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Experts say a 9.0 or above magnitude earthquake could strike off the Pacific Northwest coast any time in the next 50 years



HOLLY SCHOLZ - Don Wood points to the continuation of the subsided material that has slid southward and is visible on the south side of the Ochoco Reservoir.

It’s coming. The big one — the 9.0 magnitude Cascadia earthquake.

And with it, wonder, panic, denial and — hopefully — awareness.

It was during his Alaska days that Prineville-native Don Wood had his first run-in with earthquakes.

“I’ve been a first-hand witness of earthquakes and what earthquakes have done in Alaska,” Wood said. “I was in an earthquake in Anchorage that was assigned a 6.6. It felt like I was standing in a huge hammock being swung back and forth about 70 degrees, front to back.”

He has extensively researched earthquakes and has taken it upon himself to inform his community about the dangers lurking.

Wood earned a degree in architecture from the University of Oregon in 1974 before interning in the Eugene/Springfield area for three years. After getting his architecture license, he practiced in Bend for a while before moving to Alaska, where he worked all over the state and up and down the Aleutian Islands. He returned to Prineville in 1997 and operated his own firm, Architect Don L. Wood, until retiring a year ago.

“As an architect, I always tried to provide a better, safer, more-productive living space for people,” he said, adding that living space includes yards, streets, towns, states and countries.

Cascadia earthquake

The date was Jan. 26, 1700, more than 100 years before the United States began colonizing the Pacific Northwest. A powerful earthquake occurred along the Cascadia subduction zone, a plate boundary that stretches from Southern British Columbia to Northern California.

According to the U.S. Geological Survey, the 1700 Cascadia earthquake is estimated to be approximately magnitude 9. This “megathrust earthquake” is the most powerful caliber of quake. The longer the time period between earthquakes, the stronger the quake will be.

Over the past 10,000 years, at least 19 earthquakes have been found on the margin from Northern California to Southern British Columbia, which means an average recurrence interval of approximately 500 years. The southern margin of Cascadia south of Newport is more active with a recurrence interval of about 250 years, but quakes are of a lower magnitude in the 7.8-8.4 range, according to geologist Scott Burns, who is also a Red Cross Cascades Region board member.

“While we can’t predict the precise time and date of the region’s next large earthquake, we can take some important precautionary steps to prepare for an emergency — it could be another 600 years, or an earthquake could strike tomorrow,” Burns said.

Wood said these subduction earthquakes historically have happened 300 to 750 years apart.

“It could’ve happened, on the average, anytime in the last 50 years or it could happen in the next 300 years,” Wood said.

Daniele McKay, a geology instructor at OSU Cascades in Bend, said there is a 10 to 15 percent chance of a magnitude 9 or higher earthquake occurring in the next 50 years and a 37 percent chance of a magnitude 8 quake.

A subduction earthquake occurs when a portion of the earth’s crust, a plate, slides under the edge of an adjoining plate, causing shaking in both plates. If the subduction occurs under the ocean, a tsunami usually results.

Wood describes it as pushing two carpet rugs together with full force. They initially bunch where they meet. The more forceful rug will eventually ride over the top of the more flexible one. When that occurs along a subduction fault like off the coast of Oregon, a subduction earthquake occurs.

Burns said, “We are far from ready for the effects of shaking, landslides, liquefaction and tsunamis that are sure to occur in the event the Cascadia subduction zone shifts.”

Liquefaction

Calling himself a concerned citizen with family and friends living here, Wood wants the community to know that the Cascadia earthquake will not only hit Prineville —it could well change life as we know it.

“People need to know that when it happens, the faster they can get out of the Ochoco Valley, the safer they will be,” he said, adding that Prineville is susceptible to liquefaction. “That means it will turn into a great big mud bowl with no bottom.”

Liquefaction is the process of turning to liquid. Liquefied soils turn to muck (like a potato soup or quicksand) during an earthquake of substantial strength.

“Prineville is at risk for liquefaction,” McKay said. “The liquefaction is real. There is something there that people should know.”

Liquefaction, she explained, involves sediment — it might be water-saturated sediment or loose, uncompacted soil that has some water in it.

“The only time it becomes a problem is if it starts shaking during an earthquake,” McKay said. “It behaves as solid ground when it’s not moving, but when it’s shaking, it really behaves more as a liquid.”

This happens regardless of whether or not any dams burst or flooding has occurred. It happens to soil — especially soil that was deposited by rivers or that is along rivers, she explained.

“Most people don’t realize that the Prineville valley used to be a lake,” Wood pointed out.

The Ochoco Reservoir is getting shallower, he explained, because the silt settles out of the lake water.

“That’s what we have here —basically silt that’s settled out of lake water,” Wood said. “There’s a lot of clay, and clay makes for great pudding.”

McKay agrees. “Because Prineville has a lot of unconsolidated sediments, there will be liquefaction.”

Aftershock

So what does liquefaction mean for the structures and people of Prineville when the Cascadia earthquake strikes?

According to the OPB Aftershock website, Prineville will suffer strong shakes and panic. Heavy furniture will move, and plaster will fall from the walls and ceilings. Old brick buildings that haven’t been reinforced will fail. Most buildings constructed before the 1990s may be damaged but usable. Buildings constructed before the 1970s will likely suffer more damaged. Many bridges may be damaged and impassable.

“It doesn’t mean the whole town is flat,” McKay said, but many buildings will be damaged.

According to the OPB site, experts project it could take several weeks to restore the community to its normal function, based on damage to pipes, infrastructure, and the transportation corridors needed for recovery efforts.

Dams

Wood is concerned that the Ochoco and Bowman dams are built on fault lines.

“They don’t realize that the Ochoco Dam is built on a fault line,” he said. “They’ve driven past the Prineville Reservoir. They don’t realize that dam is built on a fault line — an active fault line.”

He said that the dams are on separate fault lines, but the fault lines meet in the center of town.

During the impending Cascadia earthquake, if the Ochoco and Prineville reservoirs are full of water, Wood said, “There’s going to be a 75-foot wall come this direction, and a 75-foot wall from that direction, and the turbulence that’s going to occur is going to wash anybody that’s on the surface, down below, most likely,” he said.

Josh Smith, a senior planner with the City of Prineville, said he’s heard there are faults below the dams but has not confirmed it.

“There’s a ton of little, small fault lines everywhere — there’s not these big plate tectonic ones —but there are small fault lines all over Oregon — all over the country, really,” he said, adding that there very well may be small faults under the dams.

Although no one wants to say the dams are going to collapse, Smith said, “They are earthen dams, and they have that potential.”

Smith also pointed out that when the Cascadia earthquake hits off the Oregon Coast, it is not necessarily going to trigger something over here.

“It may not do anything to us,” he said. “We’re in a young stage of understanding plate tectonics.”

Russ Rhoden, the manager of the Ochoco Irrigation District, said he’s been with the district for 24 years and has never heard that the dams are built on fault lines.

There is, however, a historical issue with Ochoco Dam. The north side of the dam was built up against an ancient landslide, Rhoden said.

“I’ve never heard it been called a fault line, but there’s a landslide evidently that went down in the valley and perhaps that was the narrow spot in the valley and seemed like the appropriate spot to put a dam into that position,” he said.

The Bureau of Reclamation has repaired the dam over the years and has looked at it recently in light of the Cascadia quake.

Several years ago, the Bureau of Reclamation did some core drilling, analyzing the foundation and the affect a giant earthquake would have on the dam, Rhoden said.

He feels that the Bowman Dam is pretty safe and does not think the bureau is planning to do anything with that as it relates to earthquakes.

Calls to the Bend Bureau of Reclamation office were not returned.

Evidence of seismic activity

Prineville is surrounded by evidence of past seismic activity, according to Wood.

The lava that covered the Central Oregon landscape was all deposited flat and level when it flowed across the landscape. But, Wood said, over time, the fringes of the valley have begun to sink toward the valley floor as a result of past earthquakes. For instance, Ochoco Viewpoint has slid east and downward and is also an example of past liquefaction.

Sunset Hills, a subdivision east of town on the way to the Ochoco Reservoir, is built around rimrock north of the Ochoco Dam. When the formation was new, it was level, Wood said. Now, it slopes south, and the rimrock has subsided under the newer rimrock and soils on the south side of the valley.

Wood said there’s a visible continuation of the subsided material near the Ochoco Dam that has slid to the south, visible on the south side of the Ochoco Reservoir.

The sloping rimrocks east of Prineville may not seem to be tilting further, however they tilt over long time periods, measured in centimeters, which is hard to detect visually.

Additionally, “If you look at the Prineville Reservoir, there’s a total discontinuity of the rock strata from one side of the dam to the other,” Wood pointed out. “There’s been a lot of movement there.”

Higher ground

Wood, who lives in the bottom of the valley, has his eye on some places on the heights.

“I would like to see us slowly transition from building down in the valley to up above — the whole town,” he said, adding that the top of the grade and the heights will be safer than the valley during an earthquake.

“I’m trying to talk some of my friends into moving higher — either here or somewhere else,” Wood said. “Long-term, I’d like to see us come up with the means to move the town to where we’re not in as much danger.”

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