Saturday, November 28, 2020
Nov. 28, 2020

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Similar quake due to hit Northwest

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The quake that rocked Japan and swamped the Oregon Coast is a mirror image of the tectonic rupture that occurred along the Northwest coastline 311 years ago, local geology experts said.

And our Big One is poised to strike again.

“We’re getting better about people taking this stuff seriously,” said Robert Butler, a geophysicist at the University of Portland. “I credit the 2004 tsunami in the Indian Ocean for a step-up in awareness.”

See how earthquake waves traverse the globe

The Northwest is situated atop two tectonic plates pushing against one another, just like the situation that afflicted Indonesia in 2004 and the quake that devastated Japan on Friday.

In the Northwest, the oceanic Juan de Fuca Plate is diving — or subducting — below the North American Plate in an area known as the Cascadia subduction zone. Unfortunately, the two plates are locked together. As pressure builds, scientists say it’s only a matter of time before the two plates unlock with catastrophic consequences.

This last occurred on Jan. 26, 1700.

Scientists were able to determine the date based on historical records of an unexplained “orphan” tsunami in Japan, as well as carbon dating of buried tree stumps on the Washington coast.

A decade ago, University of Washington researcher Brian Atwater documented coastal shorelines which had suddenly subsided by at least a few feet.

“If you’re in a place which is just barely above sea level and suddenly the land surface drops by a couple of meters, that matters a whole bunch,” Butler said.

Oregon State University marine geologist Chris Goldfinger, who honed a technique for analyzing core samples from the ocean bottom, has documented 41 major ruptures in the Cascadia subduction zone over the past 10,000 years.

Do the math, and that’s a major temblor roughly every 244 years.

“If you’ve been putting off getting your earthquake supply kit ready, this is a good time to be reminded of it,” said Evelyn Roeloffs, a geophysicist with the U.S. Geological Survey in Vancouver. “I think it’s obvious that these things do happen, and they happen without any warning.”

Ruptures of the subduction zone don’t run on a schedule. Scientists estimate they have come every 200 to 800 years.

Roeloffs said scientists put the odds at 1 in 7 that a major subduction-zone quake will roil the Pacific Northwest in the next 50 years.

Shock waves from Friday’s massive quake weren’t felt by humans in the Pacific Northwest.

But the region’s seismographs easily detected the huge jolt and every one of the scores of significant aftershocks that continued to rattle the Japan coast: Sixty, then 80, then at least 100 by midafternoon on Friday.

“We’ve been recording all day long, it just keeps going and going,” said Amy Wright, seismic analyst for the Pacific Northwest Seismograph Network at the University of Washington in Seattle.

More impressive “surface waves” created by such a major quake would traverse the Pacific as they circle the Earth many times, Wright said. Normally, they set off harmless, minor tremors around our Northwest volcanoes, but no such tremors were recorded this time.

Measuring instruments near the Aleutian Island chain in Alaska did pick up many small tremors around the “usual suspects” there, Wright said.

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