SGER: Field Survey of Easter Island

University Of Southern California, Los Angeles CA

Investigators

Abstract

This action is to support a field survey, in Easter Island and the Juan Fernandez Islands off Chile, of the 1946 Aleutian tsunami. The April 1,1946 Unimak Island, Alaskan tsunami remains a geophysical mystery: It was generated by a relatively moderate earthquake, yet it produced the highest waves recorded in historical times in Hawaii, where it killed 159 people and caused considerably more damage than the nearby 1957 earthquake (a much larger seismic event), or even the great 1960 Chilean tsunami that was triggered by the largest earthquake ever measured. Seismic information is scant, and the only realistic possibility of understanding this 1946 event appears to be hydrodynamic inversion from inundation data around the Pacific. Criteria for the accuracy of hydrodynamic inversions are not well established, but it appears that the geographic distribution of data is at least as important, if not more so, than the number of data points. Until recently, it was believed that the tsunami had little impact south of Hawaii. However during a field survey of an tsunami in the island of Fatu Hiva in September 1999, the International Tsunami Research Team discovered large rocks that were reportedly moved inland in 1946, at about the time of the Aleutian tsunami. Some of the rocks were found 300m inland. In the summer of 2000, a joint US/France expedition was able to document these findings. The team interviewed eyewitnesses and their accounts helped develop approximate inundation maps for the Marquesas from the April 1,1946 event. This field survey of the Aleutian tsunami in Easter Island and the Juan Fernandez Islands off Chile is very timely, since the location of these islands allows the measurement of free field inundation data - assuming that eyewitnesses can be found. During the Marquesas survey, the ITST interviewed about 40 eyewitnesses on 6 islands (and an additional 10 on 2 of the Society Islands), many quite elderly. Apart from the advanced age of the expected eyewitnesses, there is another compelling reason for performing this survey: preliminary analysis suggests that a landslide was involved in the Aleutian tsunami. If this is the case, then accurate hydrodynamic inversion is urgently needed to identify the landslide source. Inundation maps for Hawaii are currently being re-evaluated, and inundation maps for the Pacific states of the US mainland are being developed for the first time. Tsunamis from the Aleutians need to be carefully reassessed, particularly in view of the landslide trigger potential, so that the new evacuation maps are developed with the best available technology and field knowledge.