American & Japanese Clues to the 1700 Cascadia Earthquake

May 21 (Tuesday), 6 p.m., Teton Co. Library Auditorium– Open to Public. Presentation: American & Japanese clues to the 1700 Cascadia earthquake”, Presented by Brian Atwater, US Geological Survey, University of Washington.

The Geologic hazards in the United States’ Pacific northwest typically thought of are volcanoes, a repeat perhaps of the Mt. St. Helens eruption of 1980. But a much more significant hazard is that of a major earthquake and resultant tsunami. Brian Atwater’s talk will discuss the unraveling of the relatively recent discovery of this hazard and its implications for the states of Washington, Oregon, and northern California.

Not until late in the twentieth century did modern science recognize earthquake and tsunami hazards from North America’s largest active fault outside Alaska. This fault runs along and beneath the Pacific coast in the Cascadia region from southwest British Columbia to northwest California. Its very existence went unrecognized through the 1960s, and its earthquake and tsunami history received little notice before the late 1980s. Only then did geologists—tipped off by geophysicists and prompted also by the Nuclear Regulatory Commission—began identifying widespread signs of recurrent ecological catastrophes akin to an effect of the great 1964 Alaska earthquake: abrupt lowering of coastal marshes and forests into tidal water.

Atop buried marsh and forest soils geologists found sheets of sand from tsunamis generated while the land was dropping. The most recent of these inferred earthquakes was soon dated by radiocarbon methods to the decades between 1680 and 1720—a time window later narrowed, by tree-ring methods, to the dormant months between the 1699 and 1700 growing seasons. A Pacific Ocean tsunami noted in Japan gives this Cascadia earthquake an exact date of January 26, 1700 and an approximate size of magnitude 9. These and other detective stories underpin current efforts to reduce losses of life and property during the next great earthquake along the Cascadia subduction zone.