PORT TOWNSEND — The South Whidbey Island fault line could generate a 7.0-to-7.5-magnitude earthquake that would rock most of the North Olympic Peninsula, a state geologist said after a new survey.
Such as quake would hit especially hard Port Townsend’s downtown historic district, which is built on fill dirt. It would rattle to a lesser extent Port Angeles’ fill-dirt downtown.
“Port Townsend would be seriously affected,” said Tim Walsh, state Department of Natural Resources chief hazards geologist. “Port Angeles could be damaged as well.
“The intensity would be large. It would depend on exactly where on the fault it would happen. It radiates energy from where it nucleates.
“That would have a lot of influence.”
He had no predictions about when, or if, a quake would occur along the fault.
Walsh cites growing recent evidence that connects the South Whidbey fault to the Rattlesnake Mountain fault zone, linking the Whidbey fault to North Bend.
“What this means is, it could make a bigger earthquake because it’s a longer fault than we used to think,” he said.
Newly surveyed fault
State geologists recently published maps showing the newly surveyed fault, which is capable of producing earthquakes as great as 7.5 in magnitude, Walsh said.
A fault capable of producing an earthquake registering a maximum magnitude 7.5 is capable of causing serious damage over large areas.
Such a shaker could hammer Everett, Seattle and Victoria as well as the Peninsula.
“A number of faults go into the water and converge in Victoria,” Walsh said.
Tsunami danger
Walsh said a tsunami would likely result from such a quake, especially since the fault line is broad and runs as close as a half-mile north of Point Wilson at Fort Worden State Park, where the northern Quimper Peninsula tip meets the Strait of Juan de Fuca.
While geologists have confirmed the power of a potential quake along the South Whidbey fault, the size of the tidal wave that would follow a major quake has not been determined for the Quimper Peninsula and shorelines west along the Strait.
“It probably would make a tsunami, but we have not done the modeling to say how big the tsunami would be,” Walsh said.
The Rattlesnake Mountain fault zone was discovered in 1984, Walsh said, but it was not then known that it extended to the west and connected to the South Whidbey Island fault, which is just 12 miles and considered shallow.
The South Whidbey fault also could be connected to offshore faults, and the Seattle Fault, which is found mostly under Seattle, is now thought to hook around the south end of the Olympic Mountains and extend out to the coast, he added.
More detailed mapping of the fault has taken place over the past three years, funded by the U.S. Geological Survey and DNR.
‘Shake map’
A “shake map” produced by USGS in April paints a scientific scenario of a 7.0 to 8.0-magnitude quake’s shock wave from the fault across the Peninsula:
• Closest to the epicenter — most of Whidbey Island, including Coupeville, Freeland and Cllinton — some well-built wooden structures would be destroyed, while most masonry and frame structures with foundations would be destroyed.
• Severe to moderately heavy damage would occur on the Quimper Peninsula, including Port Townsend, Marrowstone and Indian islands.
• The remainder of East Jefferson County — south to Brinnon, Quilcene and Coyle — and the Miller Peninsula — which includes Diamond Point and Gardiner, as well as the Dungeness Valley in Clallam County would experience negligible damage to well-built and designed buildings, with slight to moderate damage to well-built ordinary structures.
Poorly built or badly designed structures would see severe damage with some chimneys cracked and falling. Heavy furniture would be overturned in this scenario.
• All would feel and be frightened by the quake in communities further inland from the Strait of Juan de Fuca, such as Blyn, Sequim, Port Angeles.
Some heavy furniture would move and there would be a few instances of fallen plaster. Overall damage would be slight.
• West of Port Angeles to Neah Bay, Clallam Bay and Forks, the quake would be felt indoors by many during the day, awakening some at night. Some dishes, windows would be broken or shaken and unstable objects would be overturned. Pendulum clocks might stop.
Damage might be considerably more to homes along West End rivers, which are built on or near natural fill. Thumping and cracking sounds might come from walls and standing vehicles would be rocked noticeably.
Such a temblor would be the largest in the state’s recorded history.
The Feb. 28, 2001, Nisqually quake that damaged the Capitol building in Olympia measured 6.8, according to the USGS.
Historical evidence
A 2005 study conducted by University of North Texas researchers Harry Williams and Ian Hutchinson and Alan Nelson concluded that marsh peat layers studied in Discovery Bay produced evidence of nine tsunamis over 2,500 years, not all of which originated from the Cascadia subduction zone in the Pacific Ocean, Walsh said.
Scientists believe a 9.0 quake rocked the Cascadia subduction zone, where two tectonic plates collide just off the West Coast, on Jan. 26, 1700, producing a giant tsunami that hit Japan.
A similar quake, which could produce a tsunami of 60 to 90 feet that would hit the Northwest coast in 15 minutes or less, is considered likely, though scientists aren’t sure when.
Bob Hamlin, Jefferson County Emergency Management director, said Jefferson and Clallam counties have, like California, always been at a “highest threat” level as far as quakes go.
Big red flag
“Its not a real surprise to me,” Hamlin said of the South Whidbey fault findings. “It’s just a big red flag, and just another reason to keep on what we’re doing.
“To say that our community is ready for an earthquake is really a stretch.”
A severe quake could isolate the Peninsula, he noted, as he stressed the need for individual preparedness in the event of such a disaster.
He said a major quake would leave the region without power and a chain of supplies for three to five days.
“It’s just not a matter of if, it’s a matter of when,” Hamlin said.
He referred those who want to prepare to visit www.emd.wa.gov or www.ready.gov for disaster response information.
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Port Townsend-Jefferson County Editor Jeff Chew can be reached at 360-385-2335 or at jeff.chew@peninsuladailynews.com.