April 9, 2009
By Glenn Farley / King 5 News
Northwest Cable News - Portland, OR
SEATTLE – Monday’s 6.3 magnitude earthquake that killed nearly 300 people in Italy serves as a reminder that the Pacific Northwest could suffer the same fate at any moment.
Now, scientists in California have come up with a number of scenarios that show a mega earthquake could cause high raise buildings in Seattle and other parts of the Northwest to collapse.
“Any year, it could happen. Maybe not for hundreds of years, but maybe tomorrow,” said University of Washington seismologist John Vidale.
The last so-called “megathrust” quake to hit the Northwest happened 309 years ago. The Juan de Fuca plate under the Pacific Ocean slipped some 60 feet under the North American plate. It set off a magnitude 9 earthquake that sent tsunamis into the coasts of North America and Japan.
Scientists say megathrust quakes like that happen, on average, every 400 to 500 years. When you think about how long the Earth has been around, that means we’re almost due for another.
Seismologists at San Diego State University, using a supercomputer, have created a virtual earthquake program to show the possible impacts of another megathrust quake in the Pacific Northwest.
The simulated quake spans the 600 mile long Cascadia Subduction Zone. The results show the ground in Seattle moved 1.5 feet per second in Seattle, nearly 6 inches per second in Tacoma, Olympia and Vancouver, B.C. and 3 inches per second in Portland.
Other simulations show that the ground motion, under some conditions, could be twice as big.
“We also found that these high ground velocities were accompanied by significant low-frequency shaking, like what you feel in a roller coaster, that lasted as long as five minutes – and that’s a long time,” said seismologist Kim Olsen.
What makes things worse for cities like Seattle, Tacoma and Olympia is that they sit on top of sediment-filled geological basins that tend to amplify the waves caused by major earthquakes. That means a lot of shaking for a city like Seattle, where more high rise buildings have gone up since the 2001 Nisqually quake – the one that suddenly made the safety of the Alaskan Way Viaduct a regular news story.
Scientists with the Earthquake Engineering Research Laboratory at the California Institute of Technology predict that an 8.1 magnitude quake, under the right circumstances, could cause some Seattle buildings to collapse. In 1985, several smaller buildings survived an 8.1 quake in Mexico City, but a number of high rises there failed.
The frequency of the waves is the key. At high frequency, shorter buildings move more than skyscrapers, but at low frequency, the taller buildings will move more.
Charles Roeder, a professor of civil engineering at UW, is an expert on how buildings handle earthquakes. He's read the report and says Seattle's skyscrapers will probably keep standing.
“The reason simply put is, I think we are designing for quite large seismic forces today. Engineers really do work pretty hard at making it work,” said Roeder.
Even though the study draws a line between those buildings built before 1994, including the Rainier and Columbia Tower and those built afterward, Roeder is still confident.
Olsen says something positive can be taken from these very frightening scenarios.
“Because these events will tend to occur several hundred kilometers from major cities, the study also implies that the region could benefit from an early warning system that can allow time for protective actions before the brunt of the shaking starts,” said Olsen.
Those early warning systems could provide enough to time to automatically stop trains and elevators before the worst of the shaking hits.
Olsen adds that the simulations can help in preparing for tsunamis. A 9.2 magnitude quake of Indonesia caused a massive tsunami that killed over 200,000 people.
http://www.nwcn.com/topstories/stories/NW_040909ENV-megathrust-quake-scenario-TP.bb0412eb.html