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Comunicación presntada a la 3ª Asamblea Hispano-Portuguesa de Geodesia y Geofísica = 3ª Assembleia Luso-Espanhola de Geodesia e Geofisica, celebrada en Valencia entre el 4 y el 8 de febrero de 2002., In this work, an analytical method based on the formulas by Mindlin (1936) is used to model the displacementfield and traction changes associated to the dynamic of a volcanic source. We summarized the set of analytical expressions of the displacement field due to a concentrated force and a center of dilation in a uniform halfspace. This kind of sources could be used to simulate the interaction of the magma intrusion mass with the hosting rack and the magma pressure change of the chamber due lo thermal effects or overfilling. This solution accounts for the effect of opposite signs for both types of sources. The dynamic of volcanic processes involves high strain and stress changes in the medium around the magma chamber. These local changes modify the regional tectonic stress field and can cause the failure of the rack containing the magma chamber. Modelling of such changes could be importantfor interpreting the evolution ofvolcanic unrests and the seismicity generated during unrests episodes.

Comunicación presntada a la 3ª Asamblea Hispano-Portuguesa de Geodesia y Geofísica = 3ª Assembleia Luso-Espanhola de Geodesia e Geofisica, celebrada en Valencia entre el 4 y el 8 de febrero de 2002., We have investigated the effects of the topography on the surface deformation and gravity changes caused by a magmatic infrusion in the Earth's crust. Considering axisymmetrical volcanoes and the varying depth methodology we show that topography has significant effecf on the magnitude of the predicted displacements and gravity changes. We see the reducfion of the magnitude close to the summit of the volcano. The topography effect is emphasized if we introduce the mass of the intrusion jointly with the dilation/compression of the magma chamber due to pressure change. Using the elastic-gravitational model introduced by Rundle (1980) that considers the interaciion of the intrusion mass with the ambient gravity field we show that neglecting the topographic effects may, in some cases, introduce an error greater than neglecting the ambient gravity field. We apply the described methodology to Mayon volcano, Philippines, to compute the gravity changes caused by an intrusion located in an elastic-gravitational halfspace. It can be seen that inversions based on spherical magma chamber model could gain accuracy by using varying elevation analytical solution. We show that, for inversions based on observed gravity changes, the amount of pressure or mass increment would be significantly underestimated and the depth of the chamber could be overestimated ifthe topography is not considered.

Comunicación presntada a la 3ª Asamblea Hispano-Portuguesa de Geodesia y Geofísica = 3ª Assembleia Luso-Espanhola de Geodesia e Geofisica, celebrada en Valencia entre el 4 y el 8 de febrero de 2002., Since September 1999 to December 2001 Indra Espacio and Spanish Volcanic Research Network (RIV) have been working together in the frame of DECIDE-VOLCANO project, funded by European Space Agency (ESA). The objective of this project was to demonstrate that satellite radar interferometry could be routinelly integrated in the operational volcano monitoring procedures as a complement to field collected data. For this purpose, the scenarios chosen in this project were the Canary Islands (Tenerife, Lanzarote y La Palma) and in a second stage, Ecuador (Tungurahua volcano and Galapagos islands, processing still on-going). Results obtained have been very different in every scenario: at Tenerife island we have found two deformations out of the usual monitoring areas and we have verified the results of no deformations previously obtained in Las Cañadas Caldera with ground surveys; at Lanzarote and La Palma islands we have not found any deformations greater than 3 cm.

Comunicación presntada a la 3ª Asamblea Hispano-Portuguesa de Geodesia y Geofísica = 3ª Assembleia Luso-Espanhola de Geodesia e Geofisica, celebrada en Valencia entre el 4 y el 8 de febrero de 2002., GPS techniques have demonstrated their usefulness in monitoring and detecting deformations, but a low cost-maximum gain technique such as InSAR must be found in order to assure the full surveillance of certain areas of interest. Two deformation areas have been detected using InSAR and two GPS geodetic campaigns have been designed to check these assessments. In this presentation we describe the results of two GPS campaigns conducted one year apart in Tenerife, Canary lslands, and designed to detect vertical movement. The results of these campaigns will provide future considerations about time span, number of points and general observation and documentation issues in order to obtain reliable results. We also describe in detail how the observation methodology and instrumentation were designed to avoid systematic errors in height and assure the necessary quality of the data.

Comunicación presntada a la 3ª Asamblea Hispano-Portuguesa de Geodesia y Geofísica = 3ª Assembleia Luso-Espanhola de Geodesia e Geofisica, celebrada en Valencia entre el 4 y el 8 de febrero de 2002., To define the most suitable volcano monitoring system is a particularly difficu/t task in the absence of recent activity. It must take into account not only the technical optimization of the monitoring systems, but also their economic and scientific profitability. Traditionally, observations of seismic, hydrologic, or fumarolic activity have proven useful. Since volcanic activity almost inevitably produces deformation and gravity changes before, during, between and after the events geodetic techniques are now included, in the light of the lA VCEJ's recommendations regarding routine monitoring. They are proving to be a powerful tool in the monitoring of volcanic activity detecting anomalies somefimes months or weeks before the magma jlow leads to earthquakes or other eruption precursors. Geodetic monitoring thus complements seismic monitoring by extending the study of volcanic phenomena from seconds to years and providing details on the growth of magma bodies within the volcano. Canary Islands, with a population of about 1.8 mil/ion people, are located less than /00 km from the African continent. They were formed on the continental rise and slope as independent structures. Despite being on a passive margin, they are a volcanic archipelago with a long-standing history of volcanic activity that began more than 40 m.y. ago. At least a dozen eruptions occurred on the islands ofLanzarote, Tenerife, and La Palma between /500 and 197 J. Several different types of volcanic manifestations, which follow a time pattern are to be seen on the Canary Islands. This is a typical case in which the volcanic monitoring system must be carefully designed. The system must be efficient, but must also make full use of existing facilities and have acceptable installation and running costs, in line with the current inactivity. In this presentation we described a proposal for the geodetic monitoring system in Canary lslands. This monitoring system has been develoloped with base on experimental results from different research projects and the application of theoretical analysis. This system makes used of terrestrial and space geodetic techniques.

Comunicación presntada a la 3ª Asamblea Hispano-Portuguesa de Geodesia y Geofísica = 3ª Assembleia Luso-Espanhola de Geodesia e Geofisica, celebrada en Valencia entre el 4 y el 8 de febrero de 2002., Observations of seismic, hydrologic or fumarolic activity have proven useful as reliable eruption precursors. In the last years, geodetic measurements are being used more extensively in active volcanoes because they have been shown to provide a reliable eruption precursor. Furthermore, ground deformation measurement techniques are attracting more attention, in particular with regard to pre-eruption ground displacement. The introduction of new geodetic techniques such as Global Positioning System (GPS) and Synthetic Aperture Radar Interferometry (SAR) can be highly useful in this context, because both techniques have pro ven to be a suitable tool for ground deformation monitoring of active volcanic areas. In this frame, it is possible to demonstrate the feas ibility of using space technologies for operational decision support demonstration at a volcanic risk scenario like Canary Islands. Particularly, the good existing coherence of volcanic areas makes them very proper for InSAR techniques application. By other hand, the theoretical and observational results of the analysis of sensibility of existing classical networks have made necessary the designing and observation of a GPS network in the island of Tenerife with a global coverage and monitoring purposes. This GPS network would be a complement and checking of InSAR observations performed in the island. The main objective of this job is defining and checking the applicability of a new geodetic monitoring system in Tenerife. In order to gel this, we make use of the existing facilities in the island and the new geodetic space techniques. We try to gel the possibility of detection of deformation at any part of the island. Therefore, the new system must be able to allow us the monitoring of any existing ground movement in the island.