Tsunami and earthquake Q&A

  • Herald staff
  • Saturday, March 12, 2011 12:01am
  • Local News

Q: Does the Japan earthquake signal more seismic unrest along the Pacific Ocean’s so-called “Ring of Fire?”

A: Not necessarily, according to a Joan Gomberg, a seismologist with the U.S. Geological Survey in Seattle. Recent earthquakes in Chile, New Zealand and Japan happened in different seismic zones, she said. “As far as we know, they’re isolated incidents,” she said.

Q: Snohomish County is home to major seismic faults. How large an earthquake could happen here?

A: All earthquakes are limited by the size of the fault, Gomberg said. “Something in the high 7s (in magnitude) would be very unlikely” in the Puget Sound area, she said. However, because local faults tend to be shallow, even lesser quakes can cause lots of damage.

The Cascadia subduction zone off the Pacific Coast — where two plates of the Earth’s crust meet — could generate a bigger quake and tsunami. About 200 miles offshore, it stretches from Cape Mendocino in Northern California to Vancouver Island. The last major quake on that fault happened in 1700, and it’s believed to have been about a magnitude 9.

“It appears that the entire subduction zone moved at that time,” Gomberg said.

Q: If there was a large earthquake, would there be a tsunami?

A: For a quake to generate a tsunami, “there has to be a significant amount of vertical motion,” Gomberg said. A large quake offshore likely would generate a tsunami, she said. Because of the distance and geography, the wave would be a greater threat to communities on the coast than in Puget Sound, except for the western shore of Whidbey Island, which is exposed to the Strait of Juan de Fuca.

History shows tsunamis have been generated by quakes around Puget Sound, the most recent less than 200 years ago.

Q: What sort of damage could be expected?

A: One study estimated that an earthquake and tsunami could cause $1.5 billion damage to buildings in Snohomish County. It also could wipe out 38 bridges, including the Highway 532 span connecting Stanwood and Camano Island.

People living and working along the county’s coastline would be at greatest risk, according to the county’s 2010 Natural Hazard Mitigation Plan. Vulnerable areas include Everett, Edmonds, Marysville, Mukilteo and Stanwood.

One computer simulation demonstrated that water from a Puget Sound tsunami could inundate the Snohomish River Valley to a depth of 16 feet.

Q: How prepared are we?

A: Snohomish County has several methods for warning people and the routes to safety. A reverse 911 system, for instance, has been used during recent floods and winter storms.

A regional team of more than 120 firefighters from across the county are trained as rescue technicians. They have special tools, such as equipment to stabilize buildings and find people inside collapsed structures.

Emergency management experts urge people to plan to be on their own for at least 72 hours after a disaster. Go to www.ready.gov or www.whodependsonyou.com for information.

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