SUBDUCTION-ZONE GEOHAZARDS IN THE RUSSIAN FAR EAST – HISTORIC EXAMPLES AND PALEOSEISMIC RECORDS

The Kuril-Kamchatka (K-K) subduction zone of the Russian Far East is a prolific generator of earthquakes and tsunamis. Not only do these events affect adjacent areas, but damaging tsunamis travel across the Pacific. In the central Kurils, the 2006 (M_w 8.3) and 2007 (M_w 8.1) earthquakes produced tsunamis with strongest trans-Pacific effects felt in northern California, Hawaii and Chile, including $20 million in damage in Crescent City, CA. The largest historical event is the 1952 Mw 9 earthquake (southern Kamchatka to northern Kurils) and trans-Pacific tsunami, with ~10 other K-K tsunamigenic great earthquakes (Mw >8) documented since the 19^th century.

Determining the frequency of these events, necessary for probabilistic hazard analysis, is clearly important to a broader region than the Russian Far East. Because the historical record is relatively short and also patchy, full probabilistic studies require the use of geologic records. Our paleoseismicity studies along the K-K subduction zone focus on Holocene coastal-zone records of tsunami deposits and take advantage of a well-developed tephra stratigraphy for chronology. Much of the coast has been affected by earthquake-tsunami events of known scale, with recognizable deposits, and modeling of historical events has improved our understanding of subduction-zone behavior and coastal response. Still, questions remain as to whether we have characterized the largest possible events along the K-K trench, as well as along the plate boundary south of the Komandorsky Islands, and along the Bering Sea shore of Kamchatka.

Another remaining challenge is determining the geographic extent and thus size of individual pre-historic, or unobserved historic, events. Locally, tsunami runup and inundation can be approximated by tsunami-deposit extent, but geographic correlation is problematic. Whereas correlation of ash from individual eruptions and phases of eruptions has advanced significantly by precise geochemical analyses, tsunami deposits are not so distinctive, as illustrated by the 1969/1971 Kamchatka and the 2006/2007 central Kurils tsunami-deposit pairs.