The September 2018, M w 7.5 Sulawesi earthquake occurring on the Palu-Koro strike-slip fault system was followed by an unexpected localized tsunami. We show that direct earthquakeinduced uplift and subsidence could have sourced the observed tsunami within Palu Bay. To this end, we use a physics-based, coupled earthquake-tsunami modeling framework tightly constrained by observations. The model combines rupture dynamics, seismic wave propagation, tsunami propagation and inundation. The earthquake scenario, featuring sustained supershear rupture propagation, matches key observed earthquake characteristics, including the moment magnitude, rupture duration, fault plane solution, teleseismic waveforms and inferred horizontal ground displacements. The remote stress regime reflecting regional transtension applied in the model produces a combination of up to 6 m left-lateral slip and up to 2 m normal slip on the straight fault segment dipping 65 East beneath Palu Bay. The time-dependent, 3D seafloor displacements are translated into bathymetry perturbations with a mean vertical offset of 1.5 m across the submarine fault segment. This sources a tsunami with wave amplitudes and periods that match those measured at the Pantoloan wave gauge and inundation that reproduces observations from field surveys. We conclude that a source related to earthquake displacements is probable and that landsliding may not have been the primary source of the tsunami. These results have important implications for submarine strike-slip fault systems worldwide. Physics-based modeling offers rapid response specifically in tectonic settings that are currently underrepresented in operational tsunami hazard assessment.

We present a high-resolution simulation of the 2004 Sumatra-Andaman earthquake, including non-linear frictional failure on a megathrustsplay fault system. Our method exploits unstructured meshes capturing the complicated geometries in subduction zones that are crucial to understand large earthquakes and tsunami generation. These up-to-date largest and longest dynamic rupture simulations enable analysis of dynamic source effects on the seafloor displacements.To tackle the extreme size of this scenario an end-to-end optimization of the simulation code SeisSol was necessary. We implemented a new cache-aware wave propagation scheme and optimized the dynamic rupture kernels using code generation. We established a novel clustered local-time-stepping scheme for dynamic rupture. In total, we achieved a speed-up of 13.6 compared to the previous implementation. For the Sumatra scenario with 221 million elements this reduced the time-to-solution to 13.9 hours on 86,016 Haswell cores. Furthermore, we used asynchronous output to overlap I/O and compute time.

Megathrust faults host the largest earthquakes on Earth which can trigger cascading hazards such as devastating tsunamis. Determining characteristics that control subduction zone earthquake and tsunami dynamics is critical to mitigate megathrust hazards, but is impeded by structural complexity, large spatio-temporal scales, and scarce or asymmetric instrumental coverage. Here we use high-performance computing multi-physics simulations to show that tsunamigenesis and earthquake dynamics are controlled by along-arc variability in regional tectonic stresses together with depth-dependent variations in rigidity and yield strength of near-fault sediments. We aim to identify dominant regional factors controlling megathrust hazards. To this end, we demonstrate how to unify and verify the required initial conditions for geometrically complex, multi-physics earthquake-tsunami modeling from interdisciplinary geophysical observations. We present large-scale computational models of the 2004 Sumatra-Andaman earthquake and Indian Ocean tsunami that reconcile near-and far-field seismic, geodetic, geological, and tsunami observations and reveal tsunamigenic trade-offs between slip to the trench, splay faulting, and bulk yielding of the accretionary wedge. Our computational capabilities render possible the incorporation of present and emerging high-resolution observations into dynamic-rupture-tsunami models and will be applicable to other large megathrust earthquakes. Our findings highlight the importance of regional-scale structural heterogeneity to decipher megathrust hazards.Variations in megathrust earthquake rupture behaviour are associated with tectonic, mechanical and structural factors highlighting the importance of depth-dependent and along-arc subduction zone heterogeneity (1-5). Large tsunamis may be caused by various co-seismic mechanisms including large slip to the trench, as observed during the 2011 Tohoku earthquake (6) and inferred from

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