Near field survey of the 1946 Aleutian tsunami on unimak and Senak islands

Summarization: Measurements taken on preserved watermarks of the 1946 tsunami on the islands of Unimak and Sanak Islands confirm the exceptional amplitude of runup at Scotch Cap lighthouse, 42 m, and document comparable values over a relatively short stretch of coastline extending Northeastwards 35 km to Cap Lutke. Substantial runup heights reaching 25 m are also found on Long Island, on the Southwestern shore of Sanak. Both the amplitude of the runup and its spatial distribution are characteristic of the near field of a tsunami generated by a landslide, rather than by a dislocation, a conclusion upheld by a preliminary numerical simulation, and also supported by the tentative evidence from elderly fishermen. While this scenario will have to be authentificated by in situ marine geological and geo- physical exploration, our results are in contrast to our previous work in the far field [Okal et al., 2002a] where in particular the strong directivity of the tsunami required generation by a seismic dislocation. Thus, the emerging picture of the source of this most dramatic tsunami is that of a very slow seismic dislocation, sharing the general properties of such "tsunami earthquakes" as -12- Nicaragua (1992) or Java (1994), albeit with a much larger moment probably reaching 8 × 1028 dyn-cm [Okal and Lopez, 2002], which triggered co-seismically a massive landslide at the edge of Davidson Bank. As previously modeled, the dislocation source can explain the generation of the far field tsunami, while the landslide generated the exceptional amplitudes in the near field. The presence of the landslide is not by itself singular: earthquakes routinely trigger landslides, both on land (e.g., Peru, 1970; Chi-chi, Taiwan, 1999; El Salvador, 2001) and at sea, where they can locally enhance the runup of tsunami waves to devastating amplitudes (e.g., 26 m at Riangkroko during the Flores Sea tsunami of 1992 [Imamura et al., 1995]), but only over very limited distances (usually at most a few km). The unique character of the runup along the South- ern shore of Unimak probably reflects an exceptional volume for the landslide, which in the crude model of Figure 6 would total at least 200 km3.Παρουσιάστηκε στο: Bullitin of the Seismological Society of Americ