Dr. Ron Hackney, Institut für Geowissenschaften, Abt. Geophysik, Alemanya
Sala d'Actes de l'Institut de Ciencies de la Terra "Jaume Almera", a les 12h del migdia.


Subduction zone gravity anomalies might indicate the degree of plate interface coupling and reflect the extent of rupture during great earthquakes. This is suggested by an apparent correlation between trench-parallel, negative residual gravity anomalies and regions of high seismic moment release in Circum-Pacific subduction zones. In south-central Chile (36-42°), at the northern end of the rupture zone of the 1960 Mw9.5 Valdivia earthquake, there is a prominent change in fore-arc gravity anomalies. North of 39°, fore-arc anomalies are positive, while to the south negative residual anomalies dominate. This difference suggests that north of 39° the plate interface is less coupled and less likely to be a part of extensive plate rupture.
A three-dimensional gravity model of this region, suggests that the observed differences in gravity anomalies reflect differences in slab depth beneath the fore-arc. The positive anomalies can largely be explained by a slab that north of 39° is about 5 km shallower than to the south. This, in turn, suggests that slab geometry contributes to the inferred variations in plate coupling in this region. With this in mind, we are aiming to use the geometry from the 3D gravity model to construct a detailed elastic dislocation model that explains GPS measurements of fore-arc deformation better than existing models. This modelling should help to constrain along-strike variations in the coupling properties of the thrust interface and allow further examination of the correlations between these properties and gravity anomalies.