Low-Felt Intensity of Earthquakes in the Footwall of the Vanuatu Subduction Zone, New Caledonia, Southwest Pacific

Peak ground acceleration (PGA) data from accelerometers and broadband instruments in two networks in New Caledonia and one in Vanuatu are compared with ground-motion prediction equations (GMPEs). New Caledonia is located on the outer rise of the downgoing Australian plate, which is a rare location for land and islands, and the Vanuatu volcanic arc is on the overriding Pacific plate. Deep subduction events (>50 km depth) recorded in Grande Terre (the main island of New Caledonia) fit GMPE predictions better than at Loyalty Islands, which are closer to the subduction zone, whereas shallow events have anomalously low PGAs in both regions. Source effects are ruled out because PGAs at a station on the overriding plate fit GMPE predictions. Mapped geology and horizontal-to-vertical spectral ratios indicate site amplification at some places in Grande Terre and the Isle of Pines that explains part of the observed variability. Different distance-related regional attenuation for shallow and deep earthquakes is observed, but a localized effect (barrier to energy) beneath the shallow (<50 km) outer slope of the subduction trench is required to explain our observations. We propose that a shallow barrier of fractured hydrated lithosphere is responsible for reducing ground shaking from shallow earthquakes, and this may be a common phenomenon in other subduction zones. We suspect there are local barriers beneath Grande Terre, but we do not have sufficient data to evaluate the effect. A GMPE for subduction outer rise settings is required, or some correction terms need to be considered when using existing GMPEs to evaluate ground motions for such tectonic regions. Seismic hazard in New Caledonia is lower than predicted by GMPEs, but local magnitudes of shallow earthquakes and hence real-time tsunami hazard may be underestimated using local observations unless this shallow trench path effect is accounted for.