María Charco

Inflation or deflation? New results for Mayon Volcano applying elastic-gravitational modeling

Volcanic activity produces deformation and gravity changes that many times can be used as precursors of future eruptions. Applying geodetic techniques to monitoring activity involves interpretation using deformation models. Usually, the observed changes of the deformation and gravity fields are interpreted seperately, not in a joint inversion. It can be difficult, if not impossible, to interpret the data coherently or correctly in terms of the characteristics of the intrusion or the deflation derived from the gravity changes with purely elastic models, as in the case of Mayon Volcano, Phillipines. We show that elastic-gravitational models can be used to interpret these cases simultaneously leading to a result that is more plausible on the basis of the available information. Thus, we may need to change the philosophy normally used to interpret geodetic observations. Interpretation as proposed in this work can significantly improve the possibility of predicting future eruptions.

Volcanic source inversion using a genetic algorithm and an elastic-gravitational layered earth model for magmatic intrusions

Here we present an inversion methodology using the combination of a genetic algorithm (GA) inversion program, and an elastic-gravitational earth model to determine the parameters of a volcanic intrusion. Results from the integration of the elastic-gravitational model, a suite of FORTRAN 77 programs developed to compute the displacements due to volcanic loading, with the GA inversion code, written in the C programming language, are presented. These codes allow for the calculation of displacements (horizontal and vertical), tilt, vertical strain and potential and gravity changes on the surface of an elastic-gravitational layered Earth model due to the magmatic intrusion. We detail the appropriate methodology for examining the sensitivity of the model to variation in the constituent parameters using the GA, and present, for the first time, a Monte Carlo technique for evaluating the propagation of error through the GA inversion process. One application example is given at Mayon volcano, Philippines, for the inversion program, the sensitivity analysis, and the error evaluation. The integration of the GA with the complex elastic-gravitational model is a blueprint for an efficient nonlinear inversion methodology and its implementation into an effective tool for the evaluation of parameter sensitivity. Finally, the extension of this inversion algorithm and the error assessment methodology has important implications to the modeling and data assimilation of a number of other nonlinear applications in the field of geosciences.

Joint interpretation of displacement and gravity data in volcanic areas. A test example: Long Valley Caldera, California

Volcanic activity produces deformation and gravity changes that many times can be used as precursors of future eruptions. Applying geodetic techniques to monitoring activity involves interpretation using deformation models. Usually gravity change data and displacement data are interpreted separately. We show, using modeling of deformation and gravity change data in Long Valley Caldera, California, USA, that this can lead to incorrect interpretations. The results obtained show that displacements and gravity changes must be interpreted together whenever possible and that elastic-gravitational models can be a far more appropriate approximation to problems of volcanic load in the crust than the more commonly used purely elastic models. Therefore it is necessary to change the philosophy normally used to interpret geodetic observations, improving the possibility of predicting future eruptions.

GPS monitoring in the N-W part of the volcanic island of Tenerife, Canaries, Spain: Strategy and results

This paper describes design, observation methodology, results and interpretation of the GPS surveys conducted in the áreas of the N-W of Tenerife where deformation was detected using InSAR. To avoid undesirable antenna positioning errors in the stations built using nails, we designed and used calibrated, fixed-length metal poles, allowing us to guarantee that the GPS antenna was stationed with a height repeatability of the order of 1 mm and of less than 3 millimeters on the horizontal plañe. The results demónstrate that this system is ideal for field observation, especially to detect small displacements that might be masked by accidental errors in height measurements or centering when observed with a tripod. When observations were processed, we found that using different antenna models in the same session sometimes causes errors that can lead to rather inaccurate results. We also found that it is advisable to observe one or two stations in all the sessions. The results have reconfirmed the displacement in the Chío deformation zone for the period 1995-2000 and indicate a vertical rebound from 2000 to 2002. They also confirm that the subsidence detected by InSAR to the south of the Garachico village has continued since 2000, although the magnitude of the vertical deformation has increased from around 1 cm to more than 3 cm a year. Detected displacements could be due to groundwater level variation throughout the island. A first attempt of modelling has been made using a simple model. The results indicate that the observed deformation and the groundwater level variation are related in some way. The obtained results are very important because they might affect the design of the geodetic monitoring of volcanic reactivation on the island, which will only be actually useful if it is capable of distinguishing between displacements that might be linked to volcanic activity and those produced by other causes. Even though the study was limited to a given área of Tenerife, in the Canary Islands, some conclusions apply to, and are of general interest in similar geodynamic studies.

Topography and self‐gravitation interaction in elastic‐gravitational modeling

Changes in gravity due to volcanic loading of the crust are influenced by topography. We investigate the relative importance of topography and self-gravitation in the interpretation of gravity changes. It is shown that modeling of gravity changes can be more precise with the introduction of topographic relief, although it is neglected self-gravitation of the medium. This paper exploits this result by suggesting a mathematical simplification that could be useful in the future development of a numerical technique to accurately include topographic effects in the modeling of deformation and gravity changes. Finally, we perform an inversion of the gravity changes observed at Mayon volcano (Philippines) between December 1992 and December 1996 including topographic effects by varying the depth of the source. Failure to account for topographic influences can bias estimates of source parameters particularly when the lateral extension of the relief is of the same order of magnitude as the source depth.

Short note: A revision of the FORTRAN codes GRAVW to compute deformation produced by a point magma intrusion in elastic-gravitational layered earth models

A revision of the FORTRAN codes GRAVW to compute deformation produced by a point magma intrusion in elastic-gravitational layered earth models José Fernández , Marı́a Charco, John B. Rundle, Kristy F. Tiampo Instituto de Astronomı́a y Geodesia (CSIC-UCM), Facultad de Ciencias Matemáticas, Ciudad Universitaria, Pza. de Ciencias, 3. 28040-Madrid, Spain Department of Physics, Center for Computational Science and Engineering, University of California, One Shields Avenue, Davis, CA 95616-8677, USA Department of Earth Sciences, University of Western Ontario, London, Ont., Canada

Some Insights About Volcano Deformation Interpretation

This study presents a numerical tool for solving very fast and with appreciable degree of accuracy the inverse problem to estimate the optimal parameters for the location and pressurization of a spherical magma chamber, based on geodetic data. In doing so, we propose the Finite Element Method (FEM). The forward model for deformation simulations is based on including a pressure source responsible of medium displacement as a source term in model equations. It provides a considerably reduction of the computational cost of inversion based on FEM forward models avoiding remeshing and assembling the linear system of algebraic equations that defines the numerical approach.

Finite Element Numerical Solution for Modelling Ground Deformation in Volcanic Areas

Current understanding of critical stages prior to a volcanic eruption is generally based on elastostatic analysis. We investigate the elastic response of the Earth to an internal load that simulates the effect of a pressurized magma reservoir. Firstly, equations describing the Earth’s deformation for elastic models are introduced and the corresponding boundary value problem is formulated in a weak sense. Then, a numerical tool to compute the displacement and stress fields produced by pressurized sources in volcanic areas is described. In doing so, we propose the Finite Element Method for simulating the deformation that Teide volcano (Tenerife, Canary Islands) would undergo, if a hypothetical magma intrusion would take place in a shallow magma reservoir beneath its summit. Furthermore, the numerical approach can be used to estimate the influence of parameters such as size, depth and shape of a pressurized reservoir, the topography and the medium heterogeneities over ground deformation modelling. Therefore, such numerical approaches can be useful to design and/or improve the geodetic monitoring system in volcanic areas.

3D analytical and numerical modelling of the regional topography influence on the surface deformation due to underground heat source

3D analytical and numerical modelling of the regional topography influence on the surface deformation due to underground heat source Thermo-elastic strains and stresses play a considerable role in the stress state of the lithosphere and its dynamics, especially at pronounced positive geothermal anomalies. Topography has a significant effect on ground deformation. In this paper we describe two methods for including the topographic effects in the thermo-viscoelastic model. First we use an approximate methodology which assumes that the main effect of the topography is due to distance from the source to the free surface and permits to have an analytical solution very attractive for solving the inverse problem. A numerical solution using Finite Element Method (FEM) is also computed. The numerical method allows to include the local shape of the topography in the modelling. In the numerical model the buried magmatic body is represented by a finite volume thermal source. The temperature distribution is computed by the higher-degree FEM. For analytical as well as numerical model solution only the forces of thermal origin are considered. The comparison of the results obtained using both analytical and numerical techniques shows the qualitative agreement of the vertical displacements. In the numerical values small differences were obtained. The results show that for the volcanic areas with an important relief the perturbation of the thermo-viscoelastic solution (deformation and total gravity anomaly) due to the topography can be quite significant. In consequence, neglecting topography could give erroneous results in the estimated source parameters.