Joachim H Gottsmann

4D volcano gravimetry

Time-dependentgravimetricmeasurementscandetectsubsurface processes long before magma flow leads to earthquakes or othereruptionprecursors.Theabilityofgravitymeasurementsto detect subsurface mass flow is greatly enhanced if gravity measurements are analyzed and modeled with ground-deformation data. Obtaining the maximum information from microgravity studies requires careful evaluation of the layout of network benchmarks, the gravity environmental signal, and the coupling betweengravitychangesandcrustaldeformation.Whenchanges in the system under study are fast hours to weeks, as in hydrothermal systems and restless volcanoes, continuous gravity observationsatselectedsitescanhelptocapturemanydetailsofthe dynamics of the intrusive sources. Despite the instrumental effects, mainly caused by atmospheric temperature, results from monitoring at Mt. Etna volcano show that continuous measurementsareapowerfultoolformonitoringandstudyingvolcanoes. Several analytical and numerical mathematical models can beusedtofitgravityanddeformationdata.Analyticalmodelsofferaclosed-formdescriptionofthevolcanicsource.Inprinciple, this allows one to readily infer the relative importance of the source parameters. In active volcanic sites such as Long Valley caldera California, U.S.A. and Campi Flegrei Italy, careful use of analytical models and high-quality data sets has produced good results. However, the simplifications that make analytical modelstractablemightresultinmisleadingvolcanologicalinterpretations, particularly when the real crust surrounding the sourceisfarfromthehomogeneous/isotropicassumption.Using numericalmodelsallowsconsiderationofmorerealisticdescriptions of the sources and of the crust where they are located e.g., vertical and lateral mechanical discontinuities, complex source geometries, and topography. Applications at Teide volcano Tenerifeand Campi Flegrei demonstrate the importance of this morerealisticdescriptioningravitycalculations.

Unrest at Campi Flegrei: A contribution to the magmatic versus hydrothermal debate from inverse and finite element modeling

[ 1] We present results from the modeling of ground deformation and microgravimetric data recorded at Campi Flegrei in order to assess the causative phenomena of caldera unrest between 1981 and 2001. We find that residual gravity changes during ground uplift ( 1982 - 1984) are indicative of mass changes in a hybrid of magmatic and hydrothermal sources. During deflation between 1985 and 2001, the inversion of gravity residuals for a single source does not provide convincing results. We then performed the joint inversion of gravity and deformation data for multiple spherical sources and refined source parameters by finite element modeling in order to mitigate against limitations of the analytical solutions. The data recorded during inflation and rapid deflation may be best explained by mass and pressure changes in a deep magmatic source at about 5 km depth and a shallow ( 2 km deep) hydrothermal source. Both sources contribute equally to the gravity changes observed between 1982 and 1984; the contemporary uplift appears to be mainly caused by the shallow source. The subsequent deflation is dominated by a pressure decrease in the hydrothermal source; the magmatic source contributes chiefly to the observed gravity changes. Pressure and density variations within multiple shallow-seated hydrothermal sources provide acceptable fits to the deflation and accompanying gravity changes recorded since 1988. These shallow level dynamics also appear to trigger spatially and temporarily random short-term reversals of the overall mode of ground subsidence since 1985. Our analysis does not support the idea of magmatic contributions to these short-lived periods of inflation.

Predicting shear viscosity during volcanic processes at the glass transition: a calorimetric calibration

The viscosity of volcanic melts at the glass transition has been determined for 11 compositions ranging from basanite to rhyolite. Determination of the temperature dependence of viscosity, together with the cooling rate dependence of the glass transition, permits the calibration of the value of the viscosity at the glass transition at a given cooling rate for each melt. Temperature-dependent viscosities have been obtained using micropenetration methods in the range 10 8 ^10 12 Pa s. Glass transition temperatures have been obtained using differential scanning calorimetry. For each investigated melt composition, the activation energies yielded by calorimetry and viscometry are identical. This confirms that a simple shift factor can be used for each in order to determine the viscosity at the glass transition for a given cooling rate in nature. The results of this study indicate that there is a subtle but significant compositional dependence of the shift factor of a factor of 10 (in log terms) from 10.8 to 9.8. The composition dependence of the shift factor is cast here in terms of a compositional parameter, the mol% of excess oxides (defined within). Using such a parameterisation we obtain a non-linear dependence of the shift factor upon composition that matches the 17 observed values within error. The resulting model permits the prediction of viscosity at the glass transition, during the cooling of glassy volcanic rocks to within 0.1 log units. fl 2002 Elsevier Science B.V. All rights reserved.

Toward continuous 4D microgravity monitoring of volcanoes

Four-dimensional or time-lapse microgravity monitoring has been used effectively on volcanoes for decades to characterize the changes in subsurface volcanic systems. With measurements typically lasting from a few days to weeks and then repeated a year later, the spatial resolution of theses studies is often at the expense of temporal resolution and vice versa. Continuous gravity studies with one to two instruments operating for a short period of time (weeks to months) have shown enticing evidence of very rapid changes in the volcanic plumbing system (minutes to hours) and in one case precursory signals leading to eruptive activity were detected. The need for true multi-instrument networks is clear if we are to have both the temporal and spatial reso-lution needed for effective volcano monitoring. However, the high cost of these instruments is currently limiting the implementation of continuous microgravity networks. An interim approach to consider is the development of a collaborative network of researchers able to bring multiple instruments together at key volcanoes to investigate multitemporal physical changes in a few type volcanoes. However, to truly move forward, it is imperative that new low-cost instruments are developed to increase the number of instruments available at a single site. Only in this way can both the temporal and spatial integrity of monitoring be maintained. Integration of these instruments into a multiparameter network of continuously recording sensors is essential for effective volcano monitoring and hazard mitigation.

Recognizing and tracking volcanic hazards related to non-magmatic unrest: a review

Eruption forecasting is a major goal in volcanology. Logically, but unfortunately, forecasting hazards related to non-magmatic unrest is too often overshadowed by eruption forecasting, although many volcanoes often pass through states of non-eruptive and non-magmatic unrest for various and prolonged periods of time. Volcanic hazards related to non-magmatic unrest can be highly violent and/or destructive (e.g., phreatic eruptions, secondary lahars), can lead into magmatic and eventually eruptive unrest, and can be more difficult to forecast than magmatic unrest, for various reasons. The duration of a state of non-magmatic unrest and the cause, type and locus of hazardous events can be highly variable. Moreover, non-magmatic hazards can be related to factors external to the volcano (e.g., climate, earthquake). So far, monitoring networks are often limited to the usual seismic-ground deformation-gas network, whereas recognizing indicators for non-magmatic unrest requires additional approaches. In this study we summarize non-magmatic unrest processes and potential indicators for related hazards. We propose an event-tree to classify non-magmatic unrest, which aims to cover all major hazardous outcomes. This structure could become useful for future probabilistic non-magmatic hazard assessments, and might reveal clues for future monitoring strategies.

Chapter 6 A Review on Collapse Caldera Modelling

Abstract A complete study of collapse caldera formation should ideally involve multiple aspects such as regional tectonics, system geometry, magma and host rock properties, fluid-structure interaction, pre-existing structural discontinuities, and deformation history. Due to the complexity of such a comprehensive analysis, studies so far have centred on relevant but atomised topics. From a methodological point of view, and in addition to essential field and petrological studies, collapse calderas have also been investigated through analogue and theoretical models and geophysical imaging. Each approach presents advantages and disadvantages. We review the most significant contributions, summarise the relevant outcomes, and highlight the strong points and weaknesses of each approach. Analogue models enable a qualitative study of the structural evolution of a collapse process and allow us to infer which geometric factors play a relevant role. Differences among employed models lie in the applied experimental devices, the host rock analogue material (dry quartz sand, flour, etc.), and the magma chamber analogue (water or air-filled balloons, silicone reservoirs, etc.). However, the results obtained from different experimental setups are not substantially different if basic input parameters are kept similar in the experiments. Discrepancies in results mainly stem from restrictions of experimental designs. Theoretical (mathematical) models have grown in importance during the past decades, in combination with the development of computational resources. Nowadays, these models constitute a significant source of information on caldera-forming processes and can predict semi-quantitatively general conditions for fault formation and propagation. Theoretical studies can be classified in two groups according to their objectives. One group focuses on the evolution of pressure within the magmatic reservoir during a caldera-forming eruption. The second looks more into the structural conditions for caldera collapse and hence relate to analogue models. Both analogue and theoretical models are employed to gain a fundamental understanding of caldera processes and their resulting structures. Additionally, geophysical imaging helps to construct a regional image of the subsurface at active calderas, thus imposing constraints on the structural investigations based on analogue and mathematical modelling. A revision of each of these three complementary approaches to the study of collapse calderas is given in this paper, together with a combined analysis of their main findings and restrictions.

Estimating volcanic deformation source parameters with a finite element inversion: The 2001–2002 unrest at Cotopaxi volcano, Ecuador

Deformation at Cotopaxi was observed between 2001 and 2002 along with recorded seismicity beneath the northeast (NE) flank, despite the fact that the last eruption occurred in 1942. We use electronic distance meter deformation data along with the patterns of recorded seismicity to constrain the cause of this unrest episode. To solve for the optimum deformation source parameters we employ inverse finite element (FE) models that account for material heterogeneities and surface topography. For a range of source shapes the models converge on a shallow reservoir beneath the southwest (SW) flank. The individual best fit model is a small oblate-shaped source, approximately 4-5 km beneath the summit, with a volume increase of roughly 20 × 10 6 m 3 . This SW source location contrasts with the NE seismicity locations. Subsequently, further FE models that additionally account for temperature-dependent viscoelasticity are used to reconcile the deformation and seismicity simultaneously. Comparisons of elastic and viscous timescales allude to aseismic pressurization of a small magma reservoir in the SW. Seismicity in the NE is then explained through a mechanism of fluid migration from the SW to the NE along fault systems. We extend our analyses to further show that if future unrest crises are accompanied by measurable seismicity around the deformation source, this could indicate a higher magma supply rate and increased likelihood of a forthcoming eruption.

The 3-D gravity inversion package GROWTH2.0 and its application to Tenerife Island, Spain

We present the gravity inversion software GROWTH2.0 and its application to recently obtained gravity data from the volcanic island of Tenerife (Canary Islands, Spain) to inform on its subsurface density structure. GROWTH2.0 is an inversion tool which enables the user to obtain, in a nearly automatic and nonsubjective mode, a 3D model of the subsurface density anomalies based on observed gravity anomaly data. The package is composed of three parts: (a) GRID3D to generate a 3D partition of the subsurface volume into parallelepiped elements, (b) GROWTH to perform the inversion routine and to obtain a 3D anomalous density model, and (c) VIEW for visual representation of the input data, the inversion model, and modeling residuals. The current version of the tool has been developed from an earlier code (Camacho et al., 2002) and now incorporates several novelties: (1) a Graphical User Interface (GUI), (2) an optional automated routine for determination of parameter @l, which controls the balance between model fitness and smoothness, (3) optional determination of values for minimum density contrast, (4) a robust handling of outlier data, and (5) improved automated data reduction for terrain effects based on anticorrelation with topographic data. The new capabilities and applicability of GROWTH2.0 for 3-D gravity inversion are demonstrated by a case example using new gravity data from the volcanic island of Tenerife. In a nearly automatic approach, the software provides a 3-D model informing on the location and shape of the main structural building blocks of the island. Our model results allow us to shed light on the low-density structure of the islands dominant Pico Viejo-Pico Teide (PV-PT) volcanic complex and the identification of an intrusive structure (the east bulge volcano) embedded in Teides east flank. A low-density body located at around 5.8km depth beneath PTs summit may represent a current magma or hybrid reservoir.

The effects of thermomechanical heterogeneities in island arc crust on time‐dependent preeruptive stresses and the failure of an andesitic reservoir

Using ground deformation data from Soufriere Hills volcano (SHV), we present results from numerical modeling of the temperature- and time-dependent stress evolution in a mechanically heterogeneous crust prior to reservoir failure and renewed eruptive activity. The best fit models do not allow us to discriminate between a magmatic plumbing system consisting of either a single vertically elongated reservoir or a series of stacked reservoirs. A prolate reservoir geometry with volumes between 50 and 100 km3, reservoir pressure changes between 4 and 7 MPa, and reservoir volume changes between 0.03 and 0.04 km3 with magma compressibility between 4 × 10−11 and 1 × 10−9 Pa−1 provide plausible thermomechanical model parameters to explain the deformation time series; around an order of magnitude less overpressure than is generally inferred from homogeneous, elastic crustal models. Reservoir failure is predicted to occur at the crest of the reservoir except for reservoirs with highly compressible magma (≳4×10−9 Pa) for which subhorizontal sill formation is predicted upon reservoir failure. Introducing a deep-crustal hot zone modulates the partitioning of strains into the hotter underlying crust and results in a further reduction in overpressure estimates to values of around 1–2 MPa upon reservoir failure. Deduced volume fluxes are consistent with constraints from thermal modeling of active subvolcanic systems and imply dynamic failure of a compressible magma mush column feeding eruptions at SHV. Our interpretation of the results is that the combined thermomechanical effects of a deep-crustal hot zone and hot encasing rocks around a midcrustal andesitic reservoir fundamentally alter the time-dependent subsurface stress and strain partitioning upon reservoir priming. These effects substantially influence surface strains recorded by volcano geodetic monitoring.

Thermomechanical controls on magma supply and volcanic deformation: application to Aira caldera, Japan.

Ground deformation often precedes volcanic eruptions, and results from complex interactions between source processes and the thermomechanical behaviour of surrounding rocks. Previous models aiming to constrain source processes were unable to include realistic mechanical and thermal rock properties, and the role of thermomechanical heterogeneity in magma accumulation was unclear. Here we show how spatio-temporal deformation and magma reservoir evolution are fundamentally controlled by three-dimensional thermomechanical heterogeneity. Using the example of continued inflation at Aira caldera, Japan, we demonstrate that magma is accumulating faster than it can be erupted, and the current uplift is approaching the level inferred prior to the violent 1914 Plinian eruption. Magma storage conditions coincide with estimates for the caldera-forming reservoir ~29,000 years ago, and the inferred magma supply rate indicates a ~130-year timeframe to amass enough magma to feed a future 1914-sized eruption. These new inferences are important for eruption forecasting and risk mitigation, and have significant implications for the interpretations of volcanic deformation worldwide.