INTERPLATE LOCKING AND DISSIPATION OF VISCOELASTIC POST-SEISMIC DISPLACEMENT OF THE 1960 VALDIVIA, CHILE, EARTHQUAKE

The M_w 9.5 1960 Chile earthquake ruptured the Nazca-South America subduction interface over a distance of 920±100 km, from Valdivia, in the north, to the Chile triple junction (CTJ), in the south, with mean co-seismic slip estimates varying from 15-20 m. Classical and space-based (GPS) geodetic measurements, and observations of coastal uplift, show that the 1960 earthquake produced large post-seismic displacements. Geodetically observed deformation of overriding South America (SA) includes a large component of elastic strain due to Nazca (NZ) convergence at all coastal sites and at most inland sites well north of the CTJ. This elastic deformation, caused by locking of the interplate interface, results in surficial site velocities directed in the NZ-SA convergence direction at varying rates less than the full NZ-SA convergence rate during periods of steady loading between earthquakes on the interplate interface. However, geodetic observations made in the 1990’s to mid-2000’s near and just north of the CTJ region (46.5-38°S) yielded clear evidence of westward motions relative to stable SA at a mean rate of ~4.5 mm/yr at inland sites on the overriding plate. Given strong evidence that the CTJ was the southern rupture limit of the 1960 great Chile earthquake, these motions were interpreted to represent slow, post-seismic viscoelastic mantle rebounding displacement of the SA foreland due to the 1960 earthquake. This displacement was observed at sites well east of the NZ-SA subduction trench, and was modeled as slow viscoelastic transient deformation controlled largely by the viscosity of the suprasubduction mantle wedge. However, our new continuous GPS (cGPS) observations show that since ~ 2010 this westward post-seismic displacement in the vicinity of the Chile triple junction is no longer observable at the surface. This clear change in surface site motions represent dissipation of post-seismic viscoelastic strain to levels that are superseded by elastic strain due to interplate locking of the Nazca/Antarctica-South America in the southern 1960 earthquake rupture zone.