Earthquakes, seismic hazard, and lithospheric structures beneath an ophiolite nappe, southern New Caledonia, southwest Pacific

This thesis presents the first comprehensive seismological study of southern Grande Terre, New Caledonia, southwest Pacific. Grande Terre emerges from Norfolk Ridge, which is part of northeastern Zealandia. The study region contains ophiolite nappes that were emplaced during Eocene initiation of the Tonga-Kermadec subduction system. The structure of Grande Terre, imaged by this study, provides insights into ophiolite emplacement and subduction initiation processes. Grande Terre has been isolated from tectonics since the Oligocene but currently sits in the outer rise of the Vanuatu subduction zone, providing an unusual opportunity to study the footwall of an active subduction zone. The seismic waveform data used in this study were acquired in October 2018–November 2019 during a scientific project called “Investigating the Thickness of the Peridotite Nappe of New Caledonia” (ITOPNC). This is a collaborative project between Victoria University of Wellington (VUW) in New Zealand, Service Géologique de Nouvelle-Calédonie (SGNC), University of New Caledonia, and Institut pour la Recherche et le Développement (IRD) Centre de Nouméa in New Caledonia. Twelve temporary stations were deployed to record ground motions for fourteen months. Seismic data from nine permanent stations in New Caledonia were also used in this thesis.

Studies presented in this thesis address earthquake distribution, stress state, earthquake shaking hazard, earthquake triggering, and crust-mantle structures in southern New Caledonia. Most earthquakes in southern Grande Terre are shallow (<20 km depth) and have small magnitudes (ML<3), whereas those in the nearby Vanuatu subduction zone deepen to >100 km and the largest earthquake recorded in this study was Mw 7.5 in the subduction zone on 5 December 2018. The biggest local earthquake in southern Grande Terre had ML 3.8, and occurred 15 minutes after the Mw 7.5 event. Southern Grande Terre is under a tensile stress regime with a trench-normal minimum horizontal stress, which is controlled by the adjacent Vanuatu subduction zone and topographic stresses. Earthquake shaking hazard at high peak ground accelerations (>0.1 g) in southern Grande Terre is affected more by local earthquakes than those in the Vanuatu subduction zone, because flexure results in higher rates of local seismicity than in the stable Australian plate, and peak ground accelerations from subduction zone earthquakes are anomalously lower than standard ground motion predictions. The discrepancies in observed and predicted ground shaking from subduction zone earthquakes are ~10 times different, possibly related to the deep geological structure between the subduction zone and Grande Terre. Local earthquake hazard is time-varying. The local seismicity rate strongly correlates with occurrence of Mw > 7 earthquakes in the Vanuatu subduction zone. Earthquake shaking hazard in southern Grande Terre increases by a factor of 4 immediately after a Mw > 7 earthquake nearby (200–350 km away), as a result of triggered local seismicity, and returns to background levels by Omori decay in 20–30 days.

Rayleigh wave group velocities determined using ambient noise analysis reveal that the ophiolite thickness in southern Grande Terre is ~1 km over much of its width but may reach nearly 2 km thick beneath the east coast. The ophiolite lies on a WNW-ESE oriented high velocity body (Vp 5.5–6.0 km/s, Vs 3.0–3.5 km/s) located in the upper crust beneath the centre of southern Grande Terre, with lower-velocity materials beneath the east and west coasts that are interpreted as sedimentary basins.

Receiver function analysis shows variable crustal thickness beneath southern Grande Terre: thin (25–30 km) continental crust on the west has a subhorizontal Moho; and this transitions via a west-dipping structure to oceanic crust towards the east (~10 km thick). Moho depth is confirmed by a 1D velocity model derived from local earthquake travel times that has Vp 7.4 km/s at 27–31 km depth and 7.8–8.1 km/s at 31–40 km depth. Beneath the Moho, a west-dipping structure is imaged to 65 km depth and interpreted as a fossil subduction system beneath a partially-serpentinized mantle wedge. Our results confirm the hypothesis that the slab was west-dipping beneath New Caledonia during subduction initiation and ophiolite emplacement.