Tomofumi Kozono

Magma discharge variations during the 2011 eruptions of Shinmoe-dake volcano, Japan, revealed by geodetic and satellite observations

We present precise geodetic and satellite observation-based estimations of the erupted volume and discharge rate of magma during the 2011 eruptions of Kirishima-Shinmoe-dake volcano, Japan. During these events, the type and intensity of eruption drastically changed within a week, with three major sub-Plinian eruptions on January 26 and 27, and a continuous lava extrusion from January 29 to 31. In response to each eruptive event, borehole-type tiltmeters detected deflation of a magma chamber caused by migration of magma to the surface. These measurements enabled us to estimate the geodetic volume change in the magma chamber caused by each eruptive event. Erupted volumes and discharge rates were constrained during lava extrusion using synthetic aperture radar satellite imaging of lava accumulation inside the summit crater. Combining the geodetic volume change and the volume of lava extrusion enabled the determination of the erupted volume and discharge rate during each sub-Plinian event. These precise estimates provide important information about magma storage conditions in magma chambers and eruption column dynamics, and indicate that the Shinmoe-dake eruptions occurred in a critical state between explosive and effusive eruption.

Ballistic ejecta and eruption condition of the vulcanian explosion of Shinmoedake volcano, Kyushu, Japan on 1 February, 2011

The physical condition of the 1 February, 2011, vulcanian explosion at Shinmoedake volcano, Japan, is estimated based on the size of impact craters created by ballistic ejecta, using a ballistic trajectory model and a scaling law for impact crater formation. The initial velocity, impact velocity and mass of ejecta were estimated at 240–290 m/s, 140 ± 20 m/s and 1–3 ton, respectively. The gas mass fraction at the source was calculated to be 0.04–0.1, using the initial velocity and a theoretical model of vulcanian explosion. This gas mass fraction is higher than the petrologically estimated value for pre-eruptive magma. Low-angle jets from the explosion and the estimated depth and size of a pressurized gas region suggest a shallow source inside the lava dome. The observation and results imply that segregation and accumulation of gas in a shallow conduit played a role in an increase of excess pressure immediately below the dome surface, prior to the vulcanian explosion.

Crustal deformation associated with the 2011 Shinmoe-dake eruption as observed by tiltmeters and GPS

The National Research Institute for Earth Science and Disaster Prevention (NIED) developed volcano observation stations at the Kirishima volcanic group in 2010. The stations observed remarkable crustal deformation and seismic tremors associated with the Shinmoe-dake eruption in 2011. The major eruptive activity began with sub-Plinian eruptions (January 26) before changing to explosive eruptions and continuous lava effusion into the summit crater (from January 28). The observation data combined with GEONET data of GSI indicated a magma chamber located about 7 km to the northwest of Shinmoe-dake at about 10 km depth. The tiltmeter data also quantified detailed temporal volumetric changes of the magma chamber due to the continuous eruptions. The synchronized tilt changes with the eruptions clearly show that the erupted magma was supplied from the magma chamber; nevertheless, the stations did not detect clear precursory tilt changes and earthquakes showing ascent of magma from the magma chamber just before the major eruptions. The lack of clear precursors suggests that magma had been stored in a conduit connecting the crater and the magma chamber prior to the beginning of the sub-Plinian eruptions.

Temporal variation of the Shinmoe-dake crater in the 2011 eruption revealed by spaceborne SAR observations

SAR observations to investigate the 2011 Shinmoe-dake eruption were carried out using several SAR satellites. We detected temporal variations of the Shinmoe-dake crater in the SAR images. A convex shape appeared in the PALSAR image acquired on January 27, but it is unclear whether this shape indicates a lava dome. A larger convex shape that does indicate a lava dome could be identified in SAR images acquired after that, and it seems to have grown progressively. Estimating topography so that the simulated intensity image corresponds to the acquired one, we estimated that the lava dome grew from the morning of January 29 until January 31 with a constant effusion rate of 88.7 m3/sec. The lava volume of 15 million m3 estimated from the TerraSAR-X image of February 1 was consistent with that of airborne SAR observation within 20% of its volume. From the estimated lava effusion rate and lava-covered area, we estimated that lava viscosity was less than 2.1 GPa∙s, suggesting the potential to form a lava flow. Furthermore, we corrected the foreshortening distortion using the estimated topography and suggested that the lava effusion point was around the crater lake.

Correlation between magma chamber deflation and eruption cloud height during the 2011 Shinmoe-dake eruptions

Multiple observations of subsurface and surface phenomena during volcanic eruptions provide important information about eruption styles, eruption column dynamics, and magma plumbing systems. During the 2011 eruptions of Kirishima-Shinmoe-dake volcano in Japan, borehole-type tiltmeter data and weather radar data captured the subsurface and surface phenomena, respectively; the tiltmeters detected deflation of a magma chamber caused by migration of magma to the surface, and the weather radar detected changes in the height of the eruption cloud echo. In this study, we present a method based on the correlation between magma chamber deflation and cloud echo height to identify eruption styles. The method can detect whether a column-forming eruption is accompanied by magma migration from the magma chamber (e.g., sub-Plinian eruption), or not (e.g., Vulcanian explosion). By using well-correlated chamber deflation and echo height data, we found that eruption column dynamics during the Shinmoe-dake eruptions are well described by a one-quarter power scaling relationship between cloud height and magma discharge rate, and that a clear correlation between geodetic volume change of the magma chamber and the erupted volume indicates a stable magma plumbing system connecting the magma chamber and the surface.

Silicic lava effusion controlled by the transition from viscous magma flow to friction controlled flow

Silicic magma forms a vertical fault in volcanic conduits via the brittle-ductile transition. The formation of the fault changes flow type from viscous flow to friction controlled flow. Here we investigate flow dynamics of magma by coupling a one-dimensional conduit flow model with an experimentally calibrated brittle-ductile transition. The numerical simulation demonstrates that the length of magma plug at which friction controlled flow becomes the main flow type depends on magma flux, because of crystallization kinetics and the ductile-brittle transition. At high flux, a short plug forms, because nonequilibrium crystallization inhibits an increase of magma viscosity. This results in the effusion of less viscous lava and large shear stress at the shallow part of the conduit. In contrast, the long plug that forms under low flux cannot maintain large shear stress due to weakness of the magmatic fault, which causes the extrusion of a solidified lava spine.

Numerical assessment of the potential for future limnic eruptions at lakes Nyos and Monoun, Cameroon, based on regular monitoring data

Abstract We assessed the potential for limnic eruptions at lakes Nyos and Monoun, Cameroon on the basis of numerical modelling and CO2 profiles obtained by regular monitoring of the lakes. The change through time of the profiles suggests one particular scenario for producing an eruption: a supply of CO2-undersaturated fluid from the lake bottom that induces upwards growth of the CO2-rich bottom layer, leading eventually to CO2 saturation at mid-depths of the lake. By using a numerical model for the ascent of a plume of CO2 bubbles, we found that under realistic conditions (e.g. a profile of CO2 as deduced from the regular monitoring data), a bubble plume generated from the middle depths of the lake can reach the lake surface with a high flux of CO2, which corresponds to a limnic eruption. In addition, we developed a numerical model to investigate how changes in the CO2 concentration at the lake bottom affect the dynamics of a two-phase flow in the controlled degassing pipe, using the recently observed CO2 profiles. This model enables us to estimate the CO2 concentrations at the lake bottom from the heights of fountains that are observable at the lake surface.

Long-term lava extrusion after the 2011 Shinmoe-dake eruption detected by DInSAR observations

In January 2011, the latest eruption began at Shinmoe-dake volcano, Japan, and lava accumulated in the crater. Differential interferometric synthetic aperture radar (DInSAR) observations after the end of main eruption indicated continuous uplift on the lava surface. We estimated the volume increase of lava, and the volume change rate has decreased exponentially. Results from long-term DInSAR processing indicate slow subsidence outside the crater. We interpret that this subsidence is caused by deflation of a shallow source located beneath the crater, which is a reaction to the extrusion of lava. Between November 2011 and May 2013, the total volume of extruded lava within the crater is estimated to be 6.7 ± 0.1 × 104 m3, which is comparable to the deflation volume of the shallow source. The system is not closed within the shallow source to the crater; thus, long-term lava extrusion can be explained by continuous injection from a deeper source to the shallow source.