Earthquake swarm off Oregon’s coast

U.S. Geological Survey Seismologist Paul Earle told the News-Times the swarm of earthquakes is notable, but not unheard of or cause for alarm.

He said there have been reports of “light shaking” on land, but for the most part, the earthquakes have not been not strong enough and are too distant to impact humans.

The swarm started with a magnitude 4.2 temblor at 5:21 a.m. Tuesday. By Thursday morning, the USGS latest earthquake map listed 15 of magnitude 5 or higher, considered “moderate” quakes. The National Weather Service Tsunami Warning Center has issued a dozen information statements confirming that the quakes do not pose a tsunami danger.

“Magnitude 3 is typically felt — you can feel a magnitude 2 if you’re right on top of it — but these are 400 kilometers off the coast,” Earle said. Earthquakes in between magnitudes 5 and 6 can cause minor damage to structures. Above magnitude 8, called “great” earthquakes, can cause massive loss of life and destroy whole communities near the epicenter.

Earle said seismologists don’t think it’s likely the recent swarm will trigger an earthquake in the Cascadia Subduction Zone — the fault on the eastern side of the Juan de Fuca, about 70 miles from land — and that previous, similar swarms had not done so.

A Cascadia earthquake of magnitude 8 or greater could cause substantial damage on land, but the real danger from a major seismic event on the eastern edge of the Juan de Fuca would be the subsequent tsunami. Experts predict the series of inundating waves would kill thousands in coastal areas. A great Cascadia earthquake in 1700, estimated to have been magnitude 8.7 to 9.2, caused a tsunami that flooded the seaboard on both sides of the Pacific. It’s believed such events occur once every 500 years.

And an earthquake on the Cascadia fault is much more likely to result in a tsunami than one at the location of the current swarm, Earle said, because of the direction the Juan de Fuca is moving in relation to its neighboring plates.

“There’s two reasons we haven’t seen any tsunami activity,” Earle said. “One is that these are not extremely large earthquakes.”

The other is that the Blanco Fracture Zone is a strike-slip fault, similar to the San Andreas, rather than a subduction zone, or thrust fault

“You can generate tsunamis from strike-slip earthquakes, if they’re strong enough,” Earle said, “But since the plates are sliding by each other, instead of one plate subducting beneath the other, you get less vertical offset of the sea floor.”

Earthquakes occur when two tectonic plates move suddenly and violently against each other. At a strike slip fault, the motion is mostly horizontal, but at a thrust fault, where one plate is pushing under another, the edge of one plate thrusts upward. When that occurs beneath the ocean, it raises a massive column of water that ripples in both directions, getting taller as it approaches land.

Strike-slip earthquakes can cause tsunamis at magnitudes 8 or greater. A magnitude 8 temblor is 158.5 times bigger and releases almost 2,000 times as much energy as the strongest quake measured in the Blanco Fracture Zone since Tuesday morning.

While the recent earthquake sequence is not the first observed in the fracture zone, it is somewhat remarkable, the seismologist said.

“We have seen swarms in this area before,” Earle said. “But this is the most active that we’ve seen.”

There’d been 84 earthquakes as of the News-Times print deadline Thursday, and there’s no predicting when the shaking will end.

“This could go on for days or weeks, or it could stop at any time,” Earle said.