A swarm of earthquakes continues off the US Pacific coast, but experts don’t seem concerned

(CNN) – The earthquake swarm off the Oregon coast continues, with nearly 100 earthquakes in the past 48 hours, but experts say you shouldn’t worry.

While the main concern with large earthquakes this far from earth is the possibility of tsunamis, “events of the size that have occurred so far in this swarm will not generate notable tsunamis,” writes the US Geological Survey.

Also, the fault line where the swarm occurs, the Target Fracture Zone, moves from side to side and is not the megathrust type of fault (an extremely large thrust fault) that displaces ocean water to form tsunamis. . These tsunami-producing earthquakes are most common on a nearby fault.

“The main tsunami threat to the Pacific Northwest is the Cascadia subduction zone which is ~ 360 km east of this sequence.”

Meanwhile, Blanco’s fracture zone shaking “may continue for days or weeks, or it may suddenly stop at any time,” writes the USGS.

The series of earthquakes, which began Tuesday morning and continued through Thursday, is clustered between 200 and 30 miles west of the coastal city of Newport, Oregon, far enough away not to be detected on land.

But given the area’s seismic history, it’s creating quite a stir.

“If you had asked me (this Monday) where on Earth would be most likely to have a bunch of earthquakes of magnitude 5.0 or more in a single day, this would have been one of the first on my list,” he said. to CNN Harold Tobin, director of the Pacific Northwest Seismic Network at Washington State University.

The Blanco fracture zone is more active than the infamous San Andreas fault in California, having produced more than 1,500 earthquakes of magnitude 4.0 or greater since the 1970s, according to an analysis by Oregon State University.

It is not necessarily a precursor to the “big one”

The most impressive thing about this week’s earthquakes is that the swarm has included at least nine tremors that reached a magnitude of 5.0 to 5.8, most occurring at a shallow depth of just 10 km.

The large number of earthquakes of magnitude 5.0 or greater in the region triples the annual average (three earthquakes of more than 5.0 per year) since 1980, according to the USGS database.

There were also swarms in June and July 2008 and in August 2009 according to the USGS. Although the epicenter of this week’s earthquake swarm, the Target Fracture Zone, is among the most seismically active in North America, it rarely leads to destructive earthquakes.

The fault is located about 442 km west of the Oregon coast and about 321 km west of the Cascadia subduction zone, where the largest and most destructive earthquakes in the Northwest have historically occurred.

“The White Fracture Zone earthquakes are sliding (lateral movements of the crust blocks on both sides, rather than an up-down displacement), so it is very unlikely that they represent a tsunami threat, even if a larger earthquake occurred, like a magnitude 7.0, for example, “Tobin told CNN.

There have been more than 133 earthquakes of magnitude 5.0 or greater in the Blanco Fracture Zone since 1980 and they were never followed by anything on the ground, according to seismologist Dr. Lucy Jones.

“Today’s earthquakes can be considered something of a main shock and aftershock swarm, the only distinction being that in this case, there is not much difference in magnitude between them,” Tobin said.

Although this week’s earthquakes have raised concerns that the so-called “big” could be close, Tobin said that is not necessarily the case.

“There is a lot of distance between these earthquakes and the Cascadia subduction zone,” he explained. “Our current best knowledge of how stress is transferred through the crust (and mantle) would suggest that these events do not change stress in the subduction zone appreciably.”

Although there were a notable number of 5.5+ magnitude earthquakes in a short period of time, he noted, it is not inherently alarming but seismologically interesting.