The possible projected impact area of a long-predicted 9.0 earthquake off the coast of Washington has moved closer to the North Olympic Peninsula, some scientists are saying, emphasizing it’s a “best guess.”
The fault line of the Cascadia Subduction Zone will crack farther east than earlier predicted, changing from breaking completely offshore, about 25 miles beneath the Earth’s surface, John Vidale, director of the Pacific Northwest Seismic Center at the University of Washington, said Thursday.
Seismic tests and GPS measurements indicate the fault’s eastern edge will reach under the surface of the North Olympic Peninsula, he said.
“What’s changed is the limits of where fault breakage might be,” Vidale said.
“It’s now stretching out as much as maybe 30 miles from the coast under the Olympic Peninsula.
“The most recent evidence suggests the rupture might reach perhaps as much as halfway across the Peninsula.”
But not all agree that it will make a difference when the big one hits, he said.
Only a best guess
“It’s generally accepted as a best guess, but none of us are absolutely sure,” Vidale said.
“I don’t think anyone would say they know the idea is wrong,” he added.
He said the revised projection is based on new interpretations of tremors that have been measured over the last 10 years. Scientists are understanding the information better as measuring techniques improve, he added.
“But some people would say, it’s less likely, some would say it’s more likely. We need the best guess for planning.
“It might be decades before we can nail it down. One problem is not having had a 9.0 earthquake so we can see the relationship between the edge of where the ground broke and where the tremor is.”
Impact on Peninsula
If such a earthquake occured as predicted, it would make the impact of the temblor on Clallam and Jefferson counties — with a combined population of 100,000 — that much worse than scientists first thought by about 10 percent or 15 percent, Bob Freitag, executive director of the Cascadia Region Earthquake Workgroup (www.crew.org) and director of the Institute for Hazards Mitigation Planning and Research, both at the University of Washington, said Thursday in a separate interview.
Scientists do agree a 9.0 earthquake will rattle the Pacific Northwest sometime in the next few hundred years or so when the Cascadia Subduction Zone Fault breaks, creating destruction along a 2,000-mile area from Northern California to Vancouver Island, British Columbia, Vidale said.
They don’t know when the earthquake will hit, they just know that earthquake history and the inexorable movement of the earth’s continents and the tectonic plates guarantees it, he said.
The North American continent is moving at about a fingernail length a year, which doesn’t seem like a lot, “but over a million years, it adds up,” Freitag said.
The Cascadia Suduction Zone fault is caused by the Juan de Fuca Plate sinking under the North American Plate after the plates collided, one pushing the other about 25 miles beneath the Puget Sound Basin.
Flex and then break
“It’s like a piece of wood,” Vidale said. “You can flex it for a while, but if you push it too far, it will break.”
Tremor measurements have indicated that the estimated width of the actual crack caused by the earthquake will expand farther under the Peninsula than had been thought.
Scientists have been mapping tremor patterns using seismometers to detect vibrations, and using GPS to discern ground deformations.
“[The patterns] are telling us where the rock is likely to break in the next earthquake, where plate boundaries are locked and ready to break and where it is not locked,” Vidale said.
Energy from the collision of the two plates is released once about every 200 to 1,000 years, and an average of every 500 years, bringing on a 9.0-magnitude earthquake that creates a tsunami with waves 10 to 15 feet.
The energy never stops building up and has to be released. That’s how scientists know the earthquake eventually will occur, Vidale said.
The last area subduction zone earthquake – the Great Cascadia Earthquake — struck at 9 p.m. Jan. 26, 1700.
It caused a tsunami that devastated Native American communities along the coastline, judging by remnants of buried campfires unearthed along the coast, Vidale said.
The exact time was calculated by the time of the tsunami wave that was recorded by ship-borne Japanese rice merchants who saw the wave in Japan.
Scientists determined the speed of the wave by employing physics and their knowledge of the properties of water, he said.
They calculated when the earthquake that caused the wave occurred by measuring what time it hit Japan.
When the successor to the Great Cascadia Earthquake rumbles its way off coastal California, Oregon and Washington, “it will be like a big zipper going up and down the entire coast,” Freitag said.
It would likely destroy non-earthquake proof structures, compromise fill-areas along city shorelines, and isolate rural areas by destroying bridges and roads, Freitag said.
Homes tied to their foundations are safer than those that are not, Freitag said.
A 9.0 earthquake is strong enough to make older, non-retrofitted homes like many in Port Townsend “bounce off their foundations,” he said, adding that the city of Port Townsend has an aggressive program to retrofit older residences.
New buildings OK
“New buildings built to existing codes would probably do OK,” Freitag added.
Areas where structures are built on fill, which applies to much of downtown Port Angeles and the city’s waterfront, are susceptible to collapsing in a 9.0 earthquake, he said. The same is true for shorelines and floodplains in general, which abound on Clallam County’s west end.
“Homes can actually sink,” Freitag said.
Fires would likely break out when gas lines broke, and if propane tanks explode, the flame follows the terrain.
“A moving ball of propane gas is very, very deadly,” Freitag said.
Along with having non-earthquake-resistant homes and waterways, rural areas face isolation if bridges are destroyed or roads and highways rent asunder.
The entire North Olympic Peninsula faces potential isolation if an earthquake severely damages U.S. Highway 101 or makes the Hood Canal Bridge impassable, Freitag said.
________
Staff writer Paul Gottlieb can be reached at 360-417-3536 or at paul.gottlieb@peninsuladailynews.com.
