Simulating the 1946 Aleutian far-field tsunami: The successful dislocation and the impossible landslide

H. Hébert and E.A. Okal

We present a final set of hydrodynamic simulations of the run-up of the 1946 Aleutian tsunami at the transpacific locations obtained during the field surveys taken in 1999-2001 by Okal et al. [2002]. As a source of the tsunami, we use both (i) a dislocation model based on the updated seismological study of López and Okal [2002], which features slow bilateral rupture along a 200-km long fault zone; and (ii) the asymmetric dipolar source successfully used by Okal et al. [2003] to model the near-field run-up surveyed at Unimak Island. The simulations are carried out on a series of grids featuring fine scales both in the source area, and at the receiving shores (the latter up to a final scale of 50 m), but a coarser grid on the high seas. In general, the dislocation source does fit the run-up observations in the far-field, and in particular it reproduces the strong azimuthal directivity expected in the source geometry. This extends the ressults of Titov et al. [2001], who had modeled the inundation at Hilo on the basis of a similar dislocative source, but using a different numerical method. By contrast, the dipolar source produces absolute values of run-up ranging only from a few tens of cm to 3 m on the shorelines of the Marquesas and Juan Fernández Islands, in all cases significantly less (by a factor of 3 to 5) than observed. We conclude that the near- and far-fields cannot be both modeled by a single source. The final model of the source of the 1946 Aleutian tsunami must be a composite betweem a very large, but very slow earthquake, responsible for the far field tsunami, and a landslide generating the devastating near-field tsunami.