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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]

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]

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]

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]

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]

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]

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]

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]

Simple models, like the well-known point source of dilation (Mogi’s source) in an elastic, homogeneous and isotropic half-space, are widely used to interpret geodetic and gravity data in active volcanic areas. This approach appears at odds with the real geology of volcanic regions, since the crust is not a homogeneous medium and magma chambers are not spheres. In this paper, we evaluate several more realistic source models that take into account the influence of self-gravitation effects, vertical discontinuities in the Earth’s density and elastic parameters, and non-spherical source geometries. Our results indicate that self-gravitation effects are second order over the distance and time scales normally associated with volcano monitoring. For an elastic model appropriate to Long Valley caldera, we find only minor differences between modeling the 1982–1999 caldera unrest using a point source in elastic, homogeneous half-spaces, or in elasto-gravitational, layered half-spaces. A simple experiment of matching deformation and gravity data from an ellipsoidal source using a spherical source shows that the standard approach of fitting a center of dilation to gravity and uplift data only, excluding the horizontal displacements, may yield estimates of the source parameters that are not reliable. The spherical source successfully fits the uplift and gravity changes, overestimating the depth and density of the intrusion, but is not able to fit the radial displacements. [ABSTRACT FROM AUTHOR]

Personnel from the U.S. Geological Survey's Cascades Volcano Observatory conducted first-order, class-II leveling surveys near Lassen Peak, California, in 1991 and at Newberry Volcano, Oregon, in 1985, 1986, and 1994. Near Lassen Peak no significant vertical displacements had occurred along either of two traverses, 33 and 44 km long, since second-order surveys in 1932 and 1934. At Newberry, however, the 1994 survey suggests that the volcano's summit area had risen as much as 97±22 mm with respect to a third-order survey in 1931. The 1931 and 1994 surveys measured a 37-km-long, east–west traverse across the entire volcano. The 1985 and 1986 surveys, on the other hand, measured only a 9-km-long traverse across the summit caldera with only one benchmark in common with the 1931 survey. Comparison of the 1985, 1986, and 1994 surveys revealed no significant differential displacements inside the caldera. A possible mechanism for uplift during 1931–1994 is injection of approximately 0.06 km3 of magma at a depth of approximately 10 km beneath the volcano's summit. The average magma supply rate of approximately 1×10–3 km3/year would be generally consistent with the volcano's growth rate averaged over its 600,000-year history (0.7–1.7×10–3 km3/year). [ABSTRACT FROM AUTHOR]

Leveling surveys in 1923, 1976, and each year from 1983 to 1993 have shown that the east-central part of the Yellowstone caldera, near the base of the Sour Creek resurgent dome, rose at an average rate of 14±1 mm/year from 1923 to 1976 and 22±1 mm/year from 1976 to 1984. In contrast, no detectable movement occurred in the same area from 1984 to 1985 (-2±5 mm/year), and from 1985 to 1993 the area subsided at an average rate of 19±1 mm/year. We conclude that uplift from 1923 to 1984 was caused by: (1) pressurization of the deep hydrothermal system by fluids released from a crystallizing body of rhyolite magma beneath the caldera, then trapped beneath a self-sealed zone near the base of the hydrothermal system; and (2) aseismic intrusions of magma into the lower part of the sub-caldera magma body. Subsidence since 1985 is attributed to: (1) depressurization and fluid loss from the deep hydrothermal system, and (2) sagging of the caldera floor in response to regional crustal extension. Future intrusions might trigger renewed eruptive activity at Yellowstone, but most intrusions at large silicic calderas seem to be accommodated without eruptions. Overpressurization of the deep hydrothermal system could conceivably result in a phreatic or phreatomagmatic eruption, but this hazard is mitigated by episodic rupturing of the self-sealed zone during shallow earthquake swarms. Historical ground movements, although rapid by most geologic standards, seem to be typical of inter-eruption periods at large, mature, silicic magma systems like Yellowstone. The greatest short-term hazards posed by continuing unrest in the Yellowstone region are: (1) moderate to large earthquakes (magnitude 5.5-7.5), with a recurrence interval of a few decdes; and (2) small hydrothermal explosions, most of which affect only a small area (<0.01 km), with a recurrence interval of a few years. [ABSTRACT FROM AUTHOR]

GPS measurements from a campaign network at Pacaya volcano, Guatemala, occupied from 2009 to 2015 were combined with InSAR data from 2013 to 2014 to model deformation sources for two eruptive time periods: 2009-2011 and 2013-2014. The GPS data for both of these time periods show downward vertical and outward horizontal deformation greater than 25 cm at several stations surrounding the volcano, while InSAR data shows up to 15-cm line-of-sight displacement. To better understand the dynamics of the magma storage system and sources of deformation, we inverted available geodetic data for these two periods. Our analytical modeling suggests that horizontal deformation was dominated by inflation of a shallow, subvertical dike, high within the volcanic edifice, while deflation of a deeper, spherical source embedded below the NW flank of the volcano occurred during at least part of the observation period. The source parameters for the dike feature are in good agreement with the observed alignment of recent eruptive vents, while parameters for the deeper, spherical source accommodate the downward vertical deformation observed at stations on and around the volcano. [ABSTRACT FROM AUTHOR]