Earthquakes and Seismic Hazard in Southern New Caledonia, Southwest Pacific

We use 12 temporary and 9 permanent broadband seismometers that were operating for similar to 400 days from October 2018 to November 2019 to generate the first published earthquake catalog and local magnitude function for southern New Caledonia (SNC). Local hypocenters mostly have depths <20 km, but east of the New Hebrides-Vanuatu subduction zone they deepen to >100 km. Our local magnitude estimates M-L for 107 earthquakes in the subduction zone are consistently 1.1 units smaller than M-w and mb over a range of M-w from 4.5 to 7.5, as determined by the United States Geological Survey. Our catalog has 460 earthquakes with M-w >= 3.7 in the subduction zone and the largest event in SNC has M-L 3.8. Seismicity rates in SNC are low, but M-L> 5 earthquakes are 2.7 times more frequent than elsewhere in the northern Australian plate. The probability of an M-L> 5 event in 50 years is 0.6 in SNC. The hazard of damaging seismic shaking in SNC is dominated by local moderate-magnitude earthquakes, rather than large-magnitude subduction events. The predicted peak ground acceleration for Noumea at 10% probability of exceedance in 50 years is 0.08 g. Residual analysis of ground accelerations demonstrates that the hazard for Noumea from subduction zone earthquakes is currently overestimated and that new regionally specific ground motion prediction equations (GMPEs) are needed. Our results highlight the inadequacy of current GMPEs in subduction zone footwall settings and the need for additional studies of this type of setting. Plain Language Summary We used seismometers from a local network near Noumea, which is the capital city of New Caledonia, to record earthquakes for 14 months. About 13% of the earthquakes were shallow crustal events (<20 km depth) that clustered near our network. The others clustered similar to 250 km away in the New Hebrides-Vanuatu subduction zone (NHV), where large earthquakes (recorded on global networks) regularly occur. Earthquakes beneath southern New Caledonia (SNC) are smaller and less frequent than in nearby Vanuatu, but they are more hazardous for local communities because they are at shallow depths and nearby. Our work demonstrates the necessity of long-term monitoring of local earthquakes in SNC to assess hazard there. Our analysis shows that current models used to predict shaking from large earthquakes in the NHV over-estimate hazard in New Caledonia, and this may have implications for other regions in similar tectonic settings.