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Recent earthquakes remind coastal residents to be prepared

Clusters are not correlated with a Cascadia Subduction Zone quake

By Brenna Visser

EO Media Group

Published on November 7, 2018 12:01AM

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

U.S. Geological Survey

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

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The star in the lower left corner of the map shows the location of a recent 4.5 earthquake.

U.S. Geological Survey maps

The star in the lower left corner of the map shows the location of a recent 4.5 earthquake.

Buy this photo

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

In October, 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 were registered multiple times throughout the summer, including a 6.2 quake in late August.

While seismic activity off the Columbia River 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 our area, 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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