Absolute gravity measurements along the Andean margin: A contribution to earthquake and volcano geodesy

High precision gravity measurements in active tectonic or volcanic zones may offer complementary information to geodetic studies on crustal deformation processes and their possible associated mass or density changes. The Chilean margin of the Andes belongs to the most active subduction zones where such processes can be studied. As crustal deformation processes are also well documented by previous geodetic studies based on GPS or InSAR data, it represents a suitable natural laboratory for investigating temporal gravity changes of geodynamic origin.
In 2002, we initiated a first experiment of absolute gravity measurements in Chile using the laboratory micro-g FG5 meter. We set up a repetition network in co-location with existing or new continuous GPS stations that has been reoccupied and complemented in summer 2005. Our observations allowed us to estimate the inner accuracy of this new gravity network and confirm that the required accuracy for crustal deformation studies was achieved. They also provided the first evidence of local gravity changes associated with an intermediate-depth (97 km) subduction earthquake (Mw 7,7 Tarapaca earthquake, June 13, 2005). The observed changes are consistent with the co-seismic displacements measured by GPS and are mostly explained by the crustal vertical motion as predicted by an elastic dislocation model. In 2006, we evaluate the new portable absolute meter (micro-g A10 meter), recently available for measuring absolute gravity in outdoor conditions with the aim to extend the existing network and enhance the reliability with other geodetic observables (GPS networks).
We present here the results and perspectives of these absolute gravity observations and discuss their complementarities with GPS and InSAR data to investigate the present-day crustal dynamics. Another expected contribution of this study is the determination of gravity reference base stations in South America with potential applications to structural geology, geophysics and geodesy.