Impact of Fault Geometry in dynamic modeling simulations: The case of the 2016 Mw7.8 Kaikoura.

The 2016 Mw7.8 Kaikoura earthquake presents one of most challenging natural events to model dynamically, with up to 21 faults involved in the full rupture, according to geological measurements of surface rupture ( e.g. Litchfield et al. 2018). However, most studies using static displacement observations do not resolve individual fault activation and their temporal connectivity at some parts of the fault range (e.g. Hamling et al. 2017), as suggested by the near-source strong motion data (REF). A more recent complete aftershock catalog provides improved seismological constraints on the rupture kinematics, offering new insights into the fault geometry and faulting mechanisms (Chamberlain et al. 2021). These advances motivate a re‑examination of the mysterious multi-fault rupture with complete seismological observation and physics-based dynamic rupture modeling for to better understand the governing mechanisms of multi-fault ruptures.

Compared to kinematic source inversions, dynamic modeling is a powerful numerical tool to compute realistic cases of earthquake occurrence due to complex ruptures. Yet, for earthquakes involving multiple interacting faults, even state-of-the-art dynamic models can lead to fundamentally different physical interpretations. On one hand, the corresponding dynamic modeling setup heavily depends on prior knowledge of the full system geometry, as well as on the stress-state and velocity model of the medium. On the other hand, due to the nonlinear nature of the problem, several models can produce similar results. Results show that for relatively simple ruptures, involving one or two fault planes, the solution is stable. However, when the rupture involves several faults, even minor changes to the dynamic setup result in instability and non-uniqueness of the solution.

To gain insight into how such extreme fault complexity controls rupture evolution, we perform the dynamic modeling of the 2016 Mw7.8 Kaikoura earthquake using the open-access SeisSol package. We use the New Zealand 3D velocity model and compare two different geometries. The first geometry uses the NZ Community Fault Model, while the second is based on a previously published rupture model (Ando & Kaneko 2018). For the first geometry, we analyze whether the rupture actually used secondary faults to continue its path, or if subsequent rupture was triggered by the generated wavefield. For the second geometry, we investigate the impact of rupture bifurcation onto two faults and assess whether this process generates identifiable seismic phases in the wavefield.

We analyze both dynamic scenarios using near-field and regional strong-motion records, which are expected to capture hidden features of the rupture. We further compare the simulated rupture evolution with previously published high-resolution earthquake catalogs to identify rupture patterns and evaluate potential changes in the stress field before and after the event. Our results highlight both the strengths and inherent ambiguities of dynamic rupture modeling for complex multi-fault earthquakes and provide new constraints on the physical processes governing the Kaikoura rupture.