TOMO 1 - Geodinámica Andina

ABSOLUTE GRAVITY NETWORK IN CHILE: A CONTRIBUTION TO GEODYNAMICS, GEOLOGY & GEODESY S. Bonvalot1,2, J. Hinderer3, G. Gabalda1,2, B. Luck3, F. Bondoux1, D. Comte2, L. Dorbath3, D. Legrand2, D. Remy1,2, J-C. Ruegg4 1 Institut de Recherche pour le Développement (IRD), UR154/LMTG, France ([email protected]) 2 Universidad de Chile, Departamento de Geofísica, Blanco Encalada 2085, Santiago, Chile 3 EOST/Université Louis Pasteur, Strasbourg, France 4 Institut de Physique du Globe de Paris, France

Precise absolute measurements of the earth gravity field can be of major importance in testing models for earth system processes or in monitoring the changes associated with a wide variety of natural and man-induced processes. In active subduction zones for instance, absolute gravity observations in conjunction with deformation measurements such as Global Positioning System (GPS) data, can be used to investigate temporal gravity changes and crustal deformations related with tectonic, earthquakes or volcanic processes. In most of these applications, absolute gravity measurements, instead of relative gravity measurements, are essential to assess the required accuracy to detect vertical motions or subsurface mass redistribution induced over distances in excess of several hundred kilometers and over time periods in excess of a few years.Since 2002, we carried out several field surveys in north and central Chile with the aim to set up co-located networks of geodetic (GPS) and absolute gravity reference stations that can be used to investigate the present- day crustal motions and gravity changes along the active Andean margin induced by geodynamic processes (earthquake or volcanic). A second objective is to contribute to the determination of reference gravity and GPS base stations in South America with potential applications to regional studies in geology or geodesy (structural geology, geoid computations, gravity calibration lines, etc.). Our absolute gravity measurements were acquired using the “state-of-the-art” of laboratory instruments (FG5 meter) when the highest accuracy was required (first order network) and a newly available portable instrument (A10 meter) used for network densification (second order network).

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Both FG5 and A10 instruments developed by micro-g/LaCoste company (USA) are based on free fall of a proof mass in a vacuum chamber and provide respective measurement accuracies of about 10-9 and 10-8 of the earth gravity field. The absolute gravity sites in Chile were selected according to the requirements of the study objectives and to the field conditions to ensure precise and safe data acquisition and mid to long-term stability of the stations. Special care was attempted to get measurements in co-location with other geodetic observations, such as continuous GPS data (global IGS network, TIGO Geodetic Observatory, permanent GPS monitoring networks for crustal deformation studies, Astronomic Observatories, etc.). A total number of 15 FG5 and about 40 A10 absolute gravity stations have been established in north and central Chile during 3 field surveys spanned between 2002 and 2006. The data acquired on this network allowed us first to estimate the inner accuracy of the Chilean absolute gravity network and to analyze the consistency of FG5 and A10 measurements. The repeated observations taken on this network in 2002 and 2005 also provided the first evidence of local gravity changes associated with the co-seismic displacement for the June 13, 2005 Tarapaca earthquake (Mw 7,9). Those observations are consistent with the predicted values computed from an elastic dislocation model for the Tarapaca earthquake and with other geodetic measurements (CGPS, INSAR data) also available for this event. We present here the results of these absolute gravity experiments in terms of instrument accuracy and repeatability, and environmental noise and discuss their implications for the study of geodynamic processes along the south-American plate margin and for the definition of an absolute gravity reference frame in Chile. ACKNOWLEDGMENTS Supported by IRD, ECOS-CONICYT (project n°C00U03), FONDECYT (project n°1030800), INSU (GDR G2) with contribution of SERNAGEOMIN, IGM, Univ. de Tarapacá (Arica), Univ. Arturo Prat (Iquique), Univ. Catolica del Norte (Antofagasta), Univ. La Serena, Univ. de Chile, Univ. de Concepción, , Lyceo D. Latrille (Tocopilla), TIGO Observatory, BKG Germany, ESO (Santiago, Cerro Paranal), Cerro Calán Astronomical Observatory, SHOA, Compañía Minera D.I. de Collahuasi, Carabineros de Chile, ENTEL.

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