Modelling splay fault rupture and tsunamis with self-consistent initial conditions from a geodynamic seismic cycle model of subduction

Abstract:

Observations of accretionary wedges show multiple splay faults which pose a significant tsunami hazard, but it is not yet known if they could rupture during a single earthquake or how that would affect the ensuing tsunami. Modelling dynamic splay fault rupture is complicated because of the lack of constraints on self-consistent initial conditions on both the splay and the megathrust in dynamic rupture models.

Here, we build on a two-dimensional modelling framework first presented in Van Zelst et al. (2019) that considers the different temporal and spatial scales of the geodynamics of subduction, seismic cycles, and dynamic ruptures. A geodynamic seismic cycle model provides the dynamic rupture model with self-consistent initial stress and strength conditions, material properties, and fault geometries for the megathrust and six blind splay faults. Because of the self-consistency of the input, we can study if and when multiple splay faults rupture. The resulting surface displacements of the dynamic rupture model then serve as input for a tsunami propagation and inundation model.

We find that all six splay faults are activated by different mechanisms such as slip on the megathrust and stress changes induced by seismic waves. Splay fault rupture results in distinct peaks in the vertical surface displacements with a smaller wavelength and larger amplitudes. The ensuing tsunami has one high-amplitude crest related to rupture on the longest splay fault closest to the coast and a second, broader wave packet resulting from failure on the other faults. At the coast, this results in two episodes of flooding and a larger run-up length compared to the tsunami sourced by an earthquake without splay fault rupture.

Our results suggest that larger-than-expected tsunamis could be attributed to rupture on large splay faults. However, it is difficult to pinpoint how many splay faults ruptured based on tsunami observations alone. It is therefore important to understand splay fault activation and to consider it in hazard assessment. Combining geodynamic models of subduction and dynamic rupture models gives insight into the interaction of long-term tectonic stresses and earthquake stress release and ultimately contributes to our understanding of subduction zones and seismic and tsunami hazard.