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Joint Meeting Pacific Section, AAPG & Cordilleran Section GSA April 29–May 1, 2005, San José, California

Expected and Unexpected Tsunamis Around the Pacific Rim: What Tsunami Deposits Tell Us

Joanne Bourgeois
Univ Washington, PO Box 351310, Seattle, WA 98195-1310, [email protected]

Within the historical period, the entire Pacific Rim has experienced tsunamis. Magnitude-9 subduction-zone earthquakes and their tsunamis (as large as or larger than the 26 December 04 Sumatran case) have most recently struck the Pacific in the period from 1952 to 1964 (Kamchatka, Aleutians, Chile, Alaska), producing both near-field and far-field tsunamis, and leaving tsunami deposits. Smaller but locally damaging subduction-zone earthquakes and tsunamis typically occur every few years. Moreover, historical tsunamis have been generated not only by subduction-zone ruptures, but also by other fault-zone displacements, by landslides into or under the sea, and by volcanic processes such as pyroclastic flows and eruptive explosions.

Some subduction zones and other major faults are not known historically to have produced significant large tsunamis (at least several meters runup), but Holocene stratigraphic evidence indicates their tsunamigenic potential. The best-documented case is based on buried soils and tsunami deposits along the coast of Cascadia. Also, even on coastlines known historically to be tsunamigenic, such as Hokkaido and northern Kamchatka, tsunami deposits have helped geologists recognize that these coasts have experienced larger tsunamis in pre-history than in known history.

For many parts of the Pacific Rim and beyond, events that generate large tsunamis are uncommon enough that the historical record does not permit estimates of recurrence intervals or generation of reasonable probabilistic hazard maps. The study of tsunami deposits is a major means by which such calculations and maps can be produced. For rare events such as island sector collapse or asteroid impact, risk is small, and probably not quantifiable, but the potential effects are spectacular. How should the scientific community constructively use current heightened awareness of tsunamis, without fear-mongering?

Posted with permission of The Geological Society of America; abstract also online ( © Copyright 2005 The Geological Society of America (GSA).