Recent earthquakes in Sherwood, California and Chile are part of a natural fluctuations, scientists say

Recent quakes in Chile, California and Sunday evening's magnitude 3.3 earthquake near Sherwood have people wondering if this recent uptick foretells that more, and perhaps more destructive, earthquakes are imminent.

The answer is a resounding no, according to George Priest, a geologist with the Oregon Department of Geology and Mineral Industries, and numerous other Oregon and Washington scientists. Increases or decreases in earthquake activity are all part of normal fluctuations, they said.

"This was not an unusual event," Priest said. "We have lots of 4s and magnitude 4.5s in the region, but maybe not there at that spot (in Sherwood). There is no evidence that these earthquakes mean anything for the future."

Earthquakes happen every day
In the past 24 hours, (5 p.m. Monday through 5 p.m. Tuesday) the Pacific Northwest Seismic Network at the University of Washington has recorded 17 earthquakes. The largest was a magnitude 2.4 temblor 40.5 miles southwest of Penticton, British Columbia.
But the 2.4 quake was barely felt -- like most of the 20,000 earthquakes a year documented by the National Earthquake Information Center of the U.S. Geological Survey, or about 55 a day worldwide.

Repeated earthquakes along subduction zones — where one tectonic plate dives below another — are more worrisome, Priest said, and can trigger additional quakes. The Cascadia Subduction Zone off the Oregon coast hasn't experienced a major earthquake in more than 300 years.

The Sherwood earthquake occurred deep underground along a fault in the earth's crust, unlike Chile's earthquake, which occurred along a subduction zone off the coast.

The quake in Sherwood was considered weak, with minimal shaking. Still, it was powerful enough to generate 1,501 responses in 91 ZIP codes from Kelso, Wash., to Salem on the U.S. Geological Survey's  "Did you feel it?" website.

Thanks to the large number of seismic networks and better detection, the public hears about earthquakes faster than ever.

The reason the quake generated a relatively large response is that it happened so deep underground, said Ian Madin, a state geologist who has studied the faults under the Portland metro area for more than 25 years.

A typical earthquake in California occurs at a depth of 6 to 10 miles underground. Under the Portland area, the typical depth is 12 to 15 miles underground. The Sherwood quake was about 13.6 miles underground.

The deeper the quake, the more widely it's felt because shock waves move up to the surface in a cone shape, and thus typically cover a bigger territory.

The Sherwood quake also happened quickly, which boosted its impact.

"The same amount of energy released in a shorter time makes for a sharper shake, with fewer seconds of shaking that 's more intense," Madin said.

"Every other year or so we get one these oddball quakes in the mid-2s to magnitude mid-3s that are felt much more widely that they should be,'' he said. "We had a magnitude 3.9 at Kelly Point that generated the same response as a magnitude 2.6 earthquake in the Laurelhurst neighborhood, even though the Kelly Point quake was 25 times stronger."

The faults aren't only complex, Madin said, they're closer than you might think.

"You are never more than one mile from a fault in the Portland area," he said. "And so far, we have been unable to tie any earthquake to specific fault. If these earthquakes were just a bit bigger, the recordings would be a lot better and we could actually tell a lot more about what's happening."

-- Stuart Tomlinson