The role of the Hikurangi subduction interface in enabling Kaikōura-like earthquakes: Insights from synthetic earthquake catalogues

Camilla Penney, Andrew Howell, Tim McLennan, Andrew Nicol, Bill Fry

A well-known problem in seismic hazard is the short duration of the historical record relative to the time between large earthquakes. This short record means that not all possible earthquakes have been observed, and that the statistics of earthquake recurrence intervals and magnitudes are poorly constrained. These issues are particularly acute for earthquakes involving multiple faults, such as the 2010 El Mayor-Cucapah and 2016 Kaikōura earthquakes. Such earthquakes demonstrate the potentially complex interactions of faults in single earthquakes, contrasting with the typical assumption of characteristic fault ruptures used in seismic hazard assessment. Physics-based earthquake simulators offer one approach for exploring the occurrence of, and controls on, such multi-fault earthquakes. Here we use the physics-based earthquake simulator RSQsim to generate two 450~kyr synthetic earthquake catalogues for central Aotearoa New Zealand, with and without the Hikurangi subduction interface. We improve on previous synthetic earthquake catalogues for Aotearoa New Zealand by implementing a new 3D fault modelling methodology, which is better able to account for along-fault and down-dip variations in fault geometry. We investigate the occurrence of multi-fault earthquakes in our synthetic catalogues with a particular emphasis on the role of the southern part of the Hikurangi subduction interface in these earthquakes. The synthetic catalogues show an increasing proportion of multi-fault earthquakes at higher magnitudes. We find that the subduction interface exerts a significant control on the aspect ratios, rates of occurrence, and faults involved in synthetic multi-fault earthquakes. Whilst our catalogues contain $>$100 complex multi-fault events broadly similar to the 2016 Kaik\=oura earthquake, none rupture exactly the same combination of faults, suggesting that either this fault combination is rare or that key aspects of the controlling physics or fault network are not captured by the earthquake simulator.