By LEE J. SIEGEL and CNN for YachatsNews.com
A swarm of nearly 70 small to moderate earthquakes rattled under the seafloor 250 to 290 miles off the central Oregon coast Tuesday and Wednesday. Fifteen of them measured between magnitude 5.0 and 5.8.
A few coastal residents reported feeling the largest jolts, which measured up to 5.8 in magnitude. The National Weather Service in Portland said the earthquakes were far too small to trigger a tsunami.
The epicenter of the swarm is on the Blanco Fracture Zone, which is among the most seismically active in North America. The Blanco zone is much farther west of the more concerning Cascadia subduction zone and rarely leads to destructive quakes, according to earthquake experts.
This week’s earthquake swarm apparently began with a magnitude 4.2 jolt just before 5:21 a.m. Tuesday. By 2:30 p.m. Wednesday, the U.S. Geological Survey had recorded 68 earthquakes — the largest a pair of magnitude 5.8 jolts at 4:47 p.m. and 5:21 p.m Tuesday and the smallest a 3.5 quake at 2:16 p.m. Tuesday.
Fifteen of the earthquakes measured 5.0 or stronger — including a batch of five between 9:15 p.m. and 9:51 p.m. Tuesday. Three of those measured 5.5 in magnitude. The last of the larger earthquakes measured 5.3 at 2:22 p.m. Wednesday.
By late Tuesday the USGS had received 10 reports from people who felt the two 5.8 magnitude quakes. The first quake reportedly was felt by one person each in Waldport and Brookings, Ore., Monroe, Wash., and Chilliwack, B.C. The second 5.8 reportedly was felt by six people in Florence, Waldport, Newport, Depoe Bay, Tualatin and Medford.
The jolts were centered about 6 miles deep on what is called the Blanco fracture zone, part of the boundary of the between the Pacific plate’s eastern edge and the Juan de Fuca plate’s western edge. The Cascadia subduction zone — where the Juan de Fuca plate’s eastern edges dives beneath the western edge of the North American plate — has produced magnitude 9 megaquakes and related tsunamis in prehistoric time and as recently as 1700.
But the largest quakes in the area where the latest swarm occurred were a magnitude 6.2 in 2000 a bit west of the swarm and a 6.3 in 2003 somewhat east of the swarm, said geophysicist Robert Sanders of the U.S. Geological Survey’s National Earthquake Information Center in Colorado.
Sanders said that in the six years he has worked at the center, he recalls perhaps three or four swarms of magnitude 5 jolts in various sections of the Pacific-Juan de Fuca-North American plate boundaries.
He said the latest swarm could end soon, or go on for days or weeks, and that a similar swarm, but with fewer magnitude 5 quakes, was occurring in the Gulf of California.
Interesting but not too concerning
While far enough out to sea to go mostly undetected on land, but given the area’s seismic history, it is creating quite a buzz.
“If you had asked me yesterday where on Earth would be most likely to produce a bunch of magnitude 5.0+ quakes in a single day, this would have been high on my list,” Harold Tobin, Director of the Pacific Northwest Seismic Network at the University of Washington, told CNN.
The fault line responsible for the quakes is the Blanco Fracture Zone. According to an analysis by Oregon State University, it is more active than the infamous San Andreas Fault in California, having produced more than 1,500 quakes of magnitude 4.0 or greater since the 1970s.
What has been most impressive about this week’s quakes is the swarm has included at least 13 tremors reaching a magnitude 5.0 to 5.8, with the majority occurring at a depth of six miles, which is considered shallow.
The sheer number of magnitude 5.0 or greater quakes in the region triples the annual average (three 5.0+ quakes per year) since 1980, according to the USGS database.
The activity has heightened the concern level for some, as the region is among the most earthquake-prone areas in North America and has already produced one of the largest quakes in the continental United States on January 26, 1700. The quake occurred on the neighboring Cascadia Subduction Zone, a megathrust fault much closer to land, where the Juan de Fuca plate dives underneath the North American Plate. This fault can trigger devastating tsunamis but also destructive shaking.
Though the epicenter of this week’s swarm of quakes, the Blanco Fracture Zone, is among the most seismically active in North America, it rarely leads to destructive quakes.
The fault is some 275 miles west of the Oregon coastline and about 200 miles west of the Cascadia Subduction Zone, where the largest and most destructive Northwest quakes have historically taken place.
“Blanco Fracture zone quakes are strike-slip (lateral motions of the crustal blocks on either side, rather than up-down displacement), so it is very unlikely for them to pose a tsunami threat, even if a bigger quake happened, like a magnitude 7.0 for example,” Tobin told CNN.
According to seismologist Lucy Jones there have been more than 133 quakes of magnitude 5 or greater on the Blanco Fracture Zone since 1980 and have never been followed by something on land.
“Today’s quakes can be thought of as something like a main-shock and a swarm of aftershocks, the one distinction being that in this case, there’s not a lot of magnitude difference among them,” Tobin said.
Although this week’s quakes have raised the concerns the so-called “big one” could be near, Tobin assured it is not necessarily the case.
“There’s quite a lot of distance from these quakes to the Cascadia Subduction Zone,” Tobin said. “Our best current understanding of how stress transfers through the crust (and mantle) would suggest that these events don’t change stress on the subduction zone appreciably.”
He pointed out, although we have had a remarkable number of magnitude 5.5+ quakes in a short period of time, it is not inherently alarming but rather seismologically interesting.
- Lee Siegel of Newport is a former science reporter for The Associated Press. CNN also contributed this report.
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