Interseismic, coseismic, postseismic, and slow slip event deformation above a shallow subduction thrust in the western Solomon Islands

Abstract:

The western Solomon Islands are a remarkable natural laboratory to investigate processes occurring on the shallowest (<10 km depth) portions of the subduction interface. Islands within the New Georgia Group are located <15 km from the San Cristobal Trench, with the subduction thrust located only a few km beneath the southwest coast of islands like Rannonga and Rendova. This offers a globally unique opportunity to use GPS and other land-based methods to monitor deformation processes very close to the trench at a subduction zone. We present results from a campaign GPS network in the western Solomons that has been operated from 1996-present. The data from 1996-2002 indicate interseismic coupling on the shallow portion of the interface, at a rate of nearly 100% of the relative plate motion. Coupling does not appear to extend deeper than ~20 km depth, and the relatively shallow down-dip limit of coupling is consistent with subduction of young (<6 Ma) oceanic crust of the Woodlark Basin. We also show evidence for a slow slip event in late 2000, observed at a GPS site near Gizo that was running continuously from 1999-2002. In April 2007, an Mw 8.1 earthquake occurred on the subduction thrust beneath the network, resulting in large coseismic displacements at nearby campaign GPS sites. The earthquake caused widespread coastal uplift and subsidence in the region, as revealed by studies of coral microatolls following the earthquake (Taylor et al., 2008). We invert displacements of the GPS sites jointly with vertical displacements of coral microatolls to evaluate the coseismic slip during the earthquake. The area of the interface that underwent slip in the earthquake matches well with the region that was interseismically coupled just prior to the 2007 earthquake. The data also require large coseismic slip on the shallow interface near the trench, which likely contributed to the generation of a large, damaging tsunami following the earthquake. We also show results from a recent re-measurement (April 2015) of the network to infer the distribution of crustal deformation during the 8 years following the 2007 earthquake.