Regional W-phase solutions for past large-magnitude events in the Southwest Pacific

Rapid earthquake characterisation is paramount for Tsunami Early Warning (TEW). It provides earthquake source information used as input for tsunami scenario simulations. In operational early warning, critical source information includes location and magnitude. In Aotearoa New Zealand (NZ), W-phase moment tensor analysis underpins operational tsunami forecasts. W-phase inversions can take up to 30 min to obtain stable solutions. Our work explores regional W-phase inversion to reduce the time required to retrieve a stable magnitude, location, and moment tensor. Unlike simpler automated magnitude determinations routinely used to analyse earthquakes, the W phase does not saturate with magnitude, making it better suited to quantifying the moment magnitude for the largest earthquakes with the most tsunamigenic potential. It also provides the centroid as well as a moment tensor, yielding a better understanding of the tsunami energy source. In this study, we demonstrate the use of regional W-phase inversions to speed up TEW in NZ and the Southwest Pacific. We find that, with current seismic networks, reliable solutions can be obtained within 18 min. Moreover, we show that the magnitude estimation is highly robust and can be estimated with confidence within 10 min after earthquake occurrence. This work presents a W-phase solution for 12 past regional earthquakes with $${M}_{w}\ge 6.9$$ . The regional context leads to specific processing and validation. We assess the evolution of solutions with increasing maximum epicentral distances (i.e., analysing increasingly distant stations). We also consider regional Green’s functions (GFs) in the W-phase inversion. Their use does not significantly improve the final solutions in the operational context, even though there is typically an improvement of inversion misfit. We highlight the impact of spatially clustered stations on the W-phase solutions, showing that a restricted initial list of stations improves our results.