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Historic volcanic activity in Tenerife was concentrated within two of the island's three dorsals and on the Teide-Pico Viejo complex located inside Las Cañadas caldera. Eruptions on the island are primarily characterised by basaltic and trachybasaltic fissural eruptions. However, the Teide-Pico Viejo complex also hosted explosive and effusive phonolitic eruptions. Our study focused on the analysis of the 2004-2005 unrest in Tenerife, which was characterised by an onset of a ground deformation pattern, heightened on-land seismic activity, changes in the chemical composition of fumaroles of the Teide composite volcano, an increase in diffusive emissions of carbon dioxide along the NW rift, and, significant gravity changes. We used the Envisat-ASAR satellite images from 2003 to 2010 to generate the Line-Of-Sight SBAS- DInSAR deformation time series to investigate the source responsible for ground deformation. We applied the Independent Component Analysis (ICA) to separate distinct ground deformation patterns. Specifically, we selected four components for the Independent Component Analysis decomposition: the first one mainly affects the stratovolcano's summit region of Teide and shows a circular symmetry; the second and third components are possibly related to the topography and atmospheric artifacts, while the fourth contains only a noisy signal. We employed a non-linear optimisation approach in a Finite Element modelling environment to determine the source geometry responsible for the first identified ICA pattern of ground deformation within Las Cañadas. Our results revealed the existence of an ellipsoidal ground deformation source oriented along the E-W axis, located beneath the Teide and Montaña Blanca volcanoes at 1,600 m a.s.l. This source became active during the seismic crisis of 2004-2005, which was associated with the degassing of a magma batch that intruded into the northwest rift of Tenerife island. We propose that the ground deformation observed at Teide volcano during the 2004-2005 crisis was related to hydrothermal activity within the volcano. [ABSTRACT FROM AUTHOR]

Lake Taupō, New Zealand, conceals a large caldera volcano which last erupted in 232 AD. A surveying experiment in 1979 which utilised the surface of the lake for detecting small movements of the lakebed soon became a programme to monitor vertical deformation by making regular observations at 22 fixed points – survey stations – around the lake. The programme continues today with 4 surveys being made annually. In this paper, we review the methodology and present the 42 year-long dataset in graphical forms, and draw conclusions on its implications in terms of volcanic and tectonic deformation. We also make the dataset available for download. The dataset confirms that vertical deformation is occurring in the lakebed. Long periods of slow deformation, dominated by subsidence totalling 140 mm in the Taupō Fault Belt at the northern end of the lake, and to a lesser degree at the southern end, are interrupted by uplift episodes now reaching 160 mm in the northeastern sector containing the most recent active vents. We suggest that the subsidence is primarily tectonic while inflation episodes are driven by upward migration of magma to shallow levels during periods of unrest. [ABSTRACT FROM AUTHOR]

Historic volcanic activity in Tenerife was concentrated within two of the island’s three dorsals and on the Teide-Pico Viejo complex located inside Las Cañadas caldera. Eruptions on the island are primarily characterised by basaltic and trachybasaltic fissural eruptions. However, the Teide-Pico Viejo complex also hosted explosive and effusive phonolitic eruptions. Our study focused on the analysis of the 2004–2005 unrest in Tenerife, which was characterised by an onset of a ground deformation pattern, heightened on-land seismic activity, changes in the chemical composition of fumaroles of the Teide composite volcano, an increase in diffusive emissions of carbon dioxide along the NW rift, and, significant gravity changes. We used the Envisat-ASAR satellite images from 2003 to 2010 to generate the Line-Of-Sight SBAS-DInSAR deformation time series to investigate the source responsible for ground deformation. We applied the Independent Component Analysis (ICA) to separate distinct ground deformation patterns. Specifically, we selected four components for the Independent Component Analysis decomposition: the first one mainly affects the stratovolcano’s summit region of Teide and shows a circular symmetry; the second and third components are possibly related to the topography and atmospheric artifacts, while the fourth contains only a noisy signal. We employed a non-linear optimisation approach in a Finite Element modelling environment to determine the source geometry responsible for the first identified ICA pattern of ground deformation within Las Cañadas. Our results revealed the existence of an ellipsoidal ground deformation source oriented along the E-W axis, located beneath the Teide and Montaña Blanca volcanoes at 1,600 m a.s.l. This source became active during the seismic crisis of 2004–2005, which was associated with the degassing of a magma batch that intruded into the northwest rift of Tenerife island. We propose that the ground deformation observed at Teide volcano during the 2004–2005 crisis was related to hydrothermal activity within the volcano.

Unrest began in July 2021 at Askja volcano in the Northern Volcanic Zone (NVZ) of Iceland. Its most recent eruption, in 1961, was predominantly effusive and produced ∼0.1 km3 lava field. The last plinian eruption at Askja occurred in 1875. Geodetic measurements between 1983 and 2021 detail subsidence of Askja, decaying in an exponential manner. At the end of July 2021, inflation was detected at Askja volcano, from GNSS observations and Sentinel‐1 interferograms. The inflationary episode can be divided into two periods from the onset of inflation until September 2023. An initial period until 20 September 2021 when geodetic models suggest transfer of magma (or magmatic fluids) from within the shallowest part of the magmatic system (comprising an inflating and deflating source), potentially involving silicic magma. A following period when one source of pressure increase at shallow depth can explain the observations. Plain Language Summary: Askja volcano, situated in the Northern Volcanic Zone in Iceland, has been quiet since its last eruption in 1961, with surface deformation measurements from 1983 to 2021 displaying a decaying subsidence signal within the Askja caldera. However, at the end of July 2021, the volcano began to inflate. This was detected on both GNSS and satellite observations. As of September 2023, ∼65 cm of uplift had been measured at GNSS station OLAC. Modeling of surface deformation measurements indicates that the inflation was triggered by upward migration of melt (or magmatic fluids). Key Points: At the end of July 2021, Askja volcano began to inflate—detected on both GNSS and satellite observations, ending 1983–2021 subsidenceGeodetic modeling indicates upward migration of magma, feeding a magma body at an inferred depth of 2.5–3.1 km under the main Askja calderaStart of unrest was associated with magma transfer within the upper part of the system, followed by possible additional influx from depth [ABSTRACT FROM AUTHOR]

Lake Taupō (Taupō-nui-a-Tia) infills the composite caldera above an active rhyolitic magmatic system in the central Taupō Volcanic Zone (TVZ). Ground deformation is a key unrest indicator at Taupō volcano. We present a spreadsheet tool, TaupōInflate, to calculate and plot ground deformation from magmatic inflation at depth beneath Taupō caldera. Examples show detection limits for inflating magma bodies and their ascent through the crust beneath Lake Taupō. Source locations where it is challenging to detect even substantial volumes of inflating magma bodies are as large as 20 km3, with volume changes up to 0.01 km3, owing to the restricted station placement around the lake, although a dike propagating from shallow crustal depths towards the surface is likely to be detectable. For a magma overpressure of 10 MPa, the sizes of detectable inflating bodies at depths of 5–8 km using the present monitoring system are larger than the volumes of many past eruptions, illustrating the importance of future improvements to the geodetic network. We discuss the potential for future equipment installation, including lakebed instrumentation that would require approval of local iwi Ngāti Tūwharetoa through the Tūwharetoa Māori Trust Board who oversee the health and wellbeing of Lake Taupō. [ABSTRACT FROM AUTHOR]

Volcano geodesy often involves the use of models to explain observed surface deformation. A variety of forward models are used, from analytical point sources to numerical simulations that consider complex magma system geometries, topography, and material properties. Various inversion methods can then be used to relate observed volcano data to models. Ideally, forward models should be verified through intercomparison, to check for implementation errors and quantify the error induced by any approximations used. Additionally, forward models and inversion methods should be validated through tests with synthetic and/or real data, to determine their ability to match data and estimate parameter values within uncertainty. However, to date, there have not been comprehensive verification and validation efforts in volcano geodesy. Here, we report on the first phase of the Drivers of Volcano Deformation (DVD) exercises, which were designed to build community involvement through web-based exercises involving calculations of static elastic displacement around pressurized magma reservoirs. The forward model exercises begin with a spherical reservoir in a homogeneous half space, then introduce topography, heterogeneous elastic properties, and spheroidal geometries. The inversion exercises provide synthetic noisy surface displacement data for a spherical reservoir in a homogeneous half space and assess consistency in estimates of reservoir location and volume/pressure change. There is variability in the results from both forward modeling and inversions, which highlights the strengths and limitations of different forward models, as well as the importance of inversion method choice and uncertainty quantification. This first phase of the DVD exercises serves as a community resource and will facilitate further efforts to develop standards of reproducibility. [ABSTRACT FROM AUTHOR]

Over the first century of the International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI), volcano geodesy grew from roots as an accidental and incidental system of measurements to an important method for monitoring volcanic activity and forecasting eruptions. The first practitioners in volcano geodesy were experts in other disciplines, and it was not until the latter half of the twentieth century that specialists in the field emerged—scientists who developed new methods, measured geodetic change at volcanoes, and quantitatively interpreted the results in terms of magmatic processes. Much of the early work in the field was restricted to a few volcanoes and involved techniques that had been adapted from other applications; relatively few methods were developed specifically for use on volcanoes. These volcanoes, however, provided the natural laboratories needed to advance the field. By the start of the twenty-first century, geodetic studies, especially using space-based techniques, contributed to the recognition of deformation and gravity change at hundreds of volcanoes on Earth. In coming years, IAVCEI researchers will focus on comprehensive exploitation of the growing volumes of geodetic data to better model, forecast, and track activity at volcanoes worldwide. Meanwhile, the field needs to become more diverse, better representing people who live in the shadows of volcanoes around the globe. [ABSTRACT FROM AUTHOR]

Magmatism is a recognized potential driver of volcanic flank instability. Recently, this driver was identified at Pacaya Volcano, Guatemala, where a large eruption in 2014 accelerated southwest flank creep. This work investigates the links between flank creep and eruptive behavior at Pacaya to better understand the conditions under which flank creep can be initiated, sustained, or halted at active volcanoes. Time-series of surface displacements from 2007 to 2020 computed using seven Synthetic Aperture Radar datasets are compared with volcanic activity reports, ash advisories, thermal anomalies, and lava flow maps. We find that large transient flank instabilities occurred during vigorous eruptions in 2010 and 2014, but not during times of similarly elevated activity in 2007–2009 and 2018–2020. Flank creep continued years after the two transient instability events, during the relatively quiescent intervals of 2010–2014 and 2015–2018. During periods of elevated volcanic unrest attributed to open-vent volcanic activity, as in 2007–2009 and 2018–2020, magma migrates in an open conduit with little associated deformation or flank motion. Conversely, the opening of new vents outside the summit area, irrespective of whether this marks the start or a transition in an eruption, can initiate transient flank creep as in 2010 and 2014. Thus, future opening of new magma pathways, particularly in a north-northwest to south-southeast alignment, might provide forewarning for an increased likelihood of renewed or accelerating flank creep at Pacaya. [ABSTRACT FROM AUTHOR]

In 2018 Kı̄lauea volcano erupted a decade's worth of basalt, given estimated magma supply rates, triggering caldera collapse. Yet, less than 2.5 years later Kı̄lauea re‐erupted. At the 2018 eruption onset, pressure within the summit reservoir was ∼20 MPa above magmastatic. By the onset of collapse this decreased by ∼17 MPa. Analysis of magma surges at the 2018 fissures, following collapse events, implies excess pressure at the eruption end of only ∼1 MPa. Given the new vent elevation, ∼11–12 MPa pressure increase was required to bring magma to the surface in December 2020. Analysis of Global Positioning System data between 8/2018 and 12/2020 shows there was a 73% probability that this condition was met at the onset of the 2020 eruption. Given a plausible range of possible vent elevations, there was a 40%–88% probability of sufficient pressure to bring magma to the surface 100 days before the eruption. Plain Language Summary: In 2018 Kı̄lauea volcano erupted so much lava that, based on long‐term magma supply rates, one might have anticipated a decade long quiescence. Yet Kı̄lauea erupted in December 2020 less than 2.5 years later. Volcano deformations are used to infer pressure changes within magma systems. However, large inelastic deformations during Kı̄lauea's 2018 caldera collapse prohibit this approach. Here, we utilize diverse observations to infer the pressure history within the magma system during the inter‐eruptive period. Analysis of magma surges following caldera collapse events suggests that the pressure available to drive flow was very low at the end of the 2018 eruption. Due to unique observations of lava lake drainage in 2018 eruption, we were able to use Global Positioning System displacements to estimate the pressure increase between the end of the 2018 eruption and the start of the 2020 eruption. Given that eruption was most likely between the bottom of the 2018 pit and the top of the largest down‐dropped block, we show that there was a high probability that reservoir pressure was sufficient to bring magma to the surface well before the 2020 eruption. Our approach could aid future eruption forecasting. Key Points: Time predictable estimate based on 2018 erupted volume and long‐term magma supply rate greatly overestimates the post 2018 repose periodModeling magma surges following collapse events shows driving pressures at the end of collapse cycles of only ∼1 MPaWe estimate a 73% chance of pressure sufficient to raise magma to the 20 December 2020 eruptive vents based on Global Positioning System data up to that date [ABSTRACT FROM AUTHOR]

The study of volcano deformation has grown significantly through they year 2020 since the development of interferometric synthetic aperture radar (InSAR) in the 1990s. This relatively new data source, which provides evidence of changes in subsurface magma storage and pressure without the need for ground-based equipment, has matured during the past decade. It now provides a means to address previously inaccessible questions and offers input to increasingly complex models of magmatic processes. Here, we review how technological advances in InSAR during 2010-2020 have facilitated our ability to monitor and interpret volcanic processes, primarily through rapid and accurate observations of the changing surfaces at active volcanoes worldwide. Specifically, we examine how current systems achieve excellent resolution in time and space, provide global coverage, and generate products that are easy to use by non-specialists—factors that have often limited the practical study of volcanoes using radar measurements. We also look to the future, offering our perspective about how advancements in technology and data management in the decade to come will increase the value and accessibility of InSAR applied to the geodetic study of volcanoes and monitoring of hazardous volcanic processes. New developments will include the launch of additional satellites by both public space agencies and private companies, as well as implementation of algorithms for exploiting the growing volumes of data. To meet their full potential, these efforts will require coordination between data users and data providers so that the relevant imagery is acquired, made available to volcanologists in a timely fashion, and utilized to assess and mitigate volcanic hazards. [ABSTRACT FROM AUTHOR]

We analyse spatiotemporal gravity changes observed on the Ischia island (Italy) accompanying the destructive earthquake of 21 August 2017. The 29 May 2016 to 22 September 2017 time-lapse gravity changes observed at 18 benchmarks of the Ischia gravimetric network are first corrected for the gravitational effect of the surface deformation using the deformation-induced topographic effect (DITE) correction. The co-seismic DITE is computed by Newtonian volumetric integration using the Toposk software, a high-resolution LiDAR DEM and the co-seismic vertical displacement field derived from Sentinel-1 InSAR data. We compare numerically the DITE field with its commonly used Bouguer approximation over the island of Ischia with the outcome that the Bouguer approximation of DITE is adequate and accurate in this case. The residual gravity changes are then computed at gravity benchmarks by correcting the observed gravity changes for the planar Bouguer effect of the elevation changes at benchmarks over the same period. The residual gravity changes are then inverted using an inversion approach based on model exploration and growing source bodies, making use of the Growth-dg inversion tool. The found inversion model, given as subsurface time-lapse density changes, is then interpreted as mainly due to a co-seismic or post-seismic disturbance of the hydrothermal system of the island. Pros and weak points of such interpretation are discussed. [ABSTRACT FROM AUTHOR]

We propose a multi-temporal-scale analysis of ground deformation data using both high-rate tilt and GNSS measurements and the DInSAR and daily GNSS solutions in order to investigate a sequence of four paroxysmal episodes of the Voragine crater occurring in December 2015 at Mt. Etna (Italy). The analysis aimed at inferring the magma sources feeding a sequence of very violent eruptions, in order to understand the dynamics and to image the shallow feeding system of the volcano that enabled such a rapid magma accumulation and discharge. The high-rate data allowed us to constrain the sources responsible for the fast and violent dynamics of each paroxysm, while the cumulated deformation measured by DInSAR and daily GNSS solutions, over a period of 12 days encompassing the entire eruptive sequence, also showed the deeper part of the source involved in the considered period, where magma was stored. We defined the dynamics and rates of the magma transfer, with a middle-depth storage of gas-rich magma that charges, more or less continuously, a shallower level where magma stops temporarily, accumulating pressure due to the gas exsolution. This machine-gun-like mechanism could represent a general conceptual model for similar events at Etna and at all volcanoes. [ABSTRACT FROM AUTHOR]

On January 12, 2020, Taal volcano, Philippines, erupted after 43 years of repose, affecting more than 500,000 people. Using interferometric synthetic aperture radar (InSAR) data, we present the pre‐ to post‐eruption analyses of the deformation of Taal. We find that: (1) prior to eruption, the volcano experienced long‐term deflation followed by short‐term inflation, reflecting the depressurization‐pressurization of its ∼5 km depth magma reservoir; (2) during the eruption, the magma reservoir lost a volume of 0.531 ± 0.004 km3 while a 0.643 ± 0.001 km3 lateral dike was emplaced; and (3) post‐eruption analyses reveal that the magma reservoir started recovery approximately 3 weeks after the main eruptive phase. We propose a conceptual analysis explaining the eruption and address why, despite the large volume of magma emplaced, the dike remained at depth. We also report the unique and significant contribution of InSAR data during the peak of the crisis. Plain Language Summary: Taal volcano in the Philippines erupted on January 12, 2020. Here, we present the pre‐, co‐, and post‐eruption data, model, and analyses using interferometric synthetic aperture radar (InSAR) data acquired by various satellite systems. We find that: (1) prior to the eruption, the volcano experiences a sequence of long‐term (>1 year) deflation followed by short‐term (≤1 year) inflation as a result of the depressurization‐pressurization of its ∼5 km depth magma reservoir; (2) during the eruption, the magma reservoir lost a volume of 0.531 ± 0.004 km3 while a 0.643 ± 0.001 km3 lateral dike was emplaced; and (3) post‐eruption analyses reveal that the magma reservoir is in recovery starting ∼3 weeks after the main eruptive phase. We propose a conceptual analysis to explain the 2020 Taal eruption and the dike emplacement. We also report the unique and significant contribution of remote sensing data, particularly InSAR during the peak of the crisis. Key Points: We present a comprehensive interferometric synthetic aperture radar (InSAR)‐based data, analyses, and models of Taal's pre‐ to post‐eruptive stateDuring the eruptive crisis, Taal's magma reservoir lost 0.531 ± 0.004 km3 of volume while a 0.643 ± 0.001 km3 lateral dike was emplacedLow‐latency InSAR‐derived products provided crucial and significant information to PHIVOLCS during the January 2020 eruptive event [ABSTRACT FROM AUTHOR]

Gravitational effect of surface deformation is in 4D microgravimetry treated as the deformation-induced topographic effect (DITE). The DITE field is computed using Newtonian volumetric integration which requires high resolution digital elevation model (DEM) and vertical displacement field in areal form. If only elevation changes on bench- marks of the gravimetric network are available, instead of the vertical displacement field, the DITE on benchmarks can be evaluated only approximately, using a planar Bouguer or a normal free-air-effect (nFAE) approximation. Here we analyse the adequacy and accu- racy of these two approximations in a case study for the December 2018 fissure eruption on Etna accompanied by significant surface deformation caused primarily by a relatively shallow dyke. The outcome is that in volcanic areas of similar morphology as that over the Etna summit area, and for surface deformation fields due to relatively shallow dykes, neither the Bouguer nor the nFAE approximation of the DITE is accurate enough. In such situations the residual gravity changes should be computed with both the Bouguer and nFAE corrections and interpreted as two marginal cases. In addition we analyse also a correction for the effect of benchmark elevation change based on the topographically modelled (predicted) vertical gradient of gravity (VGG) meant to approximate the in-situ VGG values at benchmarks. This correction does not appear suitable to approximate the DITE in conditions of our case study or in broader sen [ABSTRACT FROM AUTHOR]

Abstract Large silicic magma reservoirs preferentially form in the upper crust of extensional continental environments. However, our quantitative understanding of the link between mantle magmatism, silicic reservoirs, and surface deformation during rifting is very limited. Here, we focus on Corbetti, a peralkaline caldera in the densely populated Main Ethiopian Rift, which lies above a focused zone of upper mantle partial melt and has been steadily uplifting at a maximum rate of 6.6±1.2 cm yr−1 for more than 10 yr. Numerical modeling shows that a maximum concomitant residual gravity increase of 9±3 μGal yr−1 by the intrusion of mafic magma at ∼7 km depth into a compressible and inelastic crystal mush best explains the observed deformation and gravity changes. The derived magma mass flux of ∼1011 kg yr−1 is anomalously high and at least 1 order of magnitude greater than the mean long‐term mass eruption rate. This study demonstrates that periodic and high‐rate magmatic rejuvenation of upper‐crustal mush is a significant and rapid contributor to mature continental rifting.

Some geophysical or geodynamic applications require the use of true vertical gradient of gravity (VGG). This demand may be associated with reductions of or corrections to observed gravity or its spatiotemporal changes. In the absence of in situ measured VGG values, the constant value of the theoretical (normal) free air gradient (FAG) is commonly used. We propose an alternative to this practice which may significantly reduce systematic errors associated with the use of constant FAG. The true VGG appears to be better approximated, in areas with prominent and rugged topography, such as alpine or some volcanic regions, by a value based on the modelled contribution of the topographic masses to the gradient. Such prediction can be carried out with a digital elevation model (DEM) of sufficient resolution and accuracy. Here we present the VGG field computed for Mt. Etna (Italy), one of the most active and best monitored volcanoes worldwide, to illustrate how strongly the VGG deviates spatially from constant FAG. The predicted (modelled) VGG field is verified by in situ observations. We also take a look at the sensitivity of the VGG prediction to the resolution and quality of used DEMs. We conclude with discussing the applicability of the topo-predicted VGG field in near surface structural and volcanological micro-gravimetric studies. [ABSTRACT FROM AUTHOR]

Large silicic magma reservoirs preferentially form in the upper crust of extensional continental environments. However, our quantitative understanding of the link between mantle magmatism, silicic reservoirs, and surface deformation during rifting is very limited. Here, we focus on Corbetti, a peralkaline caldera in the densely populated Main Ethiopian Rift, which lies above a focused zone of upper mantle partial melt and has been steadily uplifting at a maximum rate of 6.6±1.2 cm yr−1 for more than 10 yr. Numerical modeling shows that a maximum concomitant residual gravity increase of 9±3 μGal yr−1 by the intrusion of mafic magma at ∼7 km depth into a compressible and inelastic crystal mush best explains the observed deformation and gravity changes. The derived magma mass flux of ∼1011 kg yr−1 is anomalously high and at least 1 order of magnitude greater than the mean long‐term mass eruption rate. This study demonstrates that periodic and high‐rate magmatic rejuvenation of upper‐crustal mush is a significant and rapid contributor to mature continental rifting. Key Points: Corbetti caldera is undergoing unrest since 2009 with a maximum uplift rate of 6.6 cm yr−1Net mass addition at 6–8 km below the surface by anomalously high magma flux (1.1 to 2.0×1011 kg yr−1) accommodated into compressible and inelastic upper‐crustal crystal mushDeformation of thermomechanically heterogenous host rock dominates mass redistribution and associated gravity changes [ABSTRACT FROM AUTHOR]

We conducted gravity surveys of the summit area of Kīlauea Volcano, Hawaiʻi, in November 2018 and March 2019, with the goal of determining whether there was any mass change at depth following the volcano's May–August 2018 caldera collapse. Surface deformation between the two surveys was minimal, but we measured a gravity increase (maximum 44 μGal) centered on the caldera that can be modeled as mass accumulation in a region ~1 km beneath the surface. We interpret this mass increase to be mostly magma accumulation in void space that was created during the summit collapse. Caldera uplift was evident by April 2019, indicating that the magma volume had reached a point where pressurization could be sustained. Modeled gravity change suggests a maximum magma storage rate at Kīlauea's summit during November 2018 to March 2019 that is much less than the pre‐2018 magma supply rate to the volcano. Plain Language Summary: Measuring changes in gravity over time at a volcano provides an indication of mass variation beneath the surface, which can offer evidence of whether or not magma is accumulating in, or draining from, subsurface reservoirs. After the 2018 collapse of Kīlauea Volcano, Hawaiʻi, there was uncertainty over the status of subsurface magmatic activity. We found that an increase in gravity occurred between November 2018 and March 2019. The gravity increase was not accompanied by uplift of the surface, which implies that mass, probably magma, was accumulating in void space created during the collapse. The rate of magma accumulation was much less than the pre‐2018‐collapse magma supply rate. By April 2019 Kīlauea was inflating, indicating that the magma reservoir beneath the summit had filled to the point that pressure could be sustained. Key Points: A gravity increase following the 2018 collapse of the summit of Kīlauea Volcano, Hawaiʻi, indicates mass accumulation in void space ~1 km beneath the surfaceThe maximum magma storage rate determined from gravity and deformation is much less than the pre‐collapse magma supply rate to the volcanoGravity monitoring provides an important means of assessing subsurface magma storage and supply rates as Kīlauea recovers from its 2018 collapse [ABSTRACT FROM AUTHOR]

We investigate the origin and implications of deformation-induced gravitational effects in interpretation of spatiotemporal gravity changes. We review the traditional approaches to handling the attraction of subsurface and surface deformations. These effects are relevant when inferring magmatic processes in volcano geodetic studies. We focus on the surface constituent, the deformation-induced topographic effect (DITE), which consists of a gradient effect called the free-air effect (FAE) and an attraction effect referred here as the topographic deformation effect. We present defining, alternate, as well as approximate expressions for evaluating the DITE. The alternate expressions shed light on the physical nature of DITE. By simulating numerically synthetic displacement fields of diverse shapes and areal extents imposed over terrain of various relief shapes in a referential volcanic area of prominent and rugged topographic relief, we assess the suitability and accuracy of the various approximations of DITE. Synthetic case studies are carried out using a high-resolution high-accuracy DEM and the Toposk software for evaluation of topographic attraction terms. We discuss the particularities and complications in numerical evaluation of each of the DITE expressions. We close with a conclusion that the best numerical prescription for accurate evaluation of DITE is Eq. (18) derived herein. Its numerical realization requires the knowledge of the deformation field in areal form. If the vertical displacements are known only at benchmarks, two approximations of DITE are at hand that can be numerically evaluated: the normal-FAE approximation (nFAE-DITE) and the planar Bouguer approximation (BCFAG-DITE). Based on synthetic simulations, we specify under what circumstances which approximation performs better. [ABSTRACT FROM AUTHOR]

Deception Island (South Shetland Islands) is one of the most active volcanoes in Antarctica. In the 1988–1989 austral summer, after the most recent eruptive process on the island (1967–1970), monitoring of volcanic activity through geophysical and geodetic techniques was resumed by Spanish and Argentinean scientists. In order to monitor the island's tectonic and volcanic behavior, a geodetic network was deployed. Currently, this network consists of 15 geodetic benchmarks located around Port Foster, Deception's inner bay open to the sea. Two additional geodetic benchmarks were installed outside Deception Island to be used as reference benchmarks for the differential positioning strategy. Since 1991–1992, geodetic ground-displacement velocities between the successive austral summer Antarctic campaigns have been computed and analyzed. The overall geodynamic behavior of Deception Island within the South Shetland Islands, Antarctic Peninsula and Bransfield Basin regional environment has been analyzed from geodetic ground-displacements. Results obtained demonstrate that Deception and Livingston island have a similar behavior derived from the Bransfield Basin extension and the Phoenix micro-plate subsidence processes. However, Deception Island is also highly influenced by its volcanic activity. Deception Island's volcanic behavior between 1991 and 2018 is shown by the velocity field, strain tensors and pressure source evolution obtained from the ground-displacements at the geodetic benchmarks. During this time period, it is possible to identify different inflation and deflation phases separated by transitional (or mixed) stages of extension without uplift and compression without subsidence. The most representative inflation and deflation periods were analyzed in detail, to show how they correlate with high and low seismic activity, respectively. The transitional or mixed stages, seem to be the precursors of the next inflation or deflation phase being the Bransfield basin rifting and NW-SE extension the potential related process. Finally, we have analyzed the processes that occurred prior to the volcanic unrests of 1999–2000 and 2012–2013. In both cases, an increase in detected seismic activity and/or soil and seawater temperature was observed and a mixed phase of extension without uplift seems to be precursory to the volcanic unrest. The correlation between the inflation processes, identified by ground-displacement of the network geodetic benchmarks, the increase in seismicity and the increment of soil and seawater temperature makes these transitional mixed phases potential precursors of Deception Island's volcanic unrest periods. • A GPS geodetic network of 15 geodetic stations is deployed in Deception Island. • Geodetic ground-displacement velocities have been computed and analyzed for the period 1991-2018. • Inflation-deflation processes are shown by the time evolution of the ground-displacement velocity models calculated. • Results demonstrate that Deception and Livingston islands have a similar behavior. • Deception Island is also highly influenced by its volcanic activity. [ABSTRACT FROM AUTHOR]

Borehole tiltmeters are widely used to continuously record small surface deformation of reservoirs and volcanoes. Because these instruments display unknown long-term drift, only short-term tilt signal can be used for monitoring purpose. We propose a method to invert long-term time series of tilt data induced by strain variations at depth. The assumption that tiltmeter drift is linear over time is on its own insufficient to remove the drift and uniquely determine the deformation source parameters. To overcome this problem, we first invert the data with no constrain on the drift to obtain one particular solution among all admissible. Then, using the linearity of the forward model, we use the statistical properties of the drift distributions to restore the uniqueness of the solution. We illustrate our approach with four synthetic cases simulating volume changes of a reservoir. We demonstrate the efficiency of our method and show that the accuracy of estimated volume variation dramatically improves if low drift tiltmeters are used. [ABSTRACT FROM AUTHOR]

Microgravity time series at active volcanoes can provide an indication of mass change related to subsurface magmatic processes, but uncertainty is often introduced by hydrologic variations and other noise sources that cannot easily be isolated. We empirically assessed seasonality and noise by conducting four surveys over the course of May–October 2017 at Yellowstone caldera, Wyoming. Yellowstone experiences frequent changes in the rates and styles of seismicity and surface deformation, but the mechanisms of these changes are poorly understood. Past gravity data from the caldera have yielded ambiguous results, possibly due to hydrologic noise. Given the strong visually observable changes in surface water and snow conditions over the course of our surveys, we expected to see significant variations in gravity. The net change in gravity, however, was less than 20 μGal at most sites, and there was no strong correlation with river and lake levels or snow conditions. Seasonal changes in gravity are therefore small compared to those that would be expected from magmatic activity, although they may be on the same order as those associated with Yellowstone's hydrothermal system. We did find that noise levels in gravity data were highly dependent on site characteristics, with bedrock sites away from trees yielding the lowest levels of noise, and thin concrete pads in forested areas the highest. These results can be used to plan future surveys at Yellowstone and to reinterpret past data, and they provide guidance in terms of best practices for repeat gravity work on volcanoes worldwide. Key Points: Microgravity changes due to seasonal variations at Yellowstone are much less than those expected from magmatic processesNoise levels at gravity stations are highly dependent on site setting and environmentDesign of gravity surveys at Yellowstone and elsewhere should focus on site characteristics to reduce noise and increase the chances of detecting changes caused by magmatic activity [ABSTRACT FROM AUTHOR]

We propose a multi-temporal-scale analysis of ground deformation data using both high-rate tilt and GNSS measurements and the DInSAR and daily GNSS solutions in order to investigate a sequence of four paroxysmal episodes of the Voragine crater occurring in December 2015 at Mt. Etna (Italy). The analysis aimed at inferring the magma sources feeding a sequence of very violent eruptions, in order to understand the dynamics and to image the shallow feeding system of the volcano that enabled such a rapid magma accumulation and discharge. The high-rate data allowed us to constrain the sources responsible for the fast and violent dynamics of each paroxysm, while the cumulated deformation measured by DInSAR and daily GNSS solutions, over a period of 12 days encompassing the entire eruptive sequence, also showed the deeper part of the source involved in the considered period, where magma was stored. We defined the dynamics and rates of the magma transfer, with a middle-depth storage of gas-rich magma that charges, more or less continuously, a shallower level where magma stops temporarily, accumulating pressure due to the gas exsolution. This machine-gun-like mechanism could represent a general conceptual model for similar events at Etna and at all volcanoes.

In this work, we study the subsidence of Dallol, an explosive crater and hydrothermal area along the spreading Erta Ale ridge of Afar (Ethiopia). No volcanic products exist at the surface. However, a diking episode in 2004, accompanied by dike-induced faulting, indicates that Dallol is an active volcanic area. The 2004 diking episode was followed by quiescence until subsidence started in 2008. We use InSAR to measure the deformation, and inverse, thermoelastic and poroelastic modelling to understand the possible causes of the subsidence. Analysis of InSAR data from 2004–2010 shows that subsidence, centered at Dallol, initiated in October 2008, and continued at least until February 2010 at an approximately regular rate of up to 10 cm/year. The inversion of InSAR average velocities finds that the source causing the subsidence is shallow (depth between 0.5 and 1.5 km), located under Dallol and with a volume decrease between −0.63 and −0.26 × 106 km3/year. The most likely explanation for the subsidence of Dallol volcano is a combination of outgassing (depressurization), cooling and contraction of the roof of a shallow crustal magma chamber or of the hydrothermal system.

Sabancaya is the most active volcano of the Ampato-Sabancaya Volcanic Complex (ASVC) in southern Perú and has been erupting since 2016. The analysis of ascending and descending Sentinel-1 orbits (DInSAR) and Global Navigation Satellite System (GNSS) datasets from 2014 to 2019 imaged a radially symmetric inflating area, uplifting at a rate of 35 to 50 mm/yr and centered 5 km north of Sabancaya. The DInSAR and GNSS data were modeled independently. We inverted the DInSAR data to infer the location, depth, and volume change of the deformation source. Then, we verified the DInSAR deformation model against the results from the inversion of the GNSS data. Our modelling results suggest that the imaged inflation pattern can be explained by a source 12 to 15 km deep, with a volume change rate between 26 × 106 m3/yr and 46 × 106 m3/yr, located between the Sabancaya and Hualca Hualca volcano. The observed regional inflation pattern, concentration of earthquake epicenters north of the ASVC, and inferred location of the deformation source indicate that the current eruptive activity at Sabancaya is fed by a deep regional reservoir through a lateral magmatic plumbing system.

Surface deformation measured by geodetic data is the sum of single-strain sources deforming at depth. A combination of volume changes from several analytical models (e.g. a point source or dislocation along a plane) can be used to model the different sources. However, solving for the best fit of volume variations, dislocations, position and orientation parameters of all sources is a nonlinear problem, and its solution is generally non-unique. This problem can be converted into a linear one by assimilating the sum of sources to a simplified model formed by three orthogonal planes of dislocations at fixed position and orientation. This strain source model is equivalent to having all neighbouring deformation sources contained in a small size volume. The determination of the strain tensor components can be performed by inverting geodetic data. Because of their high resolution, tiltmeters are well adapted to survey shallow deformation of volcanoes and geological reservoirs. However, they are known to display unknown long-term drift. We propose an approach to jointly estimate the temporal evolution of the strain source and time-dependent instrumental parameters. We verify the approach using synthetic data, giving confidence intervals for each component of the strain tensor. Finally, we link geological information to the internal deformation by interpreting the strain tensor as principal directions of deformation. This approach seems promising for the identification of fracture onset and fault reactivation in geothermal, hydrocarbon exploitations or volcanic systems. [ABSTRACT FROM AUTHOR]

Geodetic observations on volcanoes can reveal important aspects of crustal magma chambers. The rate of decay of deformation with distance reflects the centroid depth of the chamber. The amplitude of the deformation is proportional to the product of the pressure change and volume of the reservoir. The ratio of horizontal to vertical displacement is sensitive to chamber shape: sills are efficient at generating vertical displacement, while stocks produce more horizontal deformation. Geodesy alone cannot constrain important parameters such as chamber volume or pressure; furthermore, kinematic models have no predictive power. Elastic response combined with influx proportional to pressure gradient predicts an exponentially decaying flux, leading to saw-tooth inflation cycles observed at some volcanoes. Yet many magmatic systems exhibit more complex temporal behaviour. Wall rock adjacent to magma reservoirs cannot behave fully elastically. Modern conceptual models of magma chambers also include cumulate and/or mush zones, with potentially multi-level melt lenses. A viscoelastic shell surrounding a spherical magma chamber significantly modifies the predicted time-dependent response; post-eruptive inflation can occur without recharge if the magma is sufficiently incompressible relative to the surrounding crust (Segall P. 2016 J. Geophys. Res. Solid Earth, 121, 8501–8522). Numerical calculations confirm this behaviour for both oblate and prolate ellipsoidal chambers surrounded by viscoelastic aureoles. Interestingly, the response to a nearly instantaneous pressure drop during an explosive eruption can be non-monotonic as the rock around the chamber relaxes at different rates. Pressure-dependent recharge of a non-Newtonian magma in an elastic crust leads to an initially high rate of inflation which slows over time; behaviour that has been observed in some magmatic systems. I close by discussing future challenges in volcano geodesy. [ABSTRACT FROM AUTHOR]

Abstract: Uplift and subsidence alternate throughout the life cycle of a volcano mirroring magma accumulation, migration and/or gas pressurization, and magma cooling or depressurization, respectively. The unusual occurrence of coeval inflation and deflation is difficult to reconcile with the recharging or cooling of a single magma chamber, or hydrological processes. Here we show a persistent flank uplift and central subsidence at Colli Albani volcano near Rome (Italy), by mapping about 20 years of deformation by InSAR data. The magmatic helium signature increases in correspondence with N‐S faults along the western slope of Colli Albani, which are therefore interpreted as deep‐seated structures directly or indirectly connected with an underlying magma reservoir. Deformation modeling shows that these faults are pathways for fresh magma intrusion, whereas a residual magma is cooling below the deflating caldera. Therefore, magma recharge at depth is controlled by major faults along which the vents of the most recent activity (<200 ka ago) concentrate in the western side of the volcano and not by the caldera structures. We demonstrate that the Colli Albani magmatic system is slowly rejuvenating, posing a volcanic threat for Rome. [ABSTRACT FROM AUTHOR]

Abstract: Relative gravity measurements were made at 12 sites on Mount St. Helens and 4 sites far afield during the summers of 2010, 2012, 2014, and 2016. Positive residual gravity changes of 0.05–0.06 ± 0.01 mGal from 2010 to 2016—a sign of mass addition—remain at proximal sites after accounting for the effects of changes in Crater Glacier shape and mass. Modeling of the 2010–2016 monitoring data indicates mass addition in the volcano magma reservoir, the volcano conduit, and/or the shallow hydrothermal aquifer. Magma intrusion in the volcano's known reservoir is suggested by the joint inversion of GPS and gravity data (d = 5800 ± 710 m below sea level, ΔVm = 49.8 ± 8.6 × 106m3, ρ = 1930 ± 300 kg/m3); the modeled depth and location are consistent with that of the reservoir that fed the 2004–2008 eruption, and its mass change can explain up to 19% of the residual gravity. The other two potential sources—the conduit and shallow aquifer—are not well constrained. Magma addition along the volcano conduit can explain up to 62% of the residual gravity ( ΔVm ≅ 31 × 106m3,  ρm ≅ 2, 300 kg/m3). However, such an intrusion should have produced a measurable surface deformation, which is not observed in the GPS time series. Changes in the level of the volcano's shallow hydrothermal system ( ρw = 1, 000 kg/m3) can explain 17% ( ΔVrecharge ≅ 9 × 106m3) to 61% ( ΔVrecharge ≅ 30 × 106m3) of the residual gravity. It would therefore seem that the bulk of the mass change measured at Mount St. Helens during 2010–2016 was caused by shallow accumulation of water beneath the floor of the crater. [ABSTRACT FROM AUTHOR]

Sotará is a little known andesitic-dacitic stratovolcano in southern Colombia (Central Cordillera, Cauca Department). Its remote location and the lack of accessible roads make studying and monitoring Sotará volcano difficult. No historical eruptions are known, though there is current geothermal activity. Between the fall of 2010 and the fall of 2012 a small magma intrusion (depth ~ 1 km, ΔV ~ 10 5 m 3 ) triggered surface deformation and volcano-tectonic earthquakes but failed to reach the surface. This episode of unrest started with the volcano-tectonic earthquake of September 2010 (M L 4.4). Although one electronic tiltmeter recorded significant deformation between Jan 2011 and July 2012 (137 ± 7 μrad), seismic activity remained at background levels until May 2012. Between June and September 2012, seismicity increased dramatically with an average of 120 volcano-tectonic events per day of magnitude up to 2.2. The epicenters of these events were initially located 6 km NE of the volcano but then migrated along a linear fracture towards the NW summit of the volcano. At the same time, the hypocenters moved closer to the surface as well: from a depth of 3–6 km for the initial events to a depth of 1–4 km for the events at the end of the summer. Although the dike intrusion did not culminate in an eruption, and seismicity and deformation returned close to background levels by January 2013, this episode of unrest is a reminder that Sotará is an active volcano and should be carefully monitored. [ABSTRACT FROM AUTHOR]

Villarrica, Llaima, and Calbuco volcanoes are the most active and dangerous volcanoes in the Southern Andes, and we use Interferometric Synthetic Aperture Radar (InSAR) observations from multiple satellites (ERS-2, ENVISAT, ALOS, RADARSAT-2, COSMO-SkyMed, TerraSAR-X, Sentinel-1A and ALOS-2) to constrain ground deformation that spans episodes of unrest and eruption at all three volcanoes between 2002 and 2015. We find episodes of ground deformation at each volcano, which we invert using analytic elastic half-space models to make some of the first geophysical inferences about the source depths of potential magma chambers. At Llaima, we interpret that the VEI 2 April 3, 2009 eruption was preceded by ~ 6–15 cm of precursory ground uplift one month before from a source ~ 5 km below the surface on the western side of the edifice. The VEI 2 March 3, 2015 Villarrica eruption was followed by a short lived uplift of 5 cm in the SE part of the volcano from a source depth of ~ 6 km. The VEI 4 April 22–23, 2015 Calbuco eruption produced 12 cm of coeruptive subsidence from a source depth 8–11 km and offset ~ 2 km S from the summit. Importantly, we do not find clear evidence that the January 1, 2008, the March 3, 2015 and April 22, 2015 eruptions at Llaima, Villarrica and Calbuco volcanoes were preceded by either transient or continuous ground uplift. There are several possible explanations for the lack of precursory deformation at each volcano – it is possible that any precursory deformation occurred only hours before the eruption (e.g., at Calbuco), pre-eruptive inflation was canceled by co-eruptive subsidence (as we inferred happened during the April 2009 Llaima eruption), the pre-eruptive deformation was too small to be detectable in areas with persistent topography correlated phase delays, pressurized source are deep, or that open-vent volcanoes like Villarrica and Llaima do not pressurize. At all three volcanoes, X and C band interferograms decorrelate in a few weeks due to vegetation, snow and ice, and have persistent atmospheric phase delays that we find cannot be reliably removed with available global weather models. The low number of SAR acquisitions therefore makes it challenging to reliably measure unaliased deformation. We recommend a multi-satellite observing strategy with short repeat periods, frequently acquired high-resolution digital elevation models, and with acquisitions during every satellite overflight that may improve the temporal resolution of measurements. [ABSTRACT FROM AUTHOR]

Semipermanent GPS (SPGPS) is an alternative to conventional campaign or survey-mode GPS (SGPS) and to continuous GPS (CGPS) that offers several advantages for monitoring ground deformation. Unlike CGPS installations, SPGPS stations can be deployed quickly in response to changing volcanic conditions or earthquake activity such as a swarm or aftershock sequence. SPGPS networks can be more focused or more extensive than CGPS installations, because SPGPS equipment can be moved from station to station quickly to increase the total number of stations observed in a given time period. SPGPS networks are less intrusive on the landscape than CGPS installations, which makes it easier to satisfy land-use restrictions in ecologically sensitive areas. SPGPS observations are preferred over SGPS measurements because they provide better precision with only a modest increase in the amount of time, equipment, and personnel required in the field. We describe three applications of the SPGPS method that demonstrate its utility and flexibility. At the Yellowstone caldera, Wyoming, a 9-station SPGPS network serves to densify larger preexisting networks of CGPS and SGPS stations. At the Three Sisters volcanic center, Oregon, a 14-station SPGPS network complements an SGPS network and extends the geographic coverage provided by 3 CGPS stations permitted under wilderness land-use restrictions. In the Basin and Range province in northwest Nevada, a 6-station SPGPS network has been established in response to a prolonged earthquake swarm in an area with only sparse preexisting geodetic coverage. At Three Sisters, the estimated precision of station velocities based on annual ~ 3 month summertime SPGPS occupations from 2009 to 2015 is approximately half that for nearby CGPS stations. Conversely, SPGPS-derived station velocities are about twice as precise as those based on annual ~ 1 week SGPS measurements. After 5 years of SPGPS observations at Three Sisters, the precision of velocity determinations is estimated to be 0.5 mm/yr in longitude, 0.6 mm/yr in latitude, and 0.8 mm/yr in height. We conclude that an optimal approach to monitoring volcano deformation includes complementary CGPS and SPGPS networks, periodic InSAR observations, and measurements from in situ borehole sensors such as tiltmeters or strainmeters. This comprehensive approach provides the spatial and temporal detail necessary to adequately characterize a complex and evolving deformation pattern. Such information is essential to multi-parameter models of magmatic or tectonic processes that can help to guide research efforts, and also to inform hazards assessments and land-use planning decisions. [ABSTRACT FROM AUTHOR]

Experience during historical time throughout the Cascade arc and the lack of deep-seated deformation prior to the two most recent eruptions of Mount St. Helens might lead one to infer that Cascade volcanoes are generally quiescent and, specifically, show no signs of geodetic change until they are about to erupt. Several decades of geodetic data, however, tell a different story. Ground- and space-based deformation studies have identified surface displacements at five of the 13 major Cascade arc volcanoes that lie in the USA (Mount Baker, Mount St. Helens, South Sister, Medicine Lake, and Lassen volcanic center). No deformation has been detected at five volcanoes (Mount Rainier, Mount Hood, Newberry Volcano, Crater Lake, and Mount Shasta), and there are not sufficient data at the remaining three (Glacier Peak, Mount Adams, and Mount Jefferson) for a rigorous assessment. In addition, gravity change has been measured at two of the three locations where surveys have been repeated (Mount St. Helens and Mount Baker show changes, while South Sister does not). Broad deformation patterns associated with heavily forested and ice-clad Cascade volcanoes are generally characterized by low displacement rates, in the range of millimeters to a few centimeters per year, and are overprinted by larger tectonic motions of several centimeters per year. Continuous GPS is therefore the best means of tracking temporal changes in deformation of Cascade volcanoes and also for characterizing tectonic signals so that they may be distinguished from volcanic sources. Better spatial resolution of volcano deformation can be obtained through the use of campaign GPS, semipermanent GPS, and interferometric synthetic aperture radar observations, which leverage the accumulation of displacements over time to improve signal to noise. Deformation source mechanisms in the Cascades are diverse and include magma accumulation and withdrawal, post-emplacement cooling of recent volcanic deposits, magmatic-tectonic interactions, and loss of volatiles plus densification of magma. The Cascade Range thus offers an outstanding opportunity for investigating a wide range of volcanic processes. Indeed, there may be areas of geodetic change that have yet to be discovered, and there is good potential for addressing a number of important questions about how arc volcanoes work before, during, and after eruptions by continuing geodetic research in the Cascade Range. [ABSTRACT FROM AUTHOR]

The Canary Islands are mostly characterized by diffuse and scattered volcanism affecting a large area, with only one active stratovolcano, the Teide-Pico Viejo complex (Tenerife). More than 2 million people live and work in the 7,447 km of the archipelago, resulting in an average population density three times greater than the rest of Spain. This fact, together with the growth of exposure during the past 40 years, increases volcanic risk with respect previous eruptions, as witnessed during the recent 2011-2012 El Hierro submarine eruption. Therefore, in addition to purely scientific reasons there are economic and population-security reasons for developing and maintaining an efficient volcano monitoring system. In this scenario geodetic monitoring represents an important part of the monitoring system. We describe volcano geodetic monitoring research carried out in the Canary Islands and the results obtained. We consider for each epoch the two main existing constraints: the level of volcanic activity in the archipelago, and the limitations of the techniques available at the time. Theoretical and observational aspects are considered, as well as the implications for operational volcano surveillance. Current challenges of and future perspectives in geodetic volcano monitoring in the Canaries are also presented. [ABSTRACT FROM AUTHOR]

Erupting volcanoes commonly exhibit characteristic ground deformation that is typically interpreted in terms of pressure changes of magma reservoirs within the crust. However, other processes may also be significant. Since 1995, the Soufrière Hills Volcano, Montserrat, has erupted about 1 km 3 of magma over five discrete extrusive phases with clear cycles of associated ground deformation, recorded by GPS. Here we consider the contribution to deformation by loading of the ground surface with erupted deposits. We estimate topographic change and net deformation for the whole eruption to date, between November 1995 and February 2010. We derive the surface load distribution using differenced digital elevation data, which additionally enables us to constrain the budget of erupted lava. About a third of the lava erupted from Soufrière Hills since 1995 remains in subaerial, onshore deposits; more than previously thought. Another third is emplaced immediately offshore, and the remaining third has been transported further afield. We combine the deposit thickness map with representative deposit densities to calculate surface load and model the deformation response using finite elements. Our results show that net displacements accumulated over 14 years on Montserrat (tens of centimetres) could be explained by loading of erupted deposits on the flanks. The proportion of the observed deformation that can be explained by loading alone depends on crustal rheology. Using rheology structures favoured in the literature, our forward modelled displacements are remarkably similar to long-term observations, down to detail that we ascribe to localised load-topography interaction. Results suggest that the shallow crust beneath Montserrat is more compliant than usually assumed in geodetic models, with more rigid rheology at depth. Loading is largely accommodated by elastic strain in the shallow crust (top few kilometres) with negligible contribution from intra-crustal viscous flow over the time period investigated. We thus infer that the role of stress transfer from the surface load in metering magma reservoir behaviour must be negligible but may influence degassing in the shallow conduit, for example. Our findings suggest that, when volcanic ground deformation accompanies a voluminous eruption, geodetic model inversions will be misled if data are not appropriately corrected for the surface loading effect. [ABSTRACT FROM AUTHOR]

Abstract: Volcanic deformation during an unrest episode can provide valuable insights into potential magmatic plumbing system dynamics. Paramount to this is a model linking the recorded ground movement to the causative source at depth. Most models employ analytical techniques due to their simplicity, but these are limited in their approach due to a number of necessary assumptions, and restricted to crude subsurface representations. We address this problem by providing guidelines and example model files to benchmark against a simple, analytical model with a numerical Finite Element approach using COMSOL Multiphysics. The boundary conditions should also be applicable to other Finite Element modeling packages. Then, due to the flexibility of the Finite Element method, this allows a progression of adding increasing complexities to reproduce the likely intricacies of the subsurface. We thus provide further guidelines and accompanying model files to incorporate subsurface heterogeneity, benchmarked viscoelastic rheology and temperature-dependent mechanics. In doing so, we highlight that setting up more integrated geodetic models is not particularly difficult and can alter inferred source characteristics and dynamics. The models are applied to Uturuncu volcano in southern Bolivia to demonstrate the approach. [Copyright &y& Elsevier]

Hudson is one of the most active volcanoes in the Southern Andes-it had one of the largest eruptions of the 20th century in 1991 (VEI = 5) and smaller eruptions in 1971 (VEI = 3), maybe 1973, and 2011 (VEI of 1-2). We use satellite-based interferometric synthetic aperture radar (InSAR) and thermal imagery to characterize the activity of Hudson between 2004 and 2011 and during the 2011 eruption. InSAR data show that the volcano inflated between 2004 and 2010 with a maximum change rate of between 2 and 3 cm/yr-about half of the deformation rate observed during a previous deformation episode from 1993-1999. Inversion for an inflating point source suggests magma accumulation beneath the SW part of the caldera at an average depth of 10 km. This inferred source is deeper than both the sources estimated for the magma chamber of the 1991 eruption (from petrology) and for the 1993-1999 deformation event. Also, the deformation from 2004-2010 is centered at a slightly different location and has a smaller volume change than that between 1993-1999-further indicating that there is either a large magma reservoir or several separate ones. While the deformation center is a few km from the eruption location near the caldera rim, the two are possibly linked since the predicted static Coloumb stress changes due to the inferred inflation source would encourage unclamping on potential faults in the caldera rim. We also analize nighttime satellite thermal images from MODIS and ASTER. While MODIS did not show any unambiguous evidence for hot spots, ASTER thermal imagery show that at least four months before the eruption there were locations with temperatures 7-8ºK above background. Lahars observed by helicopter overflights on 4 March 2011 and October 2011 suggest that the hotspots may have been caused by lakes or subglacial melting. There is no InSAR data available for the months immediately preceding the eruption, but the ASTER thermal imagery results may indicate an increase in geothermal activity that could have been used to forecast the eruption. [ABSTRACT FROM AUTHOR]

Investigating the temporal development of magma reservoir pressure and associated surface displacements can reveal fundamental aspects of subsurface magmatic processes and aid in eruption forecasting. The limitation with existing volcano deformation models is that they typically ignore magma intrusion dynamics and focus on the response of surrounding rocks to source boundary pressure. Magma fluid dynamics should be incorporated into magmatic modelling to track the temporal development of a system, instead of the widely used kinematic techniques. Here, we compare analytical and numerical solutions for magma intrusion into a shallow reservoir, using two schemes of intrusion boundary condition, inlet pressure and inlet mass flow. Model sensitivity tests are conducted to explore key factors controlling the two-way coupling between solid and fluid components, assuming an incompressible magma for a first-order approach. For intrusions of viscous magma (⪆ 10 8 Pa s) or a narrow feeder conduit (5–20 m), applying an inlet pressure causes the resultant pressure and surface deformation to develop at a very slow rate; lower viscosity magmas produce faster deformation rates. The mass flow boundary condition reduces the number of model parameters as it is independent of poorly constrained parameters such as conduit and magma characteristics. For both boundary conditions, reservoir pressurization, and hence spatiotemporal surface deformation, are strongly influenced by reservoir geometry due to geometric compressibility. Our results provide fundamental knowledge to advance to more complex coupled fluid-solid mechanics models in volcano geodesy. • Two intrusion schemes are compared, examining the reservoir pressure evolution through time and resultant surface deformation • Reservoir pressure is influenced by characteristic timescales related to the dimensions of the feeder system and magma rheology [ABSTRACT FROM AUTHOR]

Abstract: We investigate active crustal deformation patterns associated with magmatic activity at Taal Volcano, an active tholeiitic volcano located in southwestern Luzon, Philippines. We present comparisons of elastic and combined elastic–viscoelastic modeling results with deformation observed by continuous GPS measurements on and near the volcano. Continuous dual-frequency and single-frequency (L1) GPS data between 1998 and 2005 provide evidence for smoothly transitioning periods of inflation and deflation centered under the volcano. Within deformation phases that typically last 3–9months (with rates exceeding 200mmyr−1), prominent inflationary phases were observed in February–November 2000 and June 2004–March 2005. The 2000 period of inflation was characterized by up to 145mmyr−1 surface extension and 220mmyr−1 of uplift of the center of Volcano Island relative to the northern caldera rim, while the 2005 inflation was characterized by as much as 116mmyr−1 horizontal extension of the volcanic edifice and 180mmyr−1 uplift. Inversions of observed surface deformation to determine the source location points to a relatively stable spherical source, situated under Volcano Island's central crater, at depths of ~5km (based on the preferred Mogi model). Using this source geometry, we develop axisymmetric finite element models with annuli of concentric shells around the magma reservoir, embedded within a multi-layered elastic lithosphere. Using simply varying pressure histories as input, a variety of forward models are fit to the time history of continuously observed deformation for GPS stations located on Volcano Island. Though the inferred source geometry, volume and near-source rheological properties are non-unique, they represent more physically appropriate material properties than those assumed in simple elastic half-space models. The sources inferred using viscoelastic shell models suggest simpler pressure histories and reduced overall pressure changes, relative to equivalent elastic half-space models. [Copyright &y& Elsevier]

Since Bezymianny Volcano resumed its activity in 1956, eruptions have been frequent; recently with up to 1–2 explosive events per year. To investigate deformation related to this activity we installed a GPS network of 8 continuous and 6 campaign stations around Bezymianny. The two striking observations for 2005–2010 are (1) rapid and continuous network-wide subsidence between 8 and 12mm/yr, which appears to affect KAMNET stations more than 40km away where we observe 4–5mm/yr of subsidence, and (2) only the summit station BZ09 shows slight deviations from the average motion in the north component at times of eruptions. The network-wide subsidence cannot be explained by tectonic deformation related to the build-up of interseismic strain due to subduction of the Pacific plate. A first order model of surface loading by eruptive products of the Kluchevskoy Group of Volcanoes also explains only a fraction of the subsidence. However, a deep sill at about 30km under Kluchevskoy that constantly discharges material fits our observations well. The sill is constrained by deep seismicity which suggests 9.5km width, 12.7km length, and a 13° dip-angle to the south-east. We infer a closing rate of 0.22m/yr, which implies a volume loss of 0.027km3/yr (0.16m/yr and 0.019km3/yr considering surface loading). Additional stations in the near and far field are required to uniquely resolve the spatial extent and likely partitioning of this source. We explain the eruption related deformation at BZ09 with a very shallow reservoir, likely within Bezymianny's edifice at a depth between 0.25km and 1.5km with a volume change of 1–4×10−4 km3. Much of the material erupted at Bezymianny may be sourced from deeper mid-crustal reservoirs with co-eruptive volume changes at or below the detection limit of the GPS network. Installation of more sensitive instruments such as tiltmeters would allow resolving of subtle co-eruptive motion. [ABSTRACT FROM AUTHOR]

Abstract: We have developed a MATLAB software package for the most common models used to interpret deformation measurements near faults and active volcanic centers. The emphasis is on analytical models of deformation that can be compared with data from the Global Positioning System (GPS), InSAR, tiltmeters and strainmeters. Source models include pressurized spherical, ellipsoidal and sill-like magma chambers in an elastic, homogeneous, flat half-space. Dikes and faults are described following the mathematical notation for rectangular dislocations in an elastic, homogeneous, flat half-space. All the expressions have been checked for typographical errors that might have been present in the original literature, extended to include deformation and strain within the Earth''s crust (as opposed to only the Earth''s surface) and verified against finite element models. A set of GPS measurements from the 2006 eruption at Augustine Volcano (Alaska) is used to test the software package. The results show that the best fit source to the GPS data is a spherical intrusion ( km3), about 880m beneath the volcano''s summit. [Copyright &y& Elsevier]

Abstract: Volcano flank instability has been recognized at many volcanoes around the globe. Structural, morphological, geodetic and geophysical evidence supports the continuous deformation of their flanks. While identification of instability has been recognized in well-documented examples, until recently the initial stages of such processes have been difficult to capture. Using a combination of geological, geodetic and geophysical data analysis, we study the stability of the Cumbre Vieja. New descending radar interferometric data, covering a volcanologically quiet period between 1992 and 2008 at Cumbre Vieja, indicate movement away from the satellite on the western volcano flank. Using an inversion of stacked velocity maps, we determine the geometry and slip for a near-horizontal dislocation beneath the western flank of Cumbre Vieja. Our ground deformation modelling results (position and depth) are in agreement with a low-density anomaly constrained by gravity data. The previously undetected intereruptive ground deformation at Cumbre Vieja volcano flanks was explained as an indicator of a kinematic passive response model of the flank, where the flanks were mobilized only during periods of magmatic activity (shallow dike intrusions) and remained stable and undeformed during intereruptive periods. Here, we present new results indicating that active creeping stress release due to gravitational loading is also a dominant deformation mechanism for (current) intereruptive periods at Cumbre Vieja, which would contribute positively to the stabilization of the edifice and reduce the associated hazard related to the volcano flank dynamics. This study at Cumbre Vieja can be considered as a prototype for similar volcanoes around the Macaronesian islands group (e.g., Fogo, Teide, El Hierro, Pico) and elsewhere. [ABSTRACT FROM AUTHOR]

Abstract: A time-dependent model for magma intrusion associated with repeated earthquake swarm activities off the east coast of the Izu peninsula was developed from precise levelling, electronic distance measurements (EDM) and global positioning system (GPS), and sea level observation data for the period 1973–1998. Significant deformations were observed in the inter-swarm period, suggesting the existence of a preparatory process for repeated earthquake swarm events. The geometry of a dyke-like tensile crack at a depth of 13km from deformation detected was estimated by precise levelling and GPS for the inter-swarm period, using a genetic algorithm. A large crack plane was considered; it had a combination of estimated deep tensile crack with a shallow dyke detected using the hypocentre distribution and calculated time-dependent inversion, which was carried out to estimate the temporal changes in the volumes of the crack in the period 1973–1998. Akaike''s Bayesian information criterion (ABIC) was used to choose the smoothing hyperparameters for the inversion. The characteristics of the estimated temporal change in magma volume were as follows. A rapid and large magma intrusion occurred at a depth of about 15km between 1976 and 1977. Despite a decrease in its expansion rate, a tensile crack at a depth greater than 8km continued to inflate between 1978 and 1988. The swarm activity started just one year after the rapid and large intrusion around the 15km depth. In addition, the tensile crack at the depth of about 15km beneath the internal region of the eastern Izu peninsula expanded in the inter-swarm period and deflated in the swarm period. This result suggests that magma was temporarily reserved in a dyke-like body beneath the eastern part of the Izu peninsula at a depth of 15km in a preparatory process for the earthquake swarm. These features strongly show that estimating the spatio-temporal distribution of this deep crack provides useful information for long-range predictions of volcanic earthquake swarms off the east coast of the Izu peninsula. [Copyright &y& Elsevier]

Abstract: Joint measurements of ground deformation and micro-gravity changes are an indispensable component for any volcano monitoring strategy. A number of analytical mathematical models are available in the literature that can be used to fit geodetic data and infer source location, depth and density. Bootstrap statistical methods allow estimations of the range of the inferred parameters. Although analytical models often assume that the crust is elastic, homogenous and isotropic, they can take into account different source geometries, the influence of topography, and gravity background noise. The careful use of analytical models, together with high quality data sets, can produce valuable insights into the nature of the deformation/gravity source. Here we present a review of various modeling methods, and use the historical unrest at Long Valley caldera (California) from 1982 to 1999 to illustrate the practical application of analytical modeling and bootstrap to constrain the source of unrest. A key question is whether the unrest at Long Valley since the late 1970s can be explained without calling upon an intrusion of magma. The answer, apparently, is no. Our modeling indicates that the inflation source is a slightly tilted prolate ellipsoid (dip angle between 91° and 105°) at a depth of 6.5 to 7.9 km beneath the caldera resurgent dome with an aspect ratio between 0.44 and 0.60, a volume change from 0.161 to 0.173 km3 and a density of 1241 to 2093 kg/m3. The larger uncertainty of the density estimate reflects the higher noise of gravity measurements. These results are consistent with the intrusion of silicic magma with a significant amount of volatiles beneath the caldera resurgent dome. [Copyright &y& Elsevier]