Earthquakes and Baker Valley

Fortunately for us, the quake-producing cracks in the Earth’s crust (geologists call them “faults”) that helped create our valley have been tranquil for a while.

How long that while has been, however, not even geologists can say.

And how long those faults will continue their quiescence is a mystery as well.

As is often the case when it comes to the restless forces at work beneath our feet, the scientists who study those forces deal in spans of time so vast as to seem meaningless to people who tend to confine their concerns to whether it’s going to snow next weekend.

“If you lived here 10,000 years, you’d go for a ride,” said Mark Ferns, regional geologist at the Oregon Department of Geology and Mineral Industries office in Baker City.

That’s interesting, but it’s not as useful as knowing whether the most recent ride happened here 9,999 years ago, or rather more recently.

“We really know nothing about the history of past intervals of earthquakes in Baker Valley, or when the next one will happen,” Ferns said.

Here’s what geologists do know:

A series of faults runs along the southwestern and western edges of Baker Valley, Ferns said.

Over time — and here we’re talking millions of years rather than mere thousands — the land to the south and west of those faults has risen (hence the Elkhorn Mountains and the sagebrush-studded foothill that rises above the golf course), and the land to the north and east has dropped (that’s Baker Valley).

This movement is of course imperceptible to us.

Well, usually.

Sometimes these huge slabs of slowly moving crust get stuck. When that happens, pressure builds along the locked section of the fault.

Consider this analogy: You’re trying to break loose a rusty bolt, really putting your back into it, when the bolt finally lets go. You can’t react that fast and so you bang your elbow against the nearest hard (and probably sharp) object.

When a stuck fault releases, you get an earthquake instead of a sore elbow.

The Geology Department’s earthquake hazard map for Oregon shows most of Northeastern Oregon having about the same level of risk.

That’s partly because, geologically speaking, Grande Ronde Valley and Baker Valley are more siblings than cousins, Ferns said.

Both valleys are bordered by faults.

The difference is that geologists know quite a lot more about the Grande Ronde Valley.

During the past couple of decades geologists have spent hundreds of hours hiking around the margins of that valley. Although their goal was to study and map the rocks there, geologists also learned much during their field work about the faults, Ferns said.

They concluded that those faults could produce earthquakes as strong as 6.3 to 6.5 on the Richter scale, he said.

Although that’s less powerful than the magnitude 7.0 quake that devastated Haiti last week, a 6.5 temblor is capable of causing extensive damage, Ferns said.

And a quake of that strength, even one along one of the Grande Ronde Valley faults, could damage buildings in Baker City, he said.

(It’s worth noting here that the Richter scale is logarithmic — a magnitude 6.0 quake is actually 10 times more powerful than a magnitude 5.0, and releases 31.6 times more energy.)

As for the faults that border Baker Valley, geologists, notwithstanding the scarcity of on-the-ground research, believe those faults are similar to ones in the Grande Ronde Valley and that they have spawned earthquakes of comparable strength, Ferns said.

How much damage?

Predicting the effects of a quake is as problematic as forecasting its timing.

Several factors affect the extent of damage on the ground, including how far below the ground is the quake’s origin (its “focus” or “hypocenter”; the more commonly used “epicenter” refers to the point on the ground directly above the focus).

Certain generalities apply, though, Ferns said.

For instance, multi-story buildings made of unreinforced stone or brick are more likely to be damaged during a quake than are single-story buildings with wood or steel frames.

Baker City has a wealth of the former — among them renowned structures such as City Hall, the County Courthouse and many commercial buildings in the downtown historic district.

The type of ground on which a building sits matters, too.

Clay soil and loose fill dirt are especially susceptible — the “shear waves” that some quakes generate can actually liquefy such soils briefly, Ferns said.

Most houses, of course, are not designed to float.

Buildings constructed on bedrock — some homes in the hilly southwest corner of Baker City, for instance — are more likely to withstand shaking.

The soil in most of Baker Valley is gravelly, which is sort of a middle ground between the two extremes — more stable than clay or fill dirt, but obviously not so rigid as bedrock.

Ferns said the most beneficial advice he can give is to remind people that in most earthquakes the greatest danger is from fire.

He recommends residents make sure natural gas-powered appliances are securely fastened so they won’t easily tip over during a quake, snapping the gas supply line and sparking a blaze.

Ferns also suggests residents assemble an emergency supply kit that includes food, warm clothes and enough drinking water to last at least 72 hours.

After all, he said, the next earthquake might not hit here for another thousand years, but blizzards, floods and windstorms can cause similar problems but aren’t likely to grant us such an extended reprieve.

What’s different west of the Cascades?

A lot, if you’re talking about earthquake risk.

The looming hazard for the westside comes from what’s known as the Cascadia “subduction zone” about 50 miles off the Oregon Coast.

A subduction zone is a different sort of fault from the ones that border Baker Valley.

In a subduction zone, one “plate” — a continent-sized chunk of the Earth’s crust — is plunging beneath another. The process is called “subduction.”

In the case of the Cascadia subduction zone, the Juan de Fuca plate is diving beneath the North American plate (that’s what you’re standing on). Right now, though, geologists believe that downward movement has stopped or slowed, and pressure is building along the subduction zone.

When the plates “unlock,” the resulting earthquake could be in the range of 9.0 on the Richter scale.

Scientists believe such massive quakes have occurred along the Cascadia subduction zone every 400 to 600 years, with the most recent happening about 310 years ago. It’s relatively easy to date those historic quakes because they generated tsunamis that left evidence along the coastline.

Panhandle has rattled recently

In both 1994 and 1998, “swarms” — series of several dozen small earthquakes — occurred along faults in the Brownlee Dam area at the eastern edge of Baker County.

The strongest of the approximately 50 quakes during the October 1994 swarm was a magnitude 3.9 temblor; it was also the most powerful quake in the Panhandle since 1966. Another quake in 1994 measures 3.7.
People in Halfway, Oxbow and other parts of the Panhandle felt both of those quakes.

In July 1998 more than a dozen quakes — the strongest one a 3.5 — happened along the Posey Valley fault near Halfway.
The strongest quake near Baker Valley in the past few decades happened in November 1998 in Bowen Valley and measured 2.9. Several people felt that tremor.