The earthquake swarm off the Oregon coast continues – with almost 100 quakes in the past 48 hours – but experts say not to worry.
The series of quakes, which began early Tuesday morning and continued into Thursday, is clustered between 200 and 250 miles west of the coastal town of Newport, Oregon – far enough away to be mostly undetected on land.
“If you had asked me (Monday) where on Earth would be most likely to produce a bunch of magnitude 5.0-plus 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 state, told CNN.
The Blanco Fracture Zone 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, an analysis by Oregon State University finds.
Not necessarily a precursor to the ‘big one’
What has been most impressive about this week’s quakes is the swarm has included at least nine tremors reaching a magnitude 5.0 to 5.8, with the majority occurring at a shallow depth of only 10 km (6.2 miles).
The sheer number of magnitude 5.0 or greater quakes in the region triples the annual average (three 5.0-plus quakes per year) since 1980, according to the USGS database.
There were also swarms in June and July of 2008 and in August 2009 according to the USGS.
Fortunately, according to the US National Tsunami Warning Center, none of this week’s quakes have triggered a tsunami alert.
However, 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 not only trigger devastating tsunamis but also destructive shaking.
This is not necessarily a precursor for the so-called ‘big one’
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.
There have been more than 133 quakes of magnitude 5.0 or greater on the Blanco Fracture Zone since 1980 and have never been followed by something on land, according to seismologist Dr. Lucy Jones.
“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.
Though 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,” he explained. “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.”
Although there have been a remarkable number of magnitude 5.5+ quakes in a short period of time, he said, it is not inherently alarming but rather seismologically interesting.
A similar swarm of quakes, though not to this week’s magnitude, occurred earlier this year near the Salton Sea in Southern California.