to be a mafic amphibolite with an 87Sr/86Sr signature of ∼0.7055 that comprised the lower crust. Melting of mafic amphibolite produced both felsic and mafic magmas by low and high degrees of partial melting, respectively. The mafic magma assimilated uppermost crustal materials and crystallized to produce an andesite magma type. The andesitic magma consists of phenocrysts (∼39 vol%) and melt with a dacitic (∼70 wt% SiO2) composition. The felsic magma mixed with ∼10% of the andesite magma and crystallized, forming the rhyolite magma. The mixing between the andesite and rhyolite magmas before the Aira CCFE produced the rhyodacite magma. The 30-ka Aira CCFE magmas were generated only by melting two kinds of crustal materials with different geochemical characteristics and had geochemical variations due to different conditions of partial melting and mixing between various crustal melts. The lack of definitive evidence of the mantle component mixing with the Aira CCFE magmas suggests that the mantle-derived magmas worked only as a heat source for crustal melting.

Solidified magma reservoir derived from active source seismic experiments in the Aira caldera, southern Kyushu, Japan

The Aira caldera, located in southern Kyushu, Japan, originally formed 100 ka, and its current shape reflects the more recent 30 ka caldera-forming eruptions (hereafter, called the AT eruptions). This study aimed to delineate the detailed two-dimensional (2D) seismic velocity structure of the Aira caldera down to approximately 15 km, by means of the travel-time tomography analysis of the seismic profile across the caldera acquired in 2017 and 2018. A substantial structural difference in thickness in the subsurface low-velocity areas in the Aira caldera between the eastern and western sides, suggest that the Aira caldera comprises at least two calderas, identified as the AT and Wakamiko calderas. The most interesting feature of the caldera structure is the existence of a substantial high-velocity zone (HVZ) with a velocity of more than 6.8 km/s at depths of about 6–11 km beneath the central area of the AT caldera. Because no high ratio of P- to S-wave velocity zones in the depth range were detected from the previous three-dimensional velocity model beneath the AT caldera region, we infer that the HVZ is not an active magma reservoir but comprises a solidified and cool remnant. In addition, a poorly resolved low-velocity zone around 15 km in depth suggests the existence of a deep active magma reservoir. By superimposing the distribution of the known pressure sources derived from the observed ground inflation and the volcanic earthquake distribution onto the 2D velocity model, the magma transportation path in the crust was imaged. This image suggested that the HVZ plays an important role in magma transportation in the upper crust. Moreover, we estimated that the AT magma reservoir in the 30 ka Aira caldera-forming eruptions has the total volume of 490 km3 DRE and is distributed in a depth range of 4–11 km.Graphical

Geochemical constraints on the evolution of the magmatic system leading to catastrophic eruptions at Aira Caldera, Japan

Caldera-forming eruptions of Aira volcano at 30 ka were among the most explosive eruptions in Japan during the late Pleistocene. In this study, new data on whole-rock major and trace elements, Sr–Nd–Pb isotopes, and 238U-230Th disequilibrium were obtained for representative volcanic products of caldera-forming and pre-caldera eruptions (31–90 ka) to acquire insights on the evolution of the felsic magma system leading to catastrophic eruptions. Voluminous rhyolitic pumices from the caldera-forming eruptions at 30 ka and rhyolitic lavas and pumices of the younger pre-caldera eruptions (31–36 ka) have similar Sr, Nd, and Pb isotopic compositions, and differ from those of dacitic and rhyolitic pumices produced by eruptions at 90 and 60 ka, respectively. These three types of felsic magmas are considered to have formed by partial melting of hornblende gabbroic rocks with distinct isotopic compositions in the middle-lower crust. Most felsic products from the pre-caldera and caldera-forming eruptions are in secular 238U-230Th equilibrium, suggesting that the original magmas were generated > ∼ 400 ka and existed as long-lived crystalline mush in the deep hot crust. Felsic magmas originating from the deep crustal mush zones ascended to form the shallow felsic magma system, which was replaced by a new one after the explosive eruptions at 90 and 60 ka. The voluminous rhyolitic magmas for the caldera-forming eruptions accumulated in a shallow magma reservoir between 60 and 30 ka. Rhyolitic magmas with compositions similar to those of the voluminous magmas erupted catastrophically at 30 ka began to erupt effusively at 36 ka and 33 ka as lava flows with significant 238U-230Th disequilibrium, suggesting that these magmas were generated in the deep crust much later than the other main rhyolitic magmas. The effusive magma eruptions presumably resulted from the relatively low content of dissolved H2O in the melt due to the low crystallinity because of the shorter residence time in the shallow felsic magma system.

Influence of a low viscosity zone on the evolution of post-eruption deformation: A case study of the crustal deformation of Aira Caldera after the 1914 eruption of Sakurajima Volcano

Spatial viscosity variations in the crust can be inferred from geophysical imaging and may be essential for interpreting volcano deformations that show more complex behaviours than what simple uni-viscous models predict. In southern Kyushu, Japan, recent seismic imaging implies the presence of a low viscosity zone (LVZ) beneath Aira Caldera, and geodetic data in and around the caldera require uni-viscous model to have lower and higher viscosities earliest and later, respectively, in the period following the 1914 eruption of Sakurajima Volcano. Here, we use a 3D finite element model comprising an elastic layer underlain by a linear Maxwell viscoelastic layer to examine the influence of a LVZ on ground surface displacement in response to two different deformation source modes, i.e., instantaneous source deflation during a major eruption and subsequent continuous source inflation due to magma recharge. A LVZ is introduced into the viscoelastic layer by gradually reducing its viscosity towards the centre. The behaviour of the LVZ model quantified by comparison with the behaviour of a uniform viscosity (UNV) model reveals that, for a given LVZ structure, an apparent UNV model that best represents the LVZ model displacement in response to the instantaneous deflation has lower viscosity than that in response to gradual inflation, i.e., the rate-controlling viscosities of the LVZ model are those in the inner and outer parts of the LVZ for the instantaneous source deflation and subsequent continuous inflation, respectively. Such LVZ model behaviour, for a LVZ spatial extent comparable with the imaged low velocity anomaly, explains well the geodetic data in and around Aira Caldera at any stage after the 1914 eruption, as the predominant viscoelastic response changes with time from that to the instantaneous deflation to that to the subsequent continuous inflation. This study highlights the need for interdisciplinary investigations that integrate geodetic and geophysical datasets to better understand volcanic unrest.

Vegetation transition from the terminal Pleistocene to early Holocene reconstructed from phytolith records in the southernmost part of mainland Japan

Phytolith records in paleosols provide valuable information on past vegetation in situ, especially for herbaceous plants. Although vegetation transition between the terminal Pleistocene and early Holocene in the southernmost parts of mainland Japan has been examined, detailed information on herbaceous plants during the period is scarce. Therefore, this study examined phytolith records in paleosols in the region to reconstruct the vegetation transition from the terminal Pleistocene to the early Holocene, focusing on herbaceous vegetation. Additionally, the combination of our phytolith and previous palynological records enable us to reconstruct vegetation transition in detail. Before ~30,000 cal BP, Andropogoneae species flourished under forest, comprising temperate pinaceous conifers, deciduous broad-leaved trees, and/or their adjacent area. In the Last Glacial Maximum (LGM), grasslands comprising Panicoideae, Chloridoideae, and Pooideae species were widely distributed under the influence of cool climate conditions and/or Aira caldera eruption. Furthermore, between ~18,000 and ~17,000 cal BP, Sasa flourished as a component of grassland or on forest floors, and then was gradually replaced by Andropogoneae species. Evergreen broad-leaved forests had developed at least by 7,300 cal BP in the study region.

Post-caldera volcanism and environmental changes in Aira Caldera, inner Kagoshima Bay, SW Japan

Post-caldera volcanism and environmental changes in Aira Caldera, inner Kagoshima Bay, SW Japan

High Resolution Three-dimensional Seismic Velocity Imaging below Aira Caldera

High Resolution Three-dimensional Seismic Velocity Imaging below Aira Caldera

Late Quaternary Crustal Movements and Volcanic Activities of Aira Caldera in Southern Kyushu, Japan

Late Quaternary Crustal Movements and Volcanic Activities of Aira Caldera in Southern Kyushu, Japan

Classification of volcanic tremors and earthquakes based on seismic correlation: application at Sakurajima volcano, Japan

SUMMARY Classification of volcanic tremors and earthquakes is an important part of volcano monitoring. Conventional classification technique relies on visual characterization of the amplitudes, frequency, or duration of seismic signals. In some cases, such classification is difficult to be maintained continuously, especially during high eruptive activities. In the present study, we develop a classification technique based on analyses of seismic wave correlation. The technique is based on a measure of spatial coherence using the distribution width of eigenvalues from the seismic covariance matrix that is referred to as ‘spectral width’. We use the spectral width characteristics to extract the frequencies where the seismic signals are the most spatially coherent. We analyse 6-month continuous seismic data at six seismic stations deployed on Sakurajima volcano, Japan. The classification is performed every 10 min to recognize volcanic tremor and B-type earthquakes, both of which show unclear onsets and are dominant at around 1–4 Hz. Their frequencies of high spatial coherence are different from each other, providing a basis for automatic classification of both types of seismic events. Our classification results show that an increase in seismic activity, particularly volcanic tremors, occurred during high eruptive activity and is well matched with the events that are routinely determined by the Japan Meteorological Agency (JMA) based on visual examination of the data. We discuss the volcanic activities by combining our classification results with those from detections and location methods based on seismic correlation. Most of the source locations of volcanic tremor and B-type earthquakes are distributed at the same region with depths of less than 4 km beneath the active craters, which are almost consistent with the source locations of explosion earthquakes and B-type earthquakes determined from P-wave onsets in the previous studies. Also, the distribution of horizontal locations is spatially elongated at shallow depths above a dyke that is inferred to extend from Aira caldera at a depth of 7–9 km. We obtain deep and shallow sources for the tremor that occurred before and after an explosive eruption, respectively. We also obtain shallow sources beneath the active crater during a continuous tremor that accompanies increasing eruptive activity. The source locations of the classified volcanic tremor and B-type earthquakes may represent the movement of magma and gas from the shallow magma chamber at 5 km depth to the active craters.

Variable inflation rate of a magmatic deformation source beneath Aira caldera after the 1914 eruption of Sakurajima volcano: Inferences from a linear Maxwell viscoelastic model constrained by geodetic data

Temporal variation of ground surface displacement rates at volcanoes may represent changes to the volumetric magma supply rate over time. If crustal rock responds elastically to a magmatic deformation source, the volume change of the source has a simple one-to-one relation to the surface deformation. For viscoelastic crustal rock, however, the correlation is not always straightforward, particularly at volcanoes where crustal viscosity is lowered by a high geothermal gradient. For a continuous inflation of the deformation source, the viscoelastic surface displacement depends on the balance between the uplift due to magma supply and subsidence due to viscoelastic relaxation. In this study, using a 3D finite element model composed of an elastic layer underlain by a linear Maxwell viscoelastic layer with a spatially uniform viscosity, we estimate the temporal variation of volumetric magma supply rate into the upper crust beneath Aira caldera in southern Kyushu, Japan, after the 1914 eruption of Sakurajima volcano. It is found that the geodetic data, including levelling and GNSS displacement fields, require a sill-like magmatic deformation source at a depth of 11 km to inflate at the following rates for six different periods: (I) ~6.9–9.4 × 106 m3/yr in 1914–1934, (II) ~9.1–16.7 × 106 m3/yr in 1934–1960, (III) ~1.6–3.8 × 106 m3/yr in 1960–1968, (IV) ~8.1–11.0 × 106 m3/yr in 1968–1976, (V) ~ −1.0–2.2 × 106 m3/yr in 1976–1997, and (VI) ~5.8–9.4 × 106 m3/yr in 1997–2007. The constrained viscosity (ηc) of the viscoelastic crust ranges from ~5 × 1018 Pa s to ~1020 Pa s or more. The lowest and highest values produce the best fit to the data in the earliest period and in period V showing surface subsidence, respectively, but for the other periods any viscosity is acceptable unless it is lower than ~1018 Pa s. The lowest viscosity providing the best fit in the earliest period indicates that viscoelastic displacement in response to the 1914 eruption plays an important role in the surface deformation field. The observed surface subsidence in period V can be explained either by inflation or deflation of the deformation source, depending on whether ηc is lower or higher than ~1019 Pa s, respectively. The subsequent surface recovery in period VI requires an increase in the inflation rate for any case of ηc. The volume of magma accumulated since the 1914 eruption (ΔV) is predicted to be ~0.6–0.8 km3 in 2020. The estimated inflation/deflation rate of the deformation source, in comparison with the observed volumetric eruption rate, reveals that a significant eruption occurs only when ΔV is ~0.4 km3 or more. However, even if ΔV is beyond the critical value, the eruptive activity does not depend on ΔV. The temporal inflation rate inferred in this study gives an opportunity to discuss eruptive dynamics in relation to the observed eruptive activity, such as a quantitative relationship between magma supply rate into, and discharge rate from, a deformation source.

Magma chamber decompression during explosive caldera-forming eruption of Aira caldera

Decompression of a magma chamber is a fundamental condition of caldera collapse. Although theoretical models have predicted the decompression of magma chambers before caldera collapse, few previous studies have demonstrated the amount of magma chamber decompression. Here, we determine water content in quartz glass embayments and inclusions from pyroclastic deposits of a caldera-forming eruption at Aira volcano approximately 30,000 years ago and apply this data to calculate decompression inside the magma chamber. We identify a pressure drop from 140–260 MPa to 20–90 MPa during the extraction of around 50 km3 of magma prior to the caldera collapse. The magma extraction may have caused down-sag subsidence at the caldera center before the onset of catastrophic caldera collapse. We propose that this deformation resulted in the fracturing and collapse of the roof rock into the magma chamber, leading to the eruption of massive ignimbrite.

Lithostratigraphy and geochemistry of Aojiki volcano and Sumiyoshiike and Yonemaru maars, Kamo Volcanic Field (Southern Kyushu), Japan

The Kamo Volcanic Field (KVF), a partly preserved Quaternary volcanic sequence of three small-volume basaltic volcanoes located about 10 km northwest of the Aira caldera in the Kagoshima graben in southwestern Kyushu, Japan. It forms part of the volcanic arc of Japan and consists of the Late Pleistocene Aojiki volcano and two Holocene maars: Sumiyoshiike and Yonemaru. Here, we report the first results of a combined lithostratigraphic and petrogenetic study of these three volcanoes. Eruptive products of Aojiki volcano comprise an initial scoriaceous tephra, a lava flow and scoria fall from the cone. Sumiyoshiike and Yonemaru maar products are mainly scoria fall and pyroclastic surge deposits, which are closely related in time and were principally emplaced by phreatomagmatic explosive eruptions. Major and trace element data of the eruptive products suggest four groups of magmas. Aojiki scoria (G1: SiO2= 50.3-51.4) are the most evolved, followed by Aojiki tephra/lava (G2: SiO2 = 48.1-50.7), while Sumiyoshiike (G3: SiO2 = 47.0-48.5) and Yonemaru (G4: SiO2= 46.3-48.5) are relatively less evolved. These magmas are derived from the same spinel bearing (lithospheric) mantle source previously metasomatized by subducting fluids and might have evolved in independent magma chambers mainly by fractional crystallization and crustal contamination except for G1 magma which may have suffered some associated mixing.

Accumulation of rhyolite magma and triggers for a caldera-forming eruption of the Aira Caldera, Japan

The sudden destabilization of voluminous magma after stable accumulation in a crustal magma chamber is a key process in the sequence of a catastrophic caldera-forming eruption. We investigated the petrological characteristics of magma that has erupted before, during and after the caldera-forming eruption of the Aira Caldera, Japan. This provides an example of the evolution of a rhyolite magma system during a catastrophic caldera-forming eruption. Stratigraphic and petrological investigations on the pre-caldera eruptive materials showed accumulation of high-silica rhyolite 4–5 km below the ground surface, which commenced at least ~ 30,000 years prior to the caldera-forming eruption. A part of the accumulated rhyolite magma leaked from the chamber and caused several pre-caldera eruptions. Phase equilibrium relationships and water concentrations in glass inclusions in phenocrysts suggest that the vertical extension of rhyolite magma chamber was 2–2.5 km. The rhyolite magma maintained a temperature between 735 and 800 °C and crystal mush state with 20–50% of crystallinity during its accumulation. The injection of mafic magmas destabilized the stored magma. Further, the heating of the rhyolite magma to 780–840 °C by the heat transfer from the mafic magma decreased the crystallinity down to 10% and induced mobilization of the stored rhyolite magmas. The remobilized magma could erupt as the Plinian eruption at the beginning and produced the Osumi pumice-fall deposit. This deposit occupies approximately 10% of the total volume of the erupted magma during the caldera-forming eruption. Though the extraction of rhyolite magma was limited to part of the magma chamber, decompression of the magma chamber by the magma extraction was enough to induce the caldera collapse and resulted in the eruption of the main ignimbrite (Ito ignimbrite) from the rest of the magma chamber. The volume of the erupted magma during the caldera-forming eruption is comparable with that of the magma chamber. New rhyolite magma with discernible composition from the rhyolite magmas of the caldera-forming eruption started accumulating in the magma chamber following the caldera-forming eruption and was the source for the post-caldera eruptions.

Using high-resolution Pb isotopes to unravel the petrogenesis of Sakurajima volcano, Japan

Sakurajima volcano on the rim of Aira caldera erupts daily, threatening the major nearby population centre of Kagoshima. Before 1955, eruptions have typically consisted of intermittent Plinian and effusive activity, but since 1955, frequent Vulcanian explosions have occurred, indicating a change in pre-eruptive processes. High-resolution Pb isotopes are used here to constrain the components, including crustal assimilants, and petrogenetic processes contributing to the composition of both pre and post 1955 magmas. Sakurajima eruptive products have more radiogenic Pb (206Pb/204Pb = 18.40–18.42) than nearby Sumiyoshiike basalts (206Pb/204Pb = 18.24), a proxy for the primitive magma feeding the Sakurajima-Aira system. Sakurajima samples lie along a mixing line between these basalts and locally exposed crustal compositions. Their Pb isotopes are consistent with addition of ~ 5% average crustal melt to the primitive basaltic magma. The narrow range of Pb isotope ratios, despite variable SiO2 contents (56.6–72.3 wt%), suggests that the final erupted magmas are derived from fractional crystallization of a mafic precursory magma displaying relatively consistent levels of crustal contamination. Andesites erupted between 4–3.7 ka and the 1995 eruption are contaminated to a greater extent, indicating that magmas with distinct compositions can feed phases of activity or individual eruptions. Post 1955 andesitic pyroclastics have lower SiO2 and higher MgO than older lavas, yet equivalent Pb isotope ratios. The more mafic composition of post 1955 eruptive products can be attributed to increased throughput of mafic magma to the system.

Characterization and correlation of the Hegawa‐Kasamori 5 tephra, a widespread tephra aged c. 450 ka associated with large‐scale pyroclastic flows from southern Kyushu, SW Japan

ABSTRACTA Middle Pleistocene widespread tephra, defined here as Hegawa‐Kasamori 5 tephra (Hgw‐Ks5), has been newly recognized over a broad area of Japan. Large‐scale pyroclastic flow deposits associated with co‐ignimbrite ash fall deposits (CAFDs) of Hgw‐Ks5 have been identified in the proximal southern Kyushu area, south‐west Japan. Hgw‐Ks5 possibly originated from the Aira caldera in southern Kyushu, and it is widely spread and intercalated with deposits of the Kasamori Formation, Honshu Island, more than 1000 km away from the source. In the north‐west area of the Aira Caldera, the tephra is sparsely distributed in the form of non‐welded ignimbrites, and is exposed stratigraphically above the well‐known Kobayashi‐Kasamori tephra. Hgw‐Ks5 is characterized through petrographic features, major element geochemistry of glass shards, and refractive indices of orthopyroxene. The results of previous stratigraphic isotope studies indicate that the eruptive age of Hgw‐Ks5 is 434–458 ka (Marine Isotope Stage 12). Assuming that the CAFDs originating from the Aira Caldera are distributed concentrically, the apparent volume of Hgw‐Ks5, estimated from the area of distribution and CAFD thickness, is ~100 km3. Therefore, a volcanic explosivity index of 7 is assigned to the Hgw‐Ks5 eruption.

Viscoelastic crustal response to magma supply and discharge in the upper crust: Implications for the uplift of the Aira caldera before and after the 1914 eruption of the Sakurajima volcano

Crustal viscoelasticity is expected to influence volcano deformation as the crustal viscosity at volcanoes is lowered from regional values by higher crustal temperatures and the eventual presence of magma. In this study, we examine viscoelastic crustal response to continuous magma supply into, and sudden discharge from, a sill emplaced in the upper crust, using a three-dimensional (3-D) finite element model composed of an elastic layer underlain by a linear Maxwell viscoelastic layer with spatially uniform viscosity. We find that viscoelastic response to sill deflation due to magma discharge is an important driving force for post-eruption surface displacement, influenced both by how much viscoelastic relaxation occurs in response to sill inflation due to magma supply in the pre-eruption period and by the amount of co-eruptive sill deflation. Following an eruption, the surface elevation is always recovered later than the sill volume, because the viscoelastic response to the sill inflation detracts from the uplift. We apply this viscoelastic model to geodetic observations in the Aira caldera, southern Kyushu, Japan, over a time span of more than 100 yr. It is found that a sill emplacement with constant inflation rate can well explain the observed first-order exponential-like surface recovery after the 1914 eruption until ∼1975 if the effective crustal viscosity is ∼5×1018 Pas, the sill emplacement with an equatorial radius of ∼2 km occurs at a depth of ∼11 km, and the sill is deflated by ∼0.4 km3 during the 1914 eruption, ∼4 times less than the eruptive volume. The sill emplacement is also required to have begun ∼50 yr before the 1914 eruption with a constant inflation rate of ∼0.009 km3/yr. In the post-eruption period, the accumulation of magma in the sill during the first ∼50 yr is lower than that estimated by an elastic model, but larger thereafter. It is necessary to introduce temporal variation of the inflation rate for higher-order improved fitting to the geodetic data after ∼1975. Our results suggest that geodetic signals observed before and after magma discharge due to an eruption need to be revisited, considering the effects of crustal viscoelasticity.

ESTIMATION OF SEDIMENTATION RATE AND FRESH-SALINE ENVIRONMENT IN A COASTAL ALLUVIAL PLAIN, USING BORING CORES OF ALLUVIUM IN THE CENTRAL PART AREA OF SETO INLAND SEA, JAPAN

To estimate the sedimentary environment between the last glacial stage and the Holocene epoch of an enclosed sea in the Okayama Plain, which constitutes a portion of the Seto Inland Sea coast, we collected 4 boring cores at depths of 6 to 19 m, performed radiocarbon dating of the organic matter contained in the clay, identified the volcanic glass from the Aira Caldera eruption, and measured the electrical conductivity of the pore water. Based on these dating analyses, it was evident that the Holocene clay layer that is widely distributed throughout the Okayama Plain had already accumulated prior to 10,000 years ago. In addition, the different hardness and the presence of Aira volcanic ash in the clay layer indicated the sedimentation since Pleistocene. Because the sea level at the time was lower than this clay sedimentation level, those facts suggest freshwater lakes existed in the Okayama Plain which was dammed up by the shallower granite basement in the mouth of Kojima Bay. Based on the long clay core and date for the last 30,000 years, the sedimentation rates were estimated to be 0.18 mm/ year before 8,100 years ago and 1.19 mm/year for the last 8,100 years, respectively. In the sea water rising after 7,000 years ago, the sea level exceeded the dam and the bay became a saline condition.

Shallow magma pre-charge during repeated Plinian eruptions at Sakurajima volcano

Vigorous explosive eruptions that produce continuous high eruption plumes (Plinian eruptions) are generally assumed to tap a magma reservoir. The 1914 Plinian eruption at the Sakurajima volcano located on the Aira caldera rim is one such case, where the main magma reservoir was assumed to be located approximately 10 km beneath the caldera. However, we report that estimated magma storage depths immediately prior to the eruption were much shallower (0.9–3.2 km) on the basis of pressure at which volatiles within the phenocryst melt inclusions and plagioclase rims were finally equilibrated. The same is observed for two historic Plinian eruptions in 1471 and 1779. This depth is even shallower than the shallowest magma reservoir estimated from the pressure source for geodetic deformation during recent Vulcanian explosions (4 km beneath the crater). We propose that the magmas were fed from a thick conduit pre-charged from deeper reservoirs. The ground subsidence observed after 1914 within the Aira caldera may have been caused by conduit recharge following the eruption. Voluminous conduit recharge could be key to forecasting the next possible large eruption at the Sakurajima volcano.

Geodetic evidence for interconnectivity between Aira and Kirishima magmatic systems, Japan

It is not known whether clustered or aligned volcanic edifices at the Earth surface have connected magmatic systems at depth. Previously suggested by geological records of paired eruptions, volcano interconnectivity still lacks proper geodetic evidence. Here we use GPS time-series and deformation modeling to show how Aira caldera and Kirishima, two adjacent volcanic centers in Kagoshima graben (southern Japan), interacted during Kirishima unrest in 2011. Whereas Aira caldera had been inflating steadily for two decades, it deflated during the eruption of Kirishima which started with a large-volume lava extrusion. This deflation, which cannot be explained by stress changes, is interpreted as the result of magma withdrawal from the Aira system during the Kirishima replenishment phase. This study highlights the behavior of connected neighboring volcanic systems before and after a large eruption, and the importance of taking into account volcano interactions in eruption probability models.

Volcanic Activity of Sakurajima Monitored Using Global Navigation Satellite System

A dense Global Navigation Satellite System (GNSS) network has been deployed at Sakurajima volcano since 1995 and extends to the surrounding area of the Aira caldera. The ground deformation obtained by GNSS observation corresponds to transient eruptive activity of Sakurajima volcano, which has produced frequent vulcanian eruptions since 1955. Inflation of the volcano was detected prior to the increase in vulcanian eruptions in 1999, and resumption of the eruptions at the Showa crater. Magma intrusion events and an increase in eruptions in late 2009, late 2011, and early 2015 suggest the existence of an open-conduit system from the Aira caldera to the vents at the summit area of the Minamidake cone, through the sub reservoir beneath the older Kitadake cone. Ground deformation induced by sudden dike intrusion is different from that of previous intrusions, as revealed by the dense GNSS network. GNSS data are useful in evaluating and forecasting volcanic activity, and are available to grasp the advection and diffusion of volcanic ash.