Adam Spencer, The Triplicate

Dangerous fault line poses a threat that only increases with time

Crescent City was forever changed when 29 city blocks were wiped from the map during the 1964 tsunami, and the consensus among scientists is that it will happen again - only worse.

The Cascadia Subduction Zone, a 700-mile fault line that lies offshore of the Pacific Northwest Coast from Cape Mendocino to Vancouver Island has produced massive earthquakes that spawn monster tsunamis on a regular basis for the past 10,000 years.

The southern end of the Cascadia fault, including the section hugging Crescent City's coast, has seen more earthquakes than the northern end, and researchers from Oregon State University say there is a 40 percent chance of a mega-quake in the next 50 years.

"Major earthquakes tend to strike more frequently along the southern end - every 240 years or so - and it has been longer than that since it last happened," said Chris Goldfinger, a professor in OSU's College of Earth, Ocean, and Atmospheric Sciences and lead author of a 13-year study on the frequency of Cascadia earthquakes.

Because of the amount of people who live and work in parts of Crescent City that would be swamped by tsunami surges (5,000 during the day and 3,000 at night), more tsunami fatalities are predicted here than anywhere else in Cascadia's shadow.

A magnitude-9.0 earthquake at high tide could immediately kill more than 900 people in Crescent City - almost a third of the total deaths predicted for the whole affected area, according to the Department of Homeland Security.

Planning for the 'big one'

Local, state and federal emergency managers are well aware of the looming risk.

In a Cascadia earthquake, up to five minutes of shaking is expected followed by a tsunami within as little as five to 10 minutes.

"That's why we're doing the planning now," said David Plance, a response planner with FEMA Region IX. "There will be as little as five minutes warning time for the tsunami, and the warning is the ground shaking."

The exhaustive Cascadia Earthquake and Tsunami Response Plan was finalized last September and "provides a framework outlining how local, tribal, state, and federal governments and private and nongovernmental organizations (NGOs) will respond and coordinate immediately following a catastrophic earthquake along the northern California coast," according to the plan's lead agencies: FEMA Region IX and California's Office of Emergency Services.

Similar plans have been adopted for Oregon, Washington and British Columbia, all part of a growing awareness that Cascadia poses an even greater risk than the San Andreas fault that is commonly thought of as the harbinger of the next "big one."

During the same week Crescent City recognizes the 50th anniversary of the 1964 tsunami, residents are being asked to practice evacuating for waves twice as tall as 1964.

On March 26 from 11 a.m. to 1 p.m., Del Norte County will conduct a live code test. Local residents located in the tsunami evacuation zone are encouraged to participate by heading to the nearest safe zone.

Emergency managers across the state will be simulating theexecution of the Cascadia Response Plan, preparing for the real thing.

Last one was in 1700

The dense tectonic plates offshore from Crescent City are slowly being forced under the North American plate at a rate of roughly 1.6 inches per year in a process called subduction.

"The slow insistent movement that forces them together causes tremendous strain to build up as the plates stick against each other," according to the Cascadia Region Earthquake Workgroup. "The sudden release of this strain causes an earthquake."

The would be followed closely by a tsunami on par with the surges that killed 19,000 people and destroyed millions of buildings in Japan in March 2011.

The last time Cascadia produced a megaquake (8.0 or bigger) was on Jan. 26, 1700, the precise date recorded by Japanese written records of a tsunami that destroyed the year's rice crop but was not preceded by an earthquake.

The effect of the 1700 event was also passed down through oral history from both Tolowa and Yurok people.

"Yurok oral history places the height of the tsunami waves at about 60 feet near the mouth of Redwood Creek," according to Lori Dengler, a professor of geology at Humboldt State University and co-founder of the interagency Redwood Coast Tsunami Workgroup.

Since 1700, the Cascadia fault has just been reloading energy, waiting to be released during the next catastrophic event.

"A very large earthquake followed by very large tsunami isquite credible in our lifetimes," Dengler said. "Probably more likely than winning the big spin."

Emergency 'muscle memory'

The threat will obviously only increase with time.

"By the year 2060, if we have not had an earthquake, we will have exceeded 85 percent of all the known intervals of earthquake recurrence in 10,000 years," said Jay Patton, a co-author of the OSU Cascadia study. "The interval between earthquakes ranges from a few decades to thousands of years. But we already have exceeded about three-fourths of them."

To save lives, Del Norte County Emergency Services Manager Cindy Henderson said people have to develop "muscle memory" - making proper tsunami response second nature.

In 2004, more than 225,000 people died from the Indian Ocean Tsunami, but the coastal village of Langi, which was closest to the epicenter of the quake and saw the first tsunami surge within 8 minutes of shaking, escaped without a single fatality.

An oral tradition passed through generations taught the people of Langi village to head to the hills after any earthquake with more than a minute of shaking.

It's this type of second-nature reaction that Henderson hopes to instill in the community through next week's evacuation drill and other methods because, she says, there is no replacement for the time-tested technique of "practice, practice, practice."

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