Chances are few residents in Shady Cove or Butte Falls felt the earth move early on the morning of Oct. 22.
But it did, according to seismic monitors at the Department of Earth and Space Sciences at the University of Washington.
The department reported a 1.2 magnitude micro-earthquake at 6:48 a.m. that day about 11 miles northeast of Shady Cove and 14 miles northwest of Butte Falls.
Barely powerful enough to cause a ripple in your morning coffee above the epicenter, it was located about six miles below the surface.
Similar small earthquakes are common throughout our region, but they aren't the ones that concern folks such as Eric Dittmer, a geologist and professor emeritus in Southern Oregon University's environmental studies department.
"Small adjustments like that are frequent," observed the Medford resident. "But it appears that big earthquakes occur in our area every 300 to 350 years. If that's the case, we are in the window now."
Noting it has been more than 310 years since the last big earthquake — estimated at magnitude 9 — rocked Western Oregon, that window is growing ever larger, added James Roddey, spokesman for the Oregon Department of Geology and Mineral Industries.
He asks you to consider this: After studying everything from "ghost forests" submerged after a tsunami to undersea landslides, scientists have determined that 40 earthquakes magnitude 8 or larger have struck Western Oregon in the past 10,000 years.
"And they found that 75 percent of those have happened less than 310 years apart on the average," Roddey said. "That suggests we are due, and due sooner rather than later."
To put the potential destruction into perspective, the April 17, 1906, quake that leveled San Francisco was magnitude 8.2, while the March 27, 1964, quake that rattled Alaska, triggering a tsunami that struck Crescent City, registered 9.2.
The large quakes in our region are triggered by gigantic tectonic plates battling it out in geological time in what scientists call the Cascadia subduction zone in Western Oregon and Washington. There have been no subduction zone earthquakes in Oregon's recorded history, Roddey noted.
In this slow-motion geological combat, the oceanic Juan de Fuca plate is being forced beneath the North American plate at about 1.6 inches per year, experts explain.
While that pace may make a garden slug seem like a runaway locomotive, earthquake specialists say the movement creates huge stress between the titanic forces, one that periodically bursts in the form of a strong earthquake.
"We know the plates are moving," said Dittmer, who continues to teach part-time at SOU, where a strong-motion seismic instrument on campus registers the larger quakes for the Pacific Northwest Seismographic Network.
"These rock forces are building up, and at some point something has to break, releasing energy," he said. "This earthquake can occur offshore, on the coastline or underneath us."
The geological evidence is rock solid, said fellow geologist Harry Smedes, 84, of Grants Pass. Retired after 35 years with the U.S. Geological Survey and an additional stint with the U.S. Department of Energy, he is now an adjunct professor at SOU. He teaches a class on volcanoes and earthquakes at the RCC/SOU Higher Education Center in Medford. His next class is scheduled for the spring term.
"The key thing is this: It isn't guesswork," Smedes said of the potential for a huge earthquake in Western Oregon. "The two plates are locked. The continental plate is bending, arching up. At some point the elastic limit of the rock will be reached, and the big one will occur."
The geologists and Roddey, the earth science officer at DOGAMI, stress they aren't trying to scare up a little Halloween fright but want Oregonians to be aware of the possibility of a large earthquake striking our region.
"We tend to put earthquake issues away on the back burner," Dittmer said. "But people need to be aware there is a potential for this to happen. In earthquake country like San Francisco or Los Angeles, you get more people listening. A lot of people here don't have earthquake experience.
"Thinking about what to do is a good thing for people — it reduces the panic," he added.
Unfortunately, no one can pinpoint precisely when the big one will hit, Smedes acknowledged.
"But we can predict that one will come because of the buildup of stress," he said. "There are some precursors, like changes in water wells and increased radon gas, as rocks begin to fracture under the pressure."
For instance, a 300-foot well in Merlin's North Valley Industrial Park percolates like a coffee pot when a major earthquake shakes the planet. The well, which serves as a monitor for quakes, is simply reflecting the changing pressures caused by an earthquake when rock deep underground expands or contracts slightly during an earth shift, experts explain.
Smedes, who has experienced a few earthquakes in his time, noted there is nothing humankind can do to prevent earthquakes.
"The most recent research shows this will happen here every 300 to 350 years," he said. "The important thing is that people need to be aware that it can happen. They need to give it some thought."
"We are trying to empower people," Roddey said.
His department is the state's central source of geologic information to reduce the loss of life and property caused by an earthquake and other natural disasters.
"Folks west of the Cascades are in a massive state of denial this could ever happen," Roddey said. "It's a tough thing to do to turn people around."
In the state's history, the largest quakes to strike Oregon were the magnitude 6.0 and 5.9 tremors that struck Klamath Falls in September 1993, he noted.
As damaging as they were, they were not the big quakes triggered by straining earth plates, he said. Rather, they were "crustal" quakes that are relatively shallow in that they occur no more than a dozen miles below the surface, he said.
"I tell people not to worry about the crustals," Roddey said. "It's the big ones you have to get ready for."
However, because the knowledge there will be a huge quake in the state's future has been around only since the early 1980s, Oregonians in general have yet to adjust to the idea, he said.
"It has to be looked at as a long-term thing," Roddey said of preparing for a big quake. "We tell folks they could be on their own for one to two weeks. Highway 101 on the coast and Interstate 5 won't exist. It will take a while to get help to some areas."
While budget-constrained state and local governments are doing what they can to create an earthquake-resistant infrastructure, from beefing up public buildings to strengthening bridges, residents should always be prepared for emergencies, whether the natural disaster is flood, fire or a big earthquake, he said.
"It becomes a matter of personal responsibility how well they survive after the quake is over," he said. "They need to remember it will take a while to get help to some areas."
Reach reporter Paul Fattig at 541-776-4496 or e-mail him at email@example.com.
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