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Enticknap: Not a matter of if, but when

## Photo: Billy Gass Photography, Depoe Bay
## Photo: Billy Gass Photography, Depoe Bay
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After service as a captain in the U.S. Army, guest writer Peter Enticknap spent 20 years in real estate and finance. Then he switched careers, serving as an adviser to conservation biology nonprofits in Alaska, Canada and Mexico. His published writings include: “Earthquakes & the Windy Craggy Mine,” “Tatshenshini Wilderness,” “Windy Craggy Mine,” “Threatened: Cabo Pulmo National Marine Park,” and “Baby Elephant Rescue-Tanzania.” Upon retirement in 2010, he moved from Alaska to McMinnville. He enjoys writing, photography, birding, camping and hiking with his partner, Linda.

 

On the night of Jan. 26, 1700, a 9.0 magnitude “megathrust” earthquake — a very large quake occurring in a subduction zone at the boundary between two tectonic plates — ruptured the sea floor off the coast of the Pacific Northwest.

A colossal tsunami raced toward Northern California, Oregon, Washington and southern British Columbia at up to 500 mph, as coastal shorelines collapsed into the ocean. A second wave moved swiftly toward Japan.

Minutes after the deep sea fracture, the tsunami inundated the Northwest coast. Ten hours later, an “orphan” tsunami of unknown origin reached northern Japan.

You can still see evidence of the devastation caused by this 321-year-old Great Quake on the Northwest coast, including the Neskowin Ghost Forest pictured above. It consists of remnants of a 2,000-year-old Sitka spruce grove rooted in ground that was thrust under the sea floor.

It took a team of American and Japanese geologists, led by Oregon State University paleoseismologist Chris Goldfinger, decades of scientific sleuthing to unravel the story behind one of the largest earthquakes ever documented in North America.

The Ghost Forest and a set of 321-year-old cedar stumps discovered at the mouth of Copalis River on the Olympic Peninsula bear witness to the great quake of 1700, as well as other megathrust earthquakes occurring along the subduction zone.

Ancient Native American legends recall when the earth trembled and massive ocean waves inundated the Northwest coast.

Carbon-14 dating, a form of tree ring analysis called dendrochronology, and historical records from Japan aided scientists in pinpointing the precise date of the quake of 1700. They excavated sand deposits carried ashore 1.2 miles inland by the tsunami it triggered, and 30 more tsunami deposits have since been discovered in Pacific Northwest estuaries up to six miles inland.

The Cascadia Subduction Zone is a 600-mile convergent tectonic plate boundary at the edge of the Continental Shelf. Running from Northern California to southern British Columbia, it lies about 70 to 100 miles off the Pacific Coast.

The CSZ marks the boundary along which the Juan de Fuca “Oceanic Plate” is sliding beneath the North American “Continental Plate.” Subduction zones like this one generate the largest megathrust earthquakes known to science.

But it was not until the 1980s that researchers recognized the full extent of the Cascadia Zone’s destructive potential.

Along this zone, the sea floor bulges up and compresses to the east at a rate of up to 1.6 inches per year. Pressure builds over centuries, as these plates are locked in place. Enormous energy is then violently released as a seismic wave megathrust earthquake.

The North American Plate suddenly slips westward over the Juan de Fuca Plate. The submerged edge of the advancing plate rebounds like a spring, releasing massive seismic waves capable of shaking the entire Northwest. This sudden rupture causes a vertical displacement of sea water, sending tsunamis in different directions.

The geologic record shows there have been 41 significant Cascadia megathrust earthquakes in the last 10,000 years, or one about every 244 years.

At least 19 were major fractures across the entire 600-mile boundary. They were likely magnitude 8.9 to 9.2,  qualifying as great quakes.

These high-magnitude quakes occur about every 300 to 500 years. They can cause a simultaneous shift of the earth’s crust of about 30 to 65 feet along the length of the zone. After a great quake, the two plates lock together until the cycle repeats.

Are we overdue for another “Big One?” Currently, scientists predict a 37 percent chance of a CSZ megathrust earthquake of 7.0 to 8.0 magnitude, or greater, will occur within the next 50 years. With the passing of each year, the risk increases.

Because of the logarithmic basis of the Richter Scale, each increase of one whole number represents a 10-fold increase in the magnitude. Each increase of one whole number also represents the release of about 31 times more energy.

Today, seismologists use the Mw Moment Magnitude Scale. It is similar to the Richter scale, but incorporates additional data that makes it more accurate for great quakes.

A 9.0 Mw earthquake could generate multiple tsunami waves of 40 feet or more. It would certainly change the earth’s topography and the Pacific Northwest coast on a permanent basis.

A megathrust event of this magnitude would be felt throughout the Pacific Northwest, with the potential for unprecedented damage and loss of life across the region. Via a process known as subsidence, portions of the coast might drop four to 10 feet below current sea levels. And until 1970, Oregon had no seismic building codes.

The Oregon Emergency Management Office advises we could be without services and help for two weeks after the next Big One strikes. The Federal Emergency Management Agency predicts significant loss of life and destruction west of Interstate 5. Millions would need emergency shelter, food and water.

FEMA predicts more than a million structures would collapse or be rendered uninhabitable, including more than 3,000 schools. More than half of all highway bridges, including those spanning the Columbia and Willamette rivers in Portland, could collapse. So could many police stations, fire stations and hospitals.

Anything not secured — your refrigerator, bookcase, TV and furniture — could  be thrown across the room, as violent shaking lasts for two to six minutes. After New Zealand’s capital of Christchurch was struck in 2011 by a 6.3 magnitude earthquake, far less powerful than projected here, it took months to rebuild the infrastructure and restore water, sewer and power.

Are you ready for the really Big One? What can you do now to prepare?

Assume you will be on your own for up to two weeks and make a plan. Visit the “2-weeks ready” section on the Oregon Emergency Management Hazards and Preparedness website for help getting started.

The agency advises you to learn basic first aid and be self-sufficient, in order to take pressure off first responders, and ensure your family can survive on its own, as roads may be impassable. To make that possible, it advises stocking up on potable water, non-perishable food, needed medications and other supplies, and encouraging neighbors to do the same, as we’re all in this together.

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