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Recent earthquakes remind the North Coast to be prepared

The clusters are not correlated with a Cascadia Subduction Zone quake
By Brenna Visser

The Daily Astorian

Published on November 9, 2018 1:00AM

A map shows the location of a 6.2 earthquake off the Oregon Coast in August.

U.S. Geological Survey

A map shows the location of a 6.2 earthquake off the Oregon Coast in August.

The star in the lower left corner of the map shows the location of a 4.5 earthquake on Wednesday off the Oregon Coast.

U.S. Geological Survey

The star in the lower left corner of the map shows the location of a 4.5 earthquake on Wednesday off the Oregon Coast.

Since mid-July, several earthquakes have rumbled off the Pacific coast, an uncomfortable reminder to get prepared for the Big One.

This month, a cluster of quakes ranging from magnitude 6.5 to 6.8 hit off Vancouver Island in British Columbia, with a 4.5 magnitude temblor striking 171 miles and 6 miles deep off the southern Oregon Coast shortly after. Similar earthquakes near Bandon have registered multiple times throughout the summer, including a 6.2 quake in late August.

While seismic activity along the North Coast has been relatively quiet, some of the shaking — and the fear of whether these earthquakes are indicators that a Cascadia Subduction Zone disaster is coming — has been felt by residents.

Though recent earthquakes have received a lot of attention, their occurrence should not be cause for any more alarm than usual, local geologist Tom Horning said.

Earthquakes with magnitudes more than 4.0 often come in clusters about once every six to 18 months, Horning said.

“This always comes up,” said Horning, who serves on the Seaside City Council. “You’ll likely be talking to me again in two years.”

The cluster happening near southern Oregon is along the Blanco Fracture Zone, a transform fault known to have frequent seismic activity. Because there are no fault zones directly pointed near the North Coast, there is less constant stress. Consider it “nature’s way of dissipating friction,” Horning said.

“It’s hard to reconcile what’s happening in Vancouver with the center part of the subduction zone where we are,” he said. “It’s only an academic exercise to estimate how much stress could be piling up or not as a result.”

If anything, the fact the Cascadia Subduction Zone fault is so quiet is more notable, said Evelyn Roeloffs, a geophysicist from the U.S. Geological Survey.

“We can’t make any association with earthquakes out in the ocean like (the ones) near southern Oregon and the timing of when earthquakes might hit closer in the coastline,” she said. “Our subduction zone is unusual because we had large earthquakes in prehistoric times, and we expect to have a big earthquake in the future … But it’s so quiet now. That’s more the mysterious thing.”

While the types of earthquakes and the frequency are not unusual, the amount of interest taken by the public and the news media has changed in recent years, Roeloffs said. Most calls or inquiries are usually timed when earthquakes hit in clusters. But public education surrounding the 9.0 earthquake expected to rock the Cascadia Subduction Zone appears to have played a role in the increase of calls or reports.

“After one of the earthquakes near Vancouver, 169 people entered a submission to our website to say they felt something,” she said. “It’s good, because I think people are thinking about the reality of earthquake hazards more.”

Horning hopes, if anything, the recent quakes have reminded North Coast residents that the Big One could come anytime.

“This activity should not encourage people to be less alarmed or more alarmed,” Horning said. “You should always be a certain amount of alarmed living here … and you should always be prepared.”


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