A Field Survey of the 1946 Aleutian Tsunami in the Far Field

• DOI: 10.1785/gssrl.73.4.490
• Publication Date: 2011-03-31T17:02:37Z
• Authors: E. A. Okal; C. E. Synolakis; G. J. Fryer; P. Heinrich; J. C. Borrero; C. Ruscher; D. Arcas; G. Guille; D. Rousseau
• Keywords: 01 natural sciences; 0105 earth and related environmental sciences

This paper reports the interviews of 48 elderly witnesses to the Aleutian tsunami of 1 April 1946, which resulted in the measurement of a database of 54 values of runup and inundation in 31 valleys of the Marquesas, Easter, and Juan Fernandez Islands. The 1946 Aleutian tsunami remains one of the most enigmatic such events of the past century. It resulted in catastrophic destruction both locally (with the annihilation of the Scotch Cap lighthouse on Unimak Island, where runup reached 42 m) and in the far field, where it claimed a total of 162 lives in California, the Marquesas, and Hawaii (where runup reached 16 m) and wrought destruction as far away as Winter Island, Antarctica, 15,000 km from its epicenter (Fuchs, 1982). Yet the parent earthquake featured a relatively low conventional magnitude of only M = 7.4 as assigned at Pasadena by Gutenberg and Richter (1954). Kanamori (1972) defined the 1946 Aleutian shock as a “tsunami earthquake”, a class of events whose tsunamis have much greater amplitude than would be predicted from their seismic magnitudes. Later work by Fukao (1979), Newman and Okal (1998), Pelayo and Wiens (1992), and Polet and Kanamori (2000) has shown that several tsunami earthquakes feature a slow moment release (most probably involving rupture in sedimentary material), which leads to the underestimation of the earthquake's size if measured at the relatively short periods characteristic of conventional magnitudes. In the case of the 1946 Aleutian event, a number of studies have suggested that the seismic moment does indeed increase at longer periods, to values as high as 3.7 × 1028 dyne-cm (Kanamori, 1972) or even 7.6 × 1028 dyne-cm (Okal, 1992). Pelayo (1990) suggested 8.5 × 1028 dyne-cm and speculated that the moment could be even ten times larger. In very general …