Géodésie, géodynamique et cycle sismique en Equateur

Géodésie, géodynamique et cycle sismique en Equateur,J. M. Nocquet,Patricia Mothes

Géodésie, géodynamique et cycle sismique en Equateur  
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Spatial geodesy now enables us to monitor the Earth's surface motion at the millimeter level in a global reference and provides new constraints on the process controlling the present-day crustal deformation. We review previously published GPS based works in the northern Andes and present preliminary results from a continuous GPS network along the Ecuadorian coast. At the plate scale, geodetic estimates indicate that the Nazca plate is converging towards South America at 58 mm/yr in a N83°E direction, along the Ecuadorian margin. This rapid oblique subduction causes two distinct effects that contribute to the observed GPS velocity field in Ecuador : (1) locking along the subduction causes elastic stress to accumulate along the plates interface and induces shortening of the overriding plate. This elastic stress will be released in future earthquakes (2) long-term motion of the Northern Andean Block towards NNE with respect to the South America plate. In order to separate the two effects, we use recently published quaternary slip rates along major faults and estimate the motion of the Northern Andean block to be 8.7 mm/yr in a N35°E direction with respect to South America. The part of the oblique convergence accommodated by the Northern Andean block motion is only 30% and reflects a partial partitioning of the oblique Nazca/South America convergence. Residual velocities in a northern Andean block reference frame are then interpreted in term of locking along the subdution. The average coupling coefficient is found to be ~40% in the rupture area of the great 1906 Mw=8.8 earthquake. It decreases to ~20% further south where
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