California Could
Trigger Major Washington
State Quakes
SEATTLE (AP) -- The land beneath California could spell trouble for Washington.
A scientist said the forces that generate Puget Sound-area earthquakes may originate within the San Andreas Fault, hundreds of kilometres to the south.
That, in turn, could mean much of western Washington state is as prone to big-time quakes as San Francisco Bay or the Los Angeles basin.
Craig Weaver, a University of Washington-based scientist who heads the Seattle U.S. Geological Survey office, presented his research Monday to the Specialty Conference on Geotechnical Earthquake Engineering and Soil Dynamics.
Weaver noted long-held assumptions about quake dangers in the increasingly urbanized Puget Sound region have been shaken repeatedly in recent years.
He said his new model is based largely on studies following the magnitude-7.2 quake that caused the death of more than 6,300 people and billions of dollars wworth of damage in Kobe, Japan, on Jan 17, 1995.
To verify it, he and other scientists plan a detailed series of studies to detect earth-movement and evidence of past quakes from northeastern California to northern Puget Sound.
I.M. Idriss, a geotechnical engineer and civil-engineering professor at the University of California in Davis, and Carlton Ho, an engineering professor at the University of Massachusetts in Amherst, said the theory is plausible but far from proven.
If borne out, Ho said, "that has some fairly significant earthquake implications for Seattle."
The Puget Sound region is susceptible to three types of quakes. The most intense, up to magnitude nine or higher, occur every 300-500 years in the Cascadia Subduction Zone roughly 130 kilometres off the coast from Cape Mendocino, Calif., to Vancouver Island, where the Juan de Fuca plate of the Earth's crust is drawn down, or subducted, beneath the North American continental plate.
At least eight of these quakes have occurred in the last 4,000 years. The last, dated by tree rings, soil deposits and historical accounts of seismic sea waves, occurred Jan. 27, 1700.
The most well-understood quakes occur deeper, where the downward edge of the Juan de Fuca plate bends and cracks under intense pressure and heat about 50 kilometres beneath the Interstate 5 corridor.
That source has generated the two most intense quakes in the region in the last 50 years, one 7.1 near Olympia in 1949 and the other 6.5 south of Renton in 1965.
The least-understood and most recently discovered faults in the area are found within the top 20 kilometres of the Earth's crust.
A quake estimated at magnitude seven altered much of the Puget Sound landscape 1,100 years ago. Jolts of about three to 4.9 beneath Bremerton were felt in June 1997. Both were within the Seattle Fault Zone, a disjointed east-west band about five kilometres wide with three principal sets of fault lines fractured by two unnamed north-south faults beneath the sound.
Weaver's "four-arc slice" model begins with the San Andreas Fault that runs beneath California from near the Mexican border east of the Salton Sea about 1,200 kilometres to Cape Mendocino, Calif., and out to sea.
Movement along that fault, which separates the Pacific and North American plates, pushes the Sierra Nevada mountains northward by almost a centimetre each year.
"The question is what happens to that centimetre," Weaver said.
He said clues may lie in a cluster of quakes in the Klamath Mountains of southern Oregon, the virtual absence of quakes throughout the Oregon Coast Range and a sudden increase north of the Columbia River.
Northward crustal movement at a rate about equal to that of the Sierra Nevadas has been detected at the Columbia but in southwestern British Columbia the bedrock has been in place for about 100 million years.
The northward movement of the Sierra Nevadas, Weaver suggested, pushes and slightly turns the Klamath Mountains, which in turn nudge the Coast Range northward.
At the end of the line, the Puget Sound basin is squished against the older rock formations to the north and buckles, "folding the crust over very long wavelengths" that produce shallow east-west fractures like the Seattle Fault Zone, Weaver said.
If the theory pans out, said Weaver, Ho and Idriss, Seattle may be raised from a zone-three earthquake rating to zone four, the highest risk level and the same as San Francisco and Los Angeles. San Diego, once rated in zone two, is now in zone four.
While those cities have had considerably more frequent, deadly and damaging quakes than Seattle in the last century, that's too short a period in which to draw seismic conclusions, Weaver said.