Tsunami hazard in Cascadia from M7-9 earthquakes: Most hazardous segments of the rupture

Cascadia tsunamis have been the primary subject of studies on tsunami scenarios along the United States west coast. However, the geographic extent as well as the final size of potential future ruptures in Cascadia are poorly known. This has caused the result of previous studies to remain mostly hypothetical and simply serve as either "reasonable" or "worst-case" scenarios.

In this study, we calculate the hazard of M7-9 earthquakes using more realistic models that systematically vary both the geographic extent and slip of the rupture. To achieve this goal, we use rupture simulations derived from locking models to provide estimates of coseismic deformation at the ocean floor, and design rupture scenarios with variable hypocenters and rupture propagation. We then apply shallow water approximation to simulate full tsunami waveforms and generate tsunami amplitude profiles along the Cascadia coastline. By varying the seismic moment thresholds of the rupture models, we find that regional maximum coastal amplitudes are not unique for a given rupture size. Numerical experiments show a heightened hazard in central Cascadia (Oregon) mostly due to the peculiar concave coastal morphology. In fact, our simulations reveal that beyond a magnitude of M_w≈8.5, increasing the rupture size will not significantly vary the tsunami hazard, especially in southern Cascadia, with the central segments playing the most crucial role. We also find that the distribution of possible coastal tsunami amplitudes in the US west coast from large Cascadia ruptures is largely insensitive to the choice of slip model. These results have significant implications in identifying the main sources of tsunami hazard along the US west coast, especially as a single worst-case rupture scenario does not uniquely correspond to the worst-case tsunami scenario at a given location.