Seismological Research Letters Volume 83, Number 1 January/February 2012 29 doi: 10.1785/gssrl .83.1.29 Re-examination of the Potential for Great Earthquakes along the Aleutian Island Arc with Implications for Tsunamis in Hawaii Rhett Butler Rhett Butler University of Hawai`i at Manoa INTRODUCTION Hawaii is threatened by tsunamis from circum-Pacifc earth- quakes and has been hit by major tsunamis from great earth- quakes in the prior century in Kamchatka (1952), the Aleutians (1946 and 1957), Chile (1960), and Alaska (1964). Of these, tsunamis generated from great earthquakes along the Alaska- Aleutian Island arc are of particular concern due to their short (4.5–hour) propagation time to Hawaii, which presents the most limited time for coastal evacuation. Historically, the Aleutian earthquakes of 1946 and 1957 produced the larg- est tele-tsunamis observed in Hawaii (Lander and Lockridge 1989). Te 1946 event (Mw 8.6) killed 159 people with waves reaching up to 16 m height on Molokai. Even though the 1957 event (Mw 8.6) caused no deaths in Hawaii, a maximum run- up of 16 m was observed on Kauai. Although these two great earthquakes in the Aleutians had substantial tsunami impacts in Hawaii, they are not among the largest seismic events to have occurred globally in the last century. To evaluate the extent of the tsunami threat to Hawaii from the Aleutian Islands, we need to consider the potential for even larger earthquakes in light of the analysis of more than a century of data measuring the Aleutian seismic zone. Te motivation for this review is the great Tohoku earth- quake of 2011, which occurred in the active seismic region of the coast of East Honshu, Japan, and caused a deadly tsunami in Japan that was felt on shorelines across the Pacifc. Tis mag- nitude Mw 9.0 event surprised the seismological community with respect to its unexpectedly large magnitude and the size of the generated tsunami ( e.g. , Normile 2011; Monastersky 2011; Showstack 2011). Since the great Sanriku earthquake of 1896 there have been nine damaging earthquakes (magnitude M 7.3 to about 8.6) in the area of the Tohoku earthquake rupture zone, including tsunamigenic events in 1896, 1897, 1915, 1933, 1936, 1938, and 1978 (U.S. Geological Survey and Japan Headquarters for Earthquake Research Promotion). Although the Tohoku tsunami caused comparatively minor coastal and harbor damage in Hawaii, both the unexpected size of the earthquake and its occurrence in a region of prior large seismicity warrant the need to reevaluate the largest tsunamigenic earthquakes that are pos- sible in the Aleutian Islands, which may impact Hawaii. Tsunami energy is directed primarily perpendicular to the strike of the earthquake fault ( e.g., Ward 1982). For the Aleutian-Alaskan tsunamigenic earthquakes of the last century, tsunami energy is projected radially to the arc and trench axis (Figure 1). Te tsunamis generated in the central part of the arc are directed at Hawaii. Te average of the tsu- nami maximum run-ups observed at about 40 locations on the northern coasts of the Hawaiian Islands were 8.2 m and 5.3 m for the 1946 and 1957 earthquakes, respectively (Walker 1994). Terefore, tsunamis from the regions near and between these events are a special concern to Hawaii. Tsunamigenesis from earthquakes in the westernmost Aleutians and eastward from the Alaska Peninsula has had a lesser impact on Hawaii. Although the great Alaska 1964 earthquake (Mw 9.2) was a much larger earthquake than the 1946 and 1957 events, a smaller tsunami was recorded owing to the more favorable ori- entation of the earthquake fault with respect to Hawaii, where the average of the tsunami maximum run-ups observed was 2.3 m (Walker 1994). Te large 1965 earthquake (Mw 8.7) that ruptured westward from the 1957 event had tsunami average maximum run-ups of 0.4 m in the Hawaiian Islands. Te 1938 earthquake generated a smaller, 0.2 m average tsunami run-up in the Islands. Both the earthquake size and its location along the arc are key factors for tsunami inundation in Hawaii. For the 1965 earthquake, the obliquity of convergence between the Pacifc and the North American tectonic plates also produces a signifcant strike-slip component in the megathrust fault slip that does not substantially contribute to tsunamigenesis. In reexamining the tsunamigenic potential for large earth- quakes along the Aleutian Islands, I begin with a review of the scientifc literature, tabulating information on source moment, fault rupture area, and average fault displacement for all earth- quakes with instrumental data since 1900 with magnitude 7.8 and greater. Te fault slip beneath the sea foor relates most directly to tsunami generation. To judge the maximum tsuna- migenic potential for Aleutian arc segments, I have used the two largest earthquakes of the last century to scale upward the average fault displacements and seismic moments observed for the Aleutians. Te great 1964 Alaska earthquake (Mw 9.2) occurred east of the Aleutian arc along the same megathrust subduction zone. Wesson et al. (1999) considered the 1964