T119 Geological Records of Earthquakes and Tsunamis on Passive and Active Margins Coasts

Monday, 5 November 2012
1:30 PM - 5:30 PM
207D

HOLOCENE TSUNAMI RECURRENCE ALONG THE EASTERN ALEUTIAN-ALASKAN MEGATHRUST, CHIRIKOF ISLAND, ALASKA

Paper number: T119-10
Program: Technical Sessions
Time: 3:45 PM – 4:00 PM

Authors

Abstract

Despite its role as the source of some of Earth’s largest historical earthquakes and tsunamis, the prehistory of the Aleutian-Alaska megathrust west of the rupture zone of the 1964 M9.2 earthquake is largely unknown. Tsunami propagation models predict that, during rupture of the megathrust between Kodiak Island and the Shumagin Islands, sea-floor displacements will direct tsunamis to the west coast of the U.S., with the maximum wave energy centered on southern California. Chirikof Island is the closest island to the Aleutian trench along this section of the eastern Aleutian-Alaska megathrust, which broke during great earthquakes in 1788 and 1938. A high tsunami was not observed in 1938, but records from Russian settlements show that a high tsunami shortly followed the 1788 earthquake. Here we report initial findings from the first study of prehistoric tsunami deposits west of Kodiak Island.

Stratigraphy in cores along transects from two boggy tundra sites 1.5 km apart on the southwest coast of Chirikof Island suggests very different numbers and times of high tsunamis. At a site 13-15 m above sea level and 400 m from the sea, two to four of 10 sandy to silty beds in a 5-m-thick sequence of freshwater peat were probably deposited by tsunamis; the oldest deposit dates from 11 ka. The freshwater peat sequence at the lower (7-15 m above sea level) site, 600 m from the sea, contains 20 sandy to silty beds deposited since 3.4 ka; nine of these beds we consider as probable, and another five as possible, tsunami deposits. Although most of the sandy beds have the consistent thicknesses (over distances of 10-270 m), sharp contacts, good sorting, and(or) upward grading typical of tsunami deposits, all samples examined so far contain freshwater diatoms. We hypothesize that tsunamis traveling >1 km inland over low dunes and boggy tundra contained primarily freshwater diatoms when sandy beds were deposited. In progress geochemical analyses (d13C, C/N, Na, Ca, Sr) may help show a marine source for sandy beds. The absence of tidal sediment in any cores from the southwest coast of the island precludes using tidal stratigraphy to estimate land-level changes during past earthquake cycles.