Quakes Off Oregon Coast, on Rainier Are Typical and Wouldn't Have Triggered Forthcoming ShakeAlert System

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A cluster of small earthquakes far off the southern Oregon coast and atop Mount Rainier in recent days are common events, and would not have triggered a seismic warning system set for a test run later this week ahead of its rollout to the public this spring, a seismologist said.

"Neither of those sequences are of particular concern," said Paul Bodin, research professor at University of Washington and manager of the Pacific Northwest Seismic Network.

The ShakeAlert system, which is designed to give people and automated systems advanced warning before significant shaking starts, would not have sent an alert for either sequence "because no one would've felt them," either because of their small magnitude or distance from population centers, Bodin said.

A handful of quakes, the largest measuring magnitude 5.1, struck about 175 miles west of Bandon, Oregon, on Saturday, with smaller aftershocks Sunday morning. These occurred on a structure called the Gorda Escarpment, at the boundary between the Pacific and Juan de Fuca plates — a strike-slip plate, which, unlike the Cascadia Subduction Zone fault, does not pose a major tsunami risk.

Mount Rainier, meanwhile, let go "a volley of really tiny little earthquakes," Bodin said, calling it a "swarmlet." The quakes, on Wednesday, Thursday and Friday, and measuring no more than magnitude 2.0, are believed by volcano scientists to stem from the movement of hydrothermal fluids that may lubricate faults and release tectonic stresses, he said.

He called both sequences normal and common. Neither changes the likelihood of the kind of major, damaging earthquakes that are overdue in the region.

(And they're nothing like the recent show put on by Mount Etna, one of the Bodin's favorite volcanoes and one of the world's most active. "I just love pictures of it when it's erupting, and there's actually rivulets of lava on it," Bodin said, describing it as "an attractive danger.")

On Thursday, the Washington Emergency Management Division and U.S. Geological Survey will test ShakeAlert, sending a test message at 11 a.m. to wireless devices opted-in to the system in King, Pierce and Thurston counties. The test comes almost exactly 20 years after the magnitude 6.1 Nisqually quake.

The ShakeAlert system, which has been operational in California for nearly a year, is set to become broadly available to people in Oregon in March and Washington in May, after years of development. Information on how to opt-in can be found here.

The system is designed to provide a few seconds or more of advance warning before shaking begins. That's enough time for a person to take protective action, such as ducking and covering, to "really reduce the chance of stuff falling on you, or you falling, which causes the majority of injuries in earthquakes," Bodin said.

Automated systems can also receive the alerts, triggering actions that could protect systems such as municipal water supplies from the forthcoming shaking. Water and gas valves can be shut, fire station doors opened, trains slowed, Bodin said.

"It's kind of prosaic, but it's going to be very important," he said. "It will absolutely save lives and misery and cost after the next big earthquake."

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