Using ruptures from an earthquake cycle simulator to test geodetic early warning system performance

New Zealand's vulnerability to seismic hazards highlights the need for systems capable of providing earthquake early warning (EEW) or rapid notice of strong shaking. Large offshore earthquakes along the subduction zone east of the North Island could also trigger catastrophic tsunamis, inundating coastal communities in under an hour. While New Zealand operates a robust seismic and geodetic network capable of monitoring moderate-to-large earthquakes, the limited observational record of large earthquakes poses challenges for EEW design and response. This study evaluates magnitude estimation from G-FAST, an early warning algorithm that uses Global Navigation Satellite System (GNSS) data to characterize earthquake sources. We analyze synthetic rupture scenarios along the Hikurangi subduction margin generated by the earthquake simulator RSQSim. For each rupture, GNSS displacements are generated at each site and compared with Peak Ground Displacement (PGD) scaling relationships to test whether they replicate real earthquakes. While we also assess PGD values from rupture scenarios produced with simpler semi-stochastic kinematic modeling, those from RSQSim yield ground motions more consistent with expected values. Given these results, synthetic displacement data from RSQSim ruptures were ingested into G-FAST to evaluate performance for rapid earthquake characterization, finding that PGD-based estimates capture moment magnitude in 90% of cases. This framework demonstrates the utility of synthetic catalogs for testing geodetic EEW performance in characterizing large subduction earthquakes in the North Island region and provides a pathway toward tsunami early warning procedures.