HR: 0800h
AN: T11A-0355    [Abstracts]
TI: Great Earthquakes, Gigantic Landslides, and the Continuing Enigma of the April Fool's Tsunami of 1946
AU: * Fryer, G J
EM: gerard@hawaii.edu
AF: University of Hawaii at Manoa, School of Ocean & Earth Science & Technology, 1680 East-West Rd, Honolulu, HI 96822-2327 United States
AU: Tryon, M D
EM: mtryon@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gillman Dr, 0244, La Jolla, CA 92093-0244 United States
AB: Paleotsunami studies can extend the record of great earthquakes back into prehistory, but what if the historical record itself is ambiguous? There is growing controversy about whether great earthquakes really occur along the Shumagin and Unimak segments of the Alaska-Aleutian system. The last great tsunami there was April 1, 1946, initiated by an earthquake whose magnitude has variously been reported from 7.1 to 8.5. Okal et al (BSSA, 2003) surveyed the near-field runup and concluded there were two sources: a magnitude 8.5 earthquake, which generated a Pacific-wide tsunami but which produced near-field runups no more than 18 m, and an earthquake-triggered slump whose tsunami reached 42 m at Scotch Cap Light near the western end of Unimak Island, but with runup rapidly decaying eastwards. An M8.5 earthquake, however, is incompatible with GPS strain measurements, which indicate that the maximum earthquake size off Unimak is M7.5. We have long contended that near- and far-field tsunamis were the result of a single earthquake-triggered debris avalanche down the Aleutian slope. In 2004 we were part of an expedition to map and explore the landslide, whose location seemed to be very tightly constrained by the known tsunami travel time to Scotch Cap Light. We found that neither our giant landslide nor Okal et al's smaller slump exist within 100 km of the presumed location. The explanation is obvious in retrospect: the tsunami was so large that it crossed the shallow Aleutian shelf as a bore travelling faster than the theoretical long-wave speed (which we had used to fix the location). Any landslide could only have occurred in an unsurveyed area farther east, off Unimak Bight, the central coast of Unimak Island. That location, however, conflicts with Okal et al's measurements of smaller runup along the Bight. We are now convinced that Okal et al confused the 1946 debris line with the lower line left by the 1957 tsunami. They were apparently unaware that the 1946 tsunami also destroyed an unmanned light on the rocky promontary of Cape Pankof, the eastern tip of Unimak Island. From photographs and pre-1946 charts, we have determined that the 1946 runup at Cape Pankof must have exceeded 25 m, a value that is incompatible with Okal et al's field measurements. We cannot avoid the conclusion that both near-field and far-field tsunamis had the same source. If the source was a great M8.5 earthquake, then the geodetic measurements from one earthquake cycle cannot be applied to another. In light of the Indian Ocean Tsunami, however, the huge runup at Scotch Cap can no longer be regarded as a problem for an earthquake source, since runups in Sumatra last December were also far larger than expected. If, on the other hand, the source was a debris avalanche, it can only have occurred in the unsurveyed region. To satisfy the timing constraints, the tsunami had to be so large that the leading bore crossed the shelf with a Froude number of at least 2. The only way to make such a large tsunami is with an immense debris avalanche of over 200 km³. Such an avalanche is quite capable of explaining the far-field observations. To solve the enigma we shall have to complete the mapping of the upper Aleutian forearc, resurvey the near-field runup knowing that the area was also affected by the 1957 tsunami, and run push-core transects across the lowlands of Unimak Bight.
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 4564 Tsunamis and storm surges
DE: 7221 Paleoseismology (8036)
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Tectonophysics [T]
MN: Fall Meeting 2005