Deflation Source Research Articles

Subsurface magma movement inferred from extensometer and tiltmeter records during the early stage of the 2018 Shinmoe-dake eruptions, Japan

We infer the temporal changes in the pressure sources that induced crustal deformation during the 2018 Shinmoe-dake eruption using strain and tilt observations and discern that the deep magmatic activity associated with the early stage of this eruption began approximately 19 h earlier than the previously defined onset of magmatic activity. Distinct tilt changes were observed from around 09:00 on 6 March to 12:00 on 8 March 2018 (JST), coincident with observed lava outflow into the crater and lava dome formation. Existing studies have attributed this tilt change to the onset of the deflation of a spherical pressure source located at ~ 7 km bsl (below sea level) to the northwest of Shinmoe-dake. Here we examine strain and tilt data that were acquired in the Kirishima volcanic group, and we find that the distinct changes in the measured strain at Isa-Yoshimatsu Observatory began at around 14:00 on 5 March. This change can be explained by the deflation of a spherical pressure source, thereby suggesting that the onset of magma ascent was earlier than previously thought. The time variation in the spherical pressure source is estimated using the time-dependent inversion of the Ensemble Kalman Filter; the deflation source ascended from ~ 11 to 7 km bsl during Phase 1 (14:00 on 5 March to 06:00 on 6 March) and descended from 7 to 8 km bsl during Phase 2 (06:00 on 6 March to 12:00 on 8 March). Interferometric synthetic aperture radar analysis suggests that a dike intrusion had occurred just below Shinmoe-dake crater until 5 March, and this inflatable crustal deformation is attributed to the emplacement of residual volcanic fluids from the 2011 eruption. It is also known that the surface eruptive activity increased during Phase 1, including an increase in ash venting from the night of 5 March. These strain and tilt observations, therefore, suggest that magma ascended from ~ 11 km bsl to the magma reservoir at 7 km bsl during Phase 1, followed by a deflation of the magma reservoir during Phase 2 due to the large magma supply to the surface.Graphical

Plate Boundary Deformation and Volcano Unrest at the Azores Triple Junction Determined From Continuous GPS Measurements, 2002–2017

AbstractGround deformation in the Azores, at the triple junction between the Eurasian, Nubian, and North American plates, has been mapped with continuous Global Positioning System (GPS) geodetic measurements to improve tectonic motion estimates and for understanding volcanic unrest. We compute daily positions, spanning almost 17 years (2000–2017), from 18 continuous GPS stations. The GPS time‐series are analyzed by searching for discontinuities and periodic functions. Results show that Flores and Graciosa islands have displacements close to predicted North American and Eurasian plate motions, respectively, while São Miguel, Terceira, São Jorge, Faial and Pico islands have displacements in between predicted Eurasian and Nubian plate motions. The Eurasian‐Nubian plate boundary in the Azores behaves as a diffuse ultra‐slow oblique spreading center with focused deformation found in the central group (CG) and São Miguel islands. The velocity field is modeled by approximating segments of the Eurasian‐Nubian plate boundary with vertical dislocations with right‐lateral motion and opening below a locking depth. Best fitting models have deep motion in the range of 2.0–2.3 mm yr−1 directed N (82.3–88.7)°E. Such displacement accounts for almost half of predicted Eurasian‐Nubian relative plate motion. The modeling results suggest that the locking depth in the CG islands is at least 20 km while in São Miguel it is about 2 km. We find transient deformation at Fogo volcano, São Miguel Island, due to unrest activity mainly during 2003–2006 and 2011–2012, and local continuous subsidence in Terceira Island, attributed to a deflation source centered on the island.

Inflation-Deflation Episodes in the Hengill and Hrómundartindur Volcanic Complexes, SW Iceland

Non-eruptive uplift and subsidence episodes remain a challenge for monitoring and hazard assessments in active volcanic systems worldwide. Sources of such deformation may relate to processes such as magma inflow and outflow, motion and phase changes of hydrothermal fluids or magma volatiles, heat transfer from magmatic bodies and heat-mining from geothermal extraction. The Hengill area, in southwest Iceland, hosts two active volcanic systems, Hengill and Hrómundartindur, and two high-temperature geothermal power plants, Hellisheiði and Nesjavellir. Using a combination of geodetic data sets (GNSS and InSAR; Global Navigation Satellite Systems and Interferometry Synthetic Aperture Radar, respectively) and a non-linear inversion scheme to estimate the optimal analytical model parameters, we investigate the ground deformation between 2017–2018. Due to other ongoing deformation processes in the area, such as plate motion, subsidence in the two geothermal production fields, and deep-seated source of contraction since 2006, we estimate 2017–2018 difference velocities by subtracting background deformation, determined from data spanning 2015–2017 (InSAR) or 2009–2017 (GNSS). This method highlights changes in ground deformation observed in 2017–2018 compared to prior years: uplift signal of ∼10 km diameter located in the eastern part of the Hengill area, and geothermal production-related temporal changes in deformation near Húsmúli, in the western part of the Hengill area. We find an inflation source located between the Hengill and Hrómundartindur volcanic complexes, lasting for ∼5 months, with a maximum uplift of ∼12 mm. Our model inversions give a source at depth of ∼6–7 km, located approximately in the same crustal volume as an inferred contracting source in 2006–2017, within the local brittle-ductile transition zone. No significant changes were observed in local seismicity, borehole temperatures and pressures during the uplift episode. These transient inflation and deflation sources are located ∼3 km NW from a source of non-eruptive uplift in the area (1993–1999). We consider possible magmatic and hydrothermal processes as the causes for these inflation-deflation episodes and conclude that further geophysical and geological studies are needed to better understand such episodes.

Observing Posteruptive Deflation of Hydrothermal System Using InSAR Time Series Analysis: An Application of ALOS‐2/PALSAR‐2 Data on the 2015 Phreatic Eruption of Hakone Volcano, Japan

AbstractFrom June 29 to July 1, 2015, a phreatic eruption occurred in Owakudani, the largest fumarole area in Hakone volcano, Japan. In this study, an interferometric synthetic aperture radar (InSAR) time series analysis of the Advanced Land Observing Satellite‐2 (ALOS‐2)/Phased Array type L‐band Synthetic Aperture Radar‐2 (PALSAR‐2) data was performed to measure deformation after the eruption. The results show that the central cones of the volcano have subsided since the eruption and its deflation source is located beneath the previously estimated bell‐shaped conductor, which is considered as a sealing layer confining a pressurized hydrothermal reservoir. Therefore, the InSAR results demonstrate the deflation of the hydrothermal system beneath the volcano. One possible cause of this deflation is compaction due to a decrease in pore pressure caused by rupture and fluid migration during and after the eruption.

Tilt and strain change during the explosion at Minami-dake, Sakurajima, on November 13, 2017

We herein propose an alternative model for deformation caused by an eruption at Sakurajima, which has been previously interpreted as being due to a Mogi-type spherical point source beneath Minami-dake. On November 13, 2017, a large explosion with a plume height of 4200 m occurred at Minami-dake. During the 3 min following the onset of the explosion (November 13, 2017, 22:07–22:10 (Japan standard time (UTC + 9); the same hereinafter), phase 1, a large strain with changes up to 120 nstrain was detected at the Arimura observation tunnel (AVOT) located approximately 2.1 km southeast from the Minami-dake crater. After the peak of the explosion (November 13, 2017, 22:10–24:00), phase 2, a large deflation was detected at every monitoring station due to the continuous Strombolian eruption. Subsidence toward Minami-dake was detected at five out of six stations, whereas subsidence toward the north of Sakurajima was detected at the newly installed Komen observation tunnel (KMT), located approximately 4.0 km northeast from the Minami-dake crater. The large strain change at AVOT as well as small tilt changes at all stations and small strain changes at the Harutayama observation tunnel (HVOT) and KMT during phase 1 can be explained by a very shallow deflation source beneath Minami-dake at 0.1 km below sea level (bsl). For phase 2, a deeper deflation source beneath Minami-dake at a depth of 3.3 km bsl was found in addition to the shallow source beneath Minami-dake, which turned inflation after the deflation that occurred during phase 1. However, this model cannot explain the tilt change of KMT. Adding a spherical deflation source beneath Kita-dake at a depth of 3.2 km bsl can explain the tilt and strain change at KMT and the other stations. The Kita-dake source was also found in a previous study of long-term ground deformation. Not only the deeper Minami-dake source MD, but also the Kita-dake source deflated due to the Minami-dake explosion.

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

The phreatic eruption of Mount Ontake in 2014 caused local-scale subsidence and a mass discharge of water–vapor plumes from vents. A previous study of InSAR data analysis modeled the local subsidence as a deflation of a shallow hydrothermal reservoir (~ 500 m beneath the vents), and speculated that it was associated with plume emission continuing just after the eruption. In addition, combination of the InSAR and GNSS data implies that another, deeper deflation source (~ 3–6 km beneath the vents) contributes to the baseline contraction of the GNSS data. In this study, we estimated daily mass flux of the emitting plumes using photographed images, and compared the temporal behavior of the discharged mass with that of deflation of the two sources in order to clarify their association. The temporal profiles of the shallow deflation volume and the discharge mass both show evidence of decay, but with different characteristics; the deflation volume progress was approximated by a single exponential decay with a long relaxation time (379–641 days), whereas the discharge mass displayed a sum of a linear trend and an exponential decay with shorter relaxation time (47 days). This discrepancy, along with GNSS data, suggests the contribution of a deep deflation source with a short relaxation time (20–40 days). Estimation of mass balance between the emitting plume and fluids discharged from both shallow and deep sources revealed that more than 70% of the discharged mass came from the deep source. Based on the estimated mass balance, phase state of the shallow reservoir was estimated as a single-phase, liquid-rich reservoir. The fast decay of the deep deflation may reflect rapid depressurization due to violent fluid discharge at the onset of the eruption. In contrast, the slow decay of the shallow deflation suggests that it had a minor role in the eruption. However, such a wet reservoir has the potential to induce volcanic hazard such as snow-melting lahar for future eruptions, requiring monitoring the volcano, which will probably shift to pre-eruptive re-pressurized phase, until the future eruption.

Multi-Sensor SAR Geodetic Imaging and Modelling of Santorini Volcano Post-Unrest Response

Volcanic history of Santorini over recent years records a seismo-volcanic unrest in 2011–12 with a non-eruptive behavior. The volcano deformation state following the unrest was investigated through multi-sensor Synthetic Aperture Radar Interferometry (InSAR) time series. We focused on the analysis of Copernicus Sentinel-1, Radarsat-2 and TerraSAR-X Multi-temporal SAR Interferometric (MT-InSAR) results, for the post-unrest period 2012–17. Data from multiple Sentinel-1 tracks and acquisition geometries were used to constrain the E-W and vertical components of the deformation field along with their evolution in time. The interpretation of the InSAR observations and modelling provided insights on the post-unrest deformation pattern of the volcano, allowing the further re-evaluation of the unrest event. The increase of subsidence rates on Nea Kameni, in accordance with the observed change of the spatial deformation pattern, compared to the pre-unrest period, suggests the superimposition of various deformation sources. Best-fitting inversion results indicate two deflation sources located at southwestern Nea Kameni at 1 km depth, and in the northern intra-caldera area at 2 km depth. A northern sill-like source interprets the post-unrest deflation attributed to the passive degassing of the magma intruded at 4 km during the unrest, while an isotropic source at Nea Kameni simulates a prevailing subsidence occurring since the pre-unrest period (1992–2010).

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

In 2014, a phreatic eruption occurred at the Ontake Volcano in Central Japan causing multiple deaths and missing persons. Interferometric Synthetic Aperture Radar data showed local-scale subsidence around the newly created eruptive vents after the eruption. Source modeling resulted in a nearly spherical deflation source emplaced at a depth of 500 m below the vents reflecting post-eruptive depressurization in a shallow hydrothermal reservoir. The cumulative deflation volume reached 7 × 105 m3 3 years after the eruption. Comparison between our source model and GNSS data indicates that this shallow reservoir could have formed by 2007 and remained stable until the 2014 eruption. The absence of significant syn-eruptive subsidence indicates that the shallow reservoir was not the main water source driving the phreatic eruption. Under simple assumptions, mass balance between the shallow reservoir and the discharge plume from the vents indicates most of the water contained in the plume comes from greater depth than the shallow reservoir. To constrain the post-eruptive process, it is necessary to track not only deformation but also plume discharge for 3 years after the eruption.

Deformation at Krafla and Bjarnarflag geothermal areas, Northern Volcanic Zone of Iceland, 1993–2015

The Krafla volcanic system has geothermal areas within the Krafla caldera and at Bjarnarflag in the Krafla fissure swarm, 9-km south of the Krafla caldera. Arrays of boreholes extract geothermal fluids for power plants in both areas. We collected and analyzed InSAR, GPS, and leveling data spanning 1993–2015 in order to investigate crustal deformation in these areas. The volcanic zone hosting the geothermal areas is also subject to large scale regional deformation processes, including plate spreading and deflation of the Krafla volcanic system. These deformation processes have to be taken into account in order to isolate the geothermal deformation signal. Plate spreading produces the largest horizontal displacements, but the regional deformation pattern also suggests readjustment of the Krafla system at depth after the 1975–1984 Krafla rifting episode. Observed deformation can be fit by an inflation source at about 20km depth north of Krafla and a deflation source at similar depth directly below the Krafla caldera. Deflation signal along the fissure swarm can be reproduced by a 1-km wide sill at 4km depth closing by 2–4cm per year. These sources are considered to approximate the combined effects of vertical deformation associated with plate spreading and post-rifting response. Local deformation at the geothermal areas is well resolved in addition to these signals. InSAR shows that deformation at Bjarnarflag is elongated along the direction of the Krafla fissure swarm (∼ 4km by ∼ 2km) while it is circular at Krafla (∼ 5km diameter). Rates of deflation at Krafla and Bjarnarflag geothermal areas have been relatively steady. Average volume decrease of about 6.6×105m3/yr for Krafla and 3.9×105m3/yr for Bjanarflag are found at sources located at ∼ 1.5km depth, when interpreted by a spherical point source of pressure. This volume change represents about 8×10−3m3/ton of the mass of geothermal fluid extracted per year, indicating important renewal of the geothermal reservoir by water flow.

Multiple-pressure-source model for ground inflation during the period of high explosivity at Sakurajima volcano, Japan — Combination analysis of continuous GNSS, tilt and strain data —

We herein propose a three-pressure-source model for ground inflation with highly eruptive activity at the Showa crater of Sakurajima volcano, Japan. We applied a model of stacked spherical sources to continuous combined geodetic data from a global navigation satellite system, tilt and strainmeters. The data were recorded during ground inflation throughout an eruptive episode that began in October 2011 and ended in March 2012. Using a genetic algorithm, we obtained the locations and volumes of three sources. A pressure source analysis of ground inflation during the period from October 2011 to March 2012 revealed inflation sources to be located at a depth of 9.6km beneath Aira caldera (A-source) and 3.3km beneath Kita-dake (K-source), and a shallow deflation source is located at a depth of 0.7km beneath Minami-dake (M-source). The A-source corresponds to the main magma reservoir at a depth of 10km beneath the Aira caldera inferred by previous geophysical studies. The K-source is a-reservoir of Sakurajima volcano, where magma intrudes from the main magma reservoir beneath the Aira caldera during the first stage of eruptive activity. The M-source is the uppermost part of a conduit from the K-source to the summit and the Showa crater. Magma injection into the A-source started in mid-November 2011, instantly triggering the migration of increased volumes of magma from the A-source to the K-source. Approximately one month later, in mid-December 2011, an increased volume of magma started migrating from the K-source to the M-source and finally erupted at the surface. The accumulation rate for the A-source is comparable to the magma supply rate for the past 100years (0.8 to 1.6×107m3). The three-pressure-source model was applied to inflation events before the 2011 event in order to reconstruct the magma migration process. Applying our source model to earlier activity phases, we found that injected magma from the A-source remained at the K-source and a small amount of magma began to move from the K-source to the M-source before the start of eruptive activity at the Showa crater in June 2006; furthermore, a large amount of magma was found to have moved toward the crater during this violent volcanic activity. We also found that the magma injection rate into the A-source sometimes reached the level of the average injection rate of the past 100years during the ground inflation events.

Real Time Tracking of Magmatic Intrusions by means of Ground Deformation Modeling during Volcanic Crises.

Volcano observatories provide near real-time information and, ultimately, forecasts about volcano activity. For this reason, multiple physical and chemical parameters are continuously monitored. Here, we present a new method to efficiently estimate the location and evolution of magmatic sources based on a stream of real-time surface deformation data, such as High-Rate GPS, and a free-geometry magmatic source model. The tool allows tracking inflation and deflation sources in time, providing estimates of where a volcano might erupt, which is important in understanding an on-going crisis. We show a successful simulated application to the pre-eruptive period of May 2008, at Mount Etna (Italy). The proposed methodology is able to track the fast dynamics of the magma migration by inverting the real-time data within seconds. This general method is suitable for integration in any volcano observatory. The method provides first order unsupervised and realistic estimates of the locations of magmatic sources and of potential eruption sites, information that is especially important for civil protection purposes.

Numerical investigation of temporal changes in volcanic deformation caused by a gas slug ascent in the conduit

Strombolian type eruptions are considered to be generated by a sudden release of a large gas slug that migrates upward in the conduit filled with a low viscous basaltic magma. We examine volcano deformations caused by such a gas slug to understand the Strombolian eruption mechanism from geodetic observation data. We model spatio-temporal pressure changes in the conduit by using a gas slug ascent model presented by James et al. (2008). As a gas slug ascends in the conduit, its volume expands because of depressurization. Hence, the magma head lifts up in the conduit and the upper part of the conduit wall is stressed. In the conduit, magma pressure increases with depth according to the bulk density of magma: the gas slug part with a low density is characterized by a small pressure gradient, while the other parts, consisting of melt, are characterized by a large pressure gradient. We numerically calculate volcano deformations caused by the spatio-temporal changes of magma pressure predicted from the basic equations representing gas slug locations in the conduit. Simulation results show that the radial and vertical displacements and tilt changes indicate volcano deformations that represent the inflation originating from the stress increase at the upper part of conduit. As the gas slug reaches the shallow part of conduit, the rate of inflation observed in the radial displacement decreases, the vertical displacement starts to move downward, and the tilt turns to show down toward the crater. These deflation signals are caused by a moving deflation source in the conduit that is formed beneath the gas slug. Since these predicted features are not observed in the tilt records associated with explosions at Stromboli volcano (Genco and Ripepe, 2010), it is necessary to modify the gas slug ascent model or to introduce other mechanisms to better understand the magma dynamics of Strombolian eruption.

Assessment of a locally-sourced loess system in Europe: The Swiss Jura Mountains

Loess of Northern, Central, and Eastern Europe cover large areas and can be of considerable thickness. However, this already well-described loess belt did not reach the Swiss Jura Mountains. Nevertheless, local deflation, transportation, and deposition processes were efficient during the Late Glacial Period. Alpine moraines and outwash deposits on the Swiss foreland worked as deflation sources under the action of winds. Aeolian particles were deposited on the easternmost ridge and summits of the Jura Mountains, forming a thin loessic cover (<50 cm), which was generally eroded and/or incorporated in periglacial sediments. Jura “loess” is enriched in Alpine silicate minerals and constitutes a clear allochthonous input in the context of Jura Mountains, where Mesozoic limestones, and their weathering products, compose the quasi-entirety of autochthonous material. The Jura loess, already recognised for more than 40 years, still needs to be characterised more accurately. Five study sites were chosen along an 85 km long transect on the easternmost ridge of the Jura Mountains. A soil pit was dug and analysed at each site, in order to identify loess layers. Grain-size distribution curves of loess permitted to discriminate four particle subpopulations, which are related to various aeolian episodes/sources and post-depositional weathering processes, respectively. The mineralogical composition is dominated by Alpine components. However, high amounts of kaolinite in the clay fraction of loess layers point to a more local origin, thus indicating the contribution of Jura moraines as potential deflation sources. Finally, the geochemical composition of loess reflects the preponderant influence of Alpine minerals.

Volcano-hydrothermal activity detected by precise levelling surveys at the Tatun volcano group in Northern Taiwan during 2006–2013

Precise levelling surveys were conducted from 2006 to 2013 on three levelling routes in the Tatun volcano group (TVG) located approximately 15km northeast of Taipei, to detect deformation in relation to the volcano-hydrothermal activities of the TVG. Uplift was detected around the most active fumarole, Tayoukeng fumarole, throughout the period 2007 to 2011; the uplift rate throughout the period from March 2009 to March 2011 was reduced in comparison to the rate between 2007 and 2009. Following this, a dormant state or a small amount of subsidence was detected in the period March 2011 to March 2013. And throughout the period from June 2006 to March 2013, subsidence was centred on an area 0.5km east of the summit of Mt. Cising, the highest peak in the TVG. A model of two spherical sources was therefore estimated from the deformation recorded from August 2007 to March 2011, using a genetic algorithm. A deflation source was obtained about 0.5km northeast of Mt. Cising at a depth of 2km; and an inflation source was situated approximately 1km south of the Tayoukeng fumarole at a depth of 0.7km. Based on previous seismic and AMT studies, the estimated sources are interpreted as being hydrothermal reservoirs. Because almost all the benchmarks around Mt. Cising show subsidence at a constant speed, we conclude that the deeper hydrothermal reservoir at a depth of 2km may have been releasing hydrothermal fluid at a constant rate throughout the period from 2006 to 2013. However, it was suggested that in 2011 the shallower hydrothermal reservoir at a depth of 0.7km changed from an inflation state to a dormant state (or small deflation) based on temporal vertical changes around Tayoukeng fumarole. A possible model for the volcano-hydrothermal system is therefore proposed. It is considered that the hydrothermal fluid may be supplied intermittently from the magma chamber to the deeper hydrothermal reservoir at a depth of 2km (although this type of fluid input event may not have occurred during our research period), and that the hydrothermal fluid upwelling between Mt. Cising and Tayoukeng fumarole flows transversally into a highly cracked and fluid-saturated area that continues to Tayoukeng fumarole. The existence of a shallower hydrothermal reservoir at a depth of 0.7km in the highly cracked and fluid-saturated area is therefore plausible.

Spatio-temporal changes in seismic velocity associated with the 2000 activity of Miyakejima volcano as inferred from cross-correlation analyses of ambient noise

Miyakejima volcano, Japan, showed strong and interesting volcanic activities in 2000 with the occurrence of dike intrusions, caldera formation and a large amount of gas emission. To understand temporal changes of the volcanic structure associated with this activity, we apply ambient noise correlation analyses to the seismic records of five seismic stations at Miyakejima volcano from July 1999 to December 2002. We calculate cross-correlation functions (CCFs) for eight available station pairs at frequency bands of 0.4–0.8Hz and 0.8–1.6Hz, and retrieve the Rayleigh waves propagating with a group velocity of about 1.0km/s. Comparing CCFs obtained for each day with the reference CCFs that are calculated from a few months of data, we estimate seismic velocity changes before and after the 2000 activity. Results at the two frequency bands show seismic velocity increase and decrease. Shallow structure at the volcanic edifice on the flanks experienced a seismic velocity increase up to 3.3%. On the other hand, the regions located close to the collapsed caldera show a seismic velocity decrease down to 2.3%. To understand the mechanisms that may introduce these seismic velocity increase and decrease, we examine several possible mechanisms of the velocity changes: stress changes due to volcanic pressure sources and caldera formation, and topographic changes. By using the deflation sources previously determined from GPS data, we calculate the stresses at depths of 0km and 1km at the volcano. The results suggest that the seismic velocity increases observed at seismic paths on the volcanic flank are explained by the compression caused by the deflation sources associated with magma activity in 2000. Stress sensitivities of the seismic velocity changes are estimated to be 1.5×10−2 to 2.2×10−2MPa−1 and 4.6×10−3 to 2.4×10−2MPa−1 at frequency bands of 0.4–0.8Hz and 0.8–1.6Hz, respectively, which are in the range of the previously reported results. Our three dimensional numerical calculations on seismic wave propagation show that the seismic velocity decreases observed at seismic paths crossing the collapsed caldera can be explained by topographic changes introduced by a caldera. We further suggest that the seismic velocity decreases detected around the regions close to the caldera may originate from the dilated regions cause by formation of the caldera.

Ground deformations and volcanic processes as imaged by CGPS data at Mt. Etna (Italy) between 2003 and 2008

We analyze the kinematic and crustal deformations of Mt. Etna from 2003 to 2008 as imaged by the Mt. Etna continuous GPS (CGPS) network (Etn@net). Through a careful analysis of GPS time series, six coherent phases of crustal deformations have been identified, three inflation phases and three deflation phases, superimposed on a major inflation of the volcanic edifice since 2001. The inversions of GPS velocities have enabled: 1) a better understanding of the evolution of the volcanic sources acting beneath the volcano; 2) analysis of the strain rate patterns; and 3) a delineation of potential coupling between volcanic sources and the observed ground deformations. The modeling of the pressure sources has shown a separation between inflation and deflation sources. The deflation sources show an upward migration, from 5.5 toward 2.0 km (b.s.l.), while the inflation sources are located within 5.5 and 4.0 km (b.s.l.). Our results indicate that the kinematic and ground deformations of the mid‐upper eastern flank are driven by the interplay between the effect of the magmatic sources and a southeastward motion. Furthermore, clockwise rotations have been detected that prevailed over the eastern motion of the flank during the inflation phases preceding the 2004–2005 and 2006 eruptions. Finally, the accordance between the higher geodetic shear strain rates and the area with the highest seismic energy release shows that measured geodetic shear strain rates can provide useful information on the potential occurrence of seismic activity.

Depth and Evolution of a Silicic Magma Chamber: Melting Experiments on a Low-K Rhyolite from Usu Volcano, Japan

We estimated the depth of the magma chamber system beneath Usu volcano, one of the most active volcanoes inJapan, by melting experiments on the eruptive product of the AD 1663 plinian eruption (Us-b pumice).This pumice is a low-K rhyolite with a K2O content of only 1wt %. The experiments were conducted using internally heated pressure vessels at 98^392MPa and 700^9008C under nickel^nickel oxide (NNO)-buffered and water-saturated conditions. The pre-eruptive conditions of the 1663 eruption were estimated to be 780 208C and around 200^250MPa. The phase equilibria, mineral compositions (plagioclase, orthopyroxene, and Fe^Ti oxides), melt composition, and melt fraction are all in agreement with this conclusion. This pressure corresponds to a depth of 8^10 km, assuming a lithostatic condition with an appropriate crustal density structure.The magma was at near water-saturated conditions. Deviation between the estimated pressure and the true total pressure was estimated to be less than 50MPa. The Us-b magma was under very uniform conditions with a temperature variation of less than c. 208C, deduced from the narrow range of the phase equilibrium conditions (crystallizing orthopyroxene without quartz) and the homogeneous composition of plagioclase (c. An 41^45).We also estimated the pre-eruptive conditions of post-1663 eruptions, where it has been proposed that two magmas (high-P and low-P members) mixed prior to each eruption. The pressure of the deep magma was also around 200^250MPa, whereas that of the shallow magma was estimated to be 100^150MPa (4^6 km depth), according to the pressure dependence of the plagioclase composition obtained in our experiments.Thus, there appear to be two magma chambers beneath Usu volcano. Our estimate is consistent with geophysical observations made during the 2000 eruption, including identification of an inflation and deflation source at a depth of 10 km, a low-frequency (12 s) tremor source at a depth of 5^6 km, and an earthquake swarm at a depth of less than 4 km. The depth of the rhyolitic magma chamber (8^10 km) is deeper than the level of neutral buoyancy for the magma, and is comparable with that of mafic magma that existed beneath the rhyolitic magma.This suggests that the depth of the 8^10 km deep magma chamber was controlled by the emplacement of the mafic magma and that the rhyolitic magma was produced at that depth. When compared with other silicic magma systems, the Usu magma system has similar characteristics, including a similar pre-eruptive pressure of around 200MPa and a pressure decrease with time, suggesting upward magma movement towards a neutral buoyancy level. The low potassium content of the Usu magma prohibited crystallization of biotite, K-feldspar or hornblende, which are common phases in magmas with intermediate to high potassium contents. The absence of hydrous minerals at near-liquidus conditions may cause a rapid increase in water content

Potential volcanological applications of the DORIS system. A geodetic study of the Socorro Island (Mexico) coordinate time-series

SUMMARY We analysed DORIS data from Socorro Island (Mexico), providing weekly coordinates in ITRF2000 from 1900 to 2006. Careful analysis of this time-series shows horizontal and vertical movements related to recent activity of the Mt. Evermannn volcano. A deflation of the volcano between 1993 and 1996 (–94 mm yr –1 in the N23 ◦ W direction, directly pointing in the direction of the crater summit) is observed. We identify also a clear horizontal discontinuity of 81 mm and a significant change in velocity on 2002 October 3. We use a Mogi formalism to provide constraints from our geodetic results on the successive locations and size of the depletion and inflation sources. Our model is consistent with a deflation source for the period 1993–1996 located beneath Mt. Evermannn at a depth of about 6.2 km below sea level. However, our data constrain only the azimuth and dip angles of the source, which could be located elsewhere along the azimuth of the summit, depth and volume varying linearly with the distance.

Structures involved in the vertical deformation at Lake Taupo (New Zealand) between 1979 and 2007: New insights from numerical modelling

Since 1979, repeat levelling measurements have been conducted on the lake filling the caldera of the dormant Taupo rhyolitic volcano in the North Island of New Zealand. Interpretation of these data through numerical modelling provides information on the structures involved in the relative vertical crustal movement throughout the southern end of the Taupo Volcanic Zone (TVZ), an area of active back-arc extension. The best-defined feature is a long term global subsidence of the northern part of the lake (7 mm yr − 1 ) due to the cumulative effect of the crust stretching and a deep deflation source. This long term subsidence is occasionally disturbed by strong short-term uplifts linked with overpressure sources located below the northern part of the lake, near active geothermal fields. Episodes of uplift can be attributed to various combinations of the following two processes taking place beneath the geothermal field (1) Movement or formation of rhyolitic magma (deepest sources) (2) Pressurization of the shallow hydrothermal fluid reservoir that traps volatiles exsolved from a crystallizing rhyolitic magma (shallowest sources). The pressurization of the shallow hydrothermal system gives rise to tensional stresses in the upper crust, resulting in seismic and aseismic fault ruptures. Slow slip motion of the Kaiapo fault decouples on a short-term time scale the ground deformation pattern on both sides of the fault. Our results, discriminating what parts of the deformation are due to the regional setting, the hydrothermal circulations and the seismic activity, reveal that each seismic swarm is preceded by 1 to 3 years of inflation of the eastern part of the lake. This systematic behaviour may allow us in the future to better predict the occurrence of the seismic swarms below the lake.

Formation of the April 2007 caldera collapse at Piton de La Fournaise volcano: Insights from GPS data

On April 5th 2007, a major collapse of the Dolomieu summit crater occurred at Piton de La Fournaise volcano (La Réunion Island). To constrain the origin and the dynamics of the collapse, we modelled continuous GPS displacements preceding and accompanying this event. Our results reveal that the Dolomieu collapse formed by a “piston-like” mechanism, in which the fast draining of the shallow magma reservoir at the beginning of the March 30th–May 1st 2007 eruption caused a gravity-driven downward displacement of its roof. During the five days preceding the collapse, summit deflation was caused by volume change through the fractured rock column above the shallow magma chamber due to the closure of voids or small-scale cavities or the upward stoping and migration of an underground cavity. Since 2000, weakening of the rock column by repeated refilling and draining of the shallow magma chamber favoured its destabilization. On April 5th, 7 h before the onset of the collapse, the deflation source was located at ~ 300 m depth revealing the initiation of the collapse at very shallow depth. The sudden collapse on April 5th was followed by continuous cyclic subsidence until April 6th. Each cycle was characterized by a progressive inward deflation of the summit zone and ended by sudden summit outward displacements caused by stress relaxation following a collapse event. Subsidence of the stoped column in the magma reservoir during the collapse of the crater on April 5th and 6th acted as a piston thus increasing the eruption rate. The collapse occurred in only 24 h by successive events and increased the depth of the Dolomieu crater by 340 m. After April 6th, only minor readjustments of the summit cone occurred with slight summit deflation and small landslides of the caldera walls.