Tags: Emerging Threats | cascadia | earthquake | japanese. plate

Understand Tsunamis by Exploring Their True Origins

Understand Tsunamis by Exploring Their True Origins
(Karin Hildebrand Lau/Dreamstime)

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Monday, 20 August 2018 02:40 PM Current | Bio | Archive

The Pacific Northwest is an area subject to some of the greatest seismic risks on Earth —while oftentimes not receiving much notice on the radar screen of the American public’s attention-span.

The fact is though that a massive adjunct of the Pacific Plate, the Juan de Fuca plate, is being thrust beneath the west coast of North America offshore from Northern California, Oregon, Washington, and Vancouver Island, British Columbia — subducted into the bowels of the Earth and creating the Cascadia Thrust Zone.

This titanic manhandling of the Earth’s crust causes the coast of the Pacific Northwest to spring back every now and again, resisting fruitless attempts to drag the entire littoral toward oblivion into the scorching magma beneath the West Coast.

Only a thrust fault like the Cascadia can produce mega-quakes that can range into the magnitude 9.0 to 9.5 categories, as the Cascadia is quite capable of accomplishing.

It has generated such seismic monsters in the past. It will do so again.

We know the last time this occurred was on Jan. 26, 1700, at around 9:00 p.m.

The historic evidence comes, however, from Japanese villages on the other side of the Pacific which were destroyed by the "Orphan Tsunami" caused by this event.

Chronicles in Japan note the previously puzzling history of a tsunami that struck seemingly out of nowhere, just before midnight on Jan. 27, 1700. Ten foot high waves flooded rice paddies, washed away buildings, knocked over lanterns that set fire to twenty houses in one town alone.

Villagers, merchants, samurai and others, primarily in the Japanese prefecture of Iwate, recorded the flooding and damage while taking note that the parent earthquake that should have given rise to the tsunami had not been felt by anyone, causing questions as to what had precipitated the waves and giving the event its name.

The tsunami did have a "mother," of course, only that this one was on the far side of a shared ocean, in North America’s Pacific Northwest. Brian Atwater, Kenji Satake and David Yamaguchi put the clues together and solved the 300 year old mystery in 2005.

By working backward from when the waves struck Japan and calculating the speed at which tsunami waves traversed the Pacific Ocean, they were able to nail down almost precisely when the great Cascadia earthquake had transpired.

Studies of tree rings in Oregon and Washington, along with sediment layers, core samples from the ocean floor offshore, debris samples from earthquake-induced landslides all dovetail (approximately, of course) with this time and date. Moreover, oral traditions of the peoples living in the area when the earthquake struck also support Jan. 26, 1700 as the date of the massive tremor.

Almost all the tribes living in the region have oral histories recounting a great earthquake around this time, far more powerful than any other, and some are buttressed further by temporal clues indicating how many generations ago the event took place.

On Vancouver Island, Kwakiutl and Cowichan stories, and from Washington, Makah and Quileute legends, all tell of a great winter quake, striking in the night-time just as people had gone to sleep — causing landslides that buried whole villages.

The Huu-ay-aht recount that almost every single person living along Pachena Bay on Vancouver Island perished; all drowned in the tsunami save for the inhabitants of the village of Masit, thanks to its location some seventy-five feet above sea level.

A woman by the name of Anacla aq sop, who happened to be away when the disaster occurred, garners a poignant place in the oral traditions as the only surviving member of her community. From the inner coastline of Vancouver Island come these words passed down from Cowichan narrative histories from three centuries ago, "The people could neither stand nor sit for the extreme motion of the earth."

"These stories bristle with information," says Professor Ruth Ludwin, University of Washington geophysicist, concerning landslides, aftershocks, inundations, etc.

It was certainly one of the most powerful earthquakes ever produced on Earth — between magnitude 8.7 and 9.2. There are more than a few seismologists who believe that such a recurrence might not be too far over the horizon; previous earthquakes are estimated to have occurred circa 1300 AD, 800 AD, 400 AD, 170 BC and 600 BC.

Many engineers and safety experts are less certain as to how the Pacific Northwest should stand up to the full brunt of what the Cascadia can dole out at its worst. Almost as concerning is the threat posed by an ancillary fault, the Seattle fault, running directly beneath some of the most populated real estate in the Pacific Northwest.

A great event on the Cascadia fault, or even a lesser one on the Seattle fault, would rival anything ever seen in California.

David Nabhan is a science writer, the author of "Earthquake Prediction: Dawn of the New Seismology" (2017) and three previous books on earthquakes. Nabhan is also a science fiction writer ("Pilots of Borealis," 2015) and the author of many scores of newspaper and magazine op-eds. Nabhan has been featured on television and talk radio all over the world. His website is www.earthquakepredictors.com. To read more of his reports — Click Here Now.

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DavidNabhan
Titanic manhandling of the Earth’s crust causes the coast of the Pacific Northwest to spring back every now and again, resisting fruitless attempts to drag the entire littoral toward oblivion into the scorching magma beneath the West Coast.
cascadia, earthquake, japanese. plate
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2018-40-20
Monday, 20 August 2018 02:40 PM
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