May 3, 1999
Geologists review history of huge Cascadia earthquakes
Major Puget Sound-area earthquakes in 1949 and 1965 are but a dim memory for most people who lived through them. More recent temblors in the Pacific Northwest, though not as severe, have raised public awareness of earthquake dangers.
But geological records going back thousands of years imply an even greater hazard in the Cascadia subduction zone than is reflected in 200 years of written history, says Brian Atwater, a University of Washington affiliate professor in geological sciences and a U.S. Geological Survey geologist.
From geological evidence, scientists have learned that in 1700 a large earthquake ruptured much of the length of the Cascadia subduction zone, the area off the coast of Oregon, Washington and British Columbia where the Juan de Fuca tectonic plate dives beneath the North America plate. There remains uncertainty about the size of the quake, the length and severity of ground motion, how far the ground sank in certain areas and the extent of the resulting tsunami, Atwater says.
The 1700 earthquake is just the most recent of seven great earthquakes that struck southwestern Washington in the past 3,500 years. The events are too few and recurrences too irregular to forecast when the next great quake might hit, he says. Geological records of great earthquakes from elsewhere in the Cascade region suggest irregular recurrence intervals averaging roughly 500 years, Atwater says. One of the longest of those records, from a lake in southern Oregon, spans 7,000 years.
“Nature is capable of more than it has shown in the region’s 200 years of written history,” says Atwater, an author of a paper about prehistoric earthquakes in Cascadia. The paper, which summarizes research by numerous scientists and discusses recent progress in the field, was presented today at the Seismological Society of America’s annual meeting in Seattle.
Atwater notes that sediments and other geological records indicate that western Washington was a hotbed of geological activity about 1,100 years ago. Between A.D. 900 and 930, an earthquake on the Seattle fault caused about 23 feet of uplift between Seattle and Bremerton, landslides into Lake Washington and a tsunami in Puget Sound.
Within a century or two of that quake:
*There was a tsunami and additional uplift northwest of Tacoma, which changed a tidal mudflat into an alder swamp.
*Land subsided and sand liquefied near Olympia.
*Faults broke to the surface and rockslides dammed streams in the eastern Olympic Mountains.
*Mount Rainier erupted explosively and its volcanic sand clogged Seattle’s Duwamish River.
*Sandy water erupted from a depth of more than 100 feet near Copalis Beach.
*The Pacific coast dropped, perhaps 2 to 6 feet, at Neah Bay, Grays Harbor, Willapa Bay and the lower Columbia River.
Seismic activity in the Northwest can affect people living thousands of miles away, Atwater says. Surviving documents, recently translated into modern Japanese and published last year, show a tsunami from the 1700 Cascadia earthquake damaged four towns on the Pacific coast of Honshu, Japan’s largest island. At Miyako, in northern Honshu, it destroyed 13 houses, set off a fire that burned 20 more houses and prompted authorities to issue government rice to 159 people.
For more information, contact Atwater at email@example.com