Earthquake source modelling for hazard assessment of the Tonga and Vanuatu subduction zones

    Understanding the complexity of earthquake source parameters, including coseismic slip distribution and rupture dimensions, is essential for local-scale seismic and tsunami hazard assessments. One effective approach is to use earthquake source models generated from synthetic earthquake catalogues via physics-based generators like RSQSim. A key factor influencing the characteristics of a synthetic earthquake catalogue is the tectonic stressing rate, calculated from the slip-deficit rate using a back-slip loading method. The slip-deficit rate can be calculated by integrating the geodetically-inferred convergence rate from Euler Pole rotations with seismic coupling models. Unfortunately, some of the world’s subduction zones have insufficient geodetic data to significantly constrain coupling models. Such is the case with our focus area in the southwest Pacific. To overcome this challenge, we estimate coupling factors on subduction interfaces by adjusting them according to the seismicity rate ratios between the instrumental and synthetic earthquake catalogues of the baseline models. The subduction interfaces are divided into several segments for calculating the seismicity rate ratios along strike. To incorporate sufficient instrumental earthquakes for seismicity rate estimates and to avoid artificial segmentation, we test the segment window lengths and shifting distance. Our new method is applied to the Tonga and Vanuatu subduction zones, which exhibit the highest convergence rates among subduction zones worldwide of approximately 240 mm/year. The coupling factor in this area was poorly defined in previous studies, leading to debate about whether the coupling was weak or strong in each segment. The ideal coupling distribution occurs when adjusted by seismicity rate ratios calculated with a 500 km moving window shifted 50 km along the strike for the Tonga and Vanuatu subduction zones. The results show weak coupling at northern Tonga and strong coupling at northern Vanuatu interfaces. We use this model to develop a synthetic catalogue of finite fault earthquakes spanning ~60,000 years.