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The BIG “Big One”

Published by Steve Campion. Category: Geography & Geology

Californians know earthquakes all too well. But where do you suppose the largest known earthquake struck the United States’ Lower 48?  It was not in the Golden State, where the San Francisco quake of 1906 agitated the needle at magnitude 7.8.  That’s powerful, to be sure, but there was an even bigger “Big One” in the Evergreen State.

It was on this date many years ago — January 26, 1700 — that a megathrust earthquake tore a 600-mile crack in the seafloor off the Northwest coast, rattled the inhabitants of the region with a 9.0 temblor, and sent a tsunami across the Pacific that flooded villages in Japan.

Never heard of it?  Few people had until recently when an international team of scientists and scholars pieced the evidence together.  Their research, in fact, is a fascinating story in itself.  But the quake DID happen and — are you sitting down? — will likely happen again.

The ever-churning molten material that lies beneath the earth’s surface puts pressure on the large tectonic plates that blanket the planet, crunching one against another or one beneath another. The giant rocky plates might snag for decades or centuries or millennia, but the pressure builds until the massive plates can move past the obstruction and release the ground-shaking energy we know as an earthquake.  Washington sits next to the Cascadia Subduction Zone where the enormous Pacific plate rubs against the Juan de Fuca plate which, in turn, is being pushed beneath the North American plate. Subduction zones like this have produced the largest seismic events ever recorded: megathrust earthquakes.  Examples: Chile in 1960 and 2010, Alaska in 1964, Sumatra (Indian Ocean) in 2004.  And though the Northwest hasn’t had one in a while — 312 years — our area might be due for another.

FACTS ABOUT THE CASCADIA MEGATHRUST EARTHQUAKE OF JANUARY 26, 1700

  • At 9 pm on Jan 26, 1700, a rupture began along the Cascadia Subduction Zone a few dozen miles off the coast of Washington and Oregon.
  • Within seconds, the break ripped along 600 miles paralleling the coast of northern California, Oregon, Washington, and southern Vancouver Island in British Columbia.
  • The magnitude of the quake was estimated to be between 8.7 and 9.2.
  • The shaking was intense and lasted for perhaps five minutes.
  • The ground shifted about 50 to 60 feet.
  • A tsunami was created and it immediately began moving at high speed in all directions from the rupture on the sea floor.  The sea rose thirty or more feet on the Washington coast, and even caused a noticeable surge in Japan on the other side of the Pacific Ocean.

Here’s how we know about it:

  • In the 1970s a team of Japanese scholars began a detailed study of historic tsunamis that had been observed and written about in various village records and personal narratives.  This was an astoundingly complex project accessing old and rare archives from all over the country.
  • The team was able to assemble timelines and match the tsunamis to known earthquakes.
  • One tsunami was an “orphan.”  It had no identifiable origin.  Many coastal villages recorded a tsunami and major flooding after midnight and into the morning of January 28, 1700, but there was no corresponding earthquake.
  • In the 1990s, Kenji Satake of the Japanese team and American scientist Brian Atwater met and compared notes.
  • There was some evidence of a major earthquake in the Northwest region of North America in the decades either before or after 1700.
  • As further scientific studies were conducted and analyzed over a decade, that wide date range began to narrow.
  • The Makah people in Washington had experienced great flooding.  Written records from the mid 19th Century tell of oral traditions in the “not very remote past.”  The Hoh Indians told stories about a great shaking of the earth.
  • Oral traditions of the First Nations people on Vancouver Island told of landslides and collapsed houses among the Cowichan.  Winter villages had been destroyed with no survivors.
  • Soil samples revealed deposits consistent with major flooding.  Some sediments dated to about 1700, revealed human campsites smothered beneath layers of sand consistent with sudden burial seen at other tsunami sites around the world.
  • Ocean floor core samples pointed to shifting and massive underwater landslides about 1700.
  • Ghost forests (i.e., still-standing dead trees) on the coast gave scientists the chance to date their demise with radiocarbon tests.
  • Tree ring studies showed that a significant number of red-cedars in coastal marshlands were killed between August 1699 and May 1700.
  • Careful reconstruction of events in Japan (according to the historical documents) permitted scientists to determine the direction, height, and timing of the orphan tsunami as it hit each village and the direction from which it came.  The results pointed back across the Pacific Ocean to a massive megathrust earthquake at 9 pm on January 26, 1700.

Here’s what you might NOT like to think about (but probably should):

  • Unlike earthquakes that are more localized and shake for a minute or less, megathrust quakes rip along fractures hundreds of miles long and shake violently for several minutes.
  • If Cascadia ripped today, it could rock Washington, Oregon, and parts of California and British Columbia all at once.
  • Coastal areas in the Northwest (think Cannon Beach, Long Beach, Westport, Ocean Shores) would be swamped by the tsunami within minutes.
  • The large metropolitan cities of Portland, Seattle, and Vancouver would be shaken for several minutes.  No one really knows how modern skyscrapers would fare against an earthquake of that power and duration.
  • Evidence shows that the Cascadia fault has ripped more than 40 times in the past; about 7 times in the last 5000 years.
  • That means the Northwest has a powerful megathrust earthquake about every 300 to 600 years.
  • It’s been 312 years since the last one.
  • Have a nice day.

SOURCES: If you want the nitty gritty science that went into tying the Cascadia earthquake to the orphan tsunami of Japan, go to the source.  The Orphan Tsunami of 1700: Japanese Clues to a Parent Earthquake in North America (2005) was written by Brian Atwater, Kenji Satake, and the international team of scientists who pieced this together.  An engaging book on this topic written for a popular audience is Cascadia’s Fault (2011) by Jerry Thompson.  Chapter 17 is specifically about the detective story that found the connection.  You’ll also find some resources on the websites of the USGS and Natural Resources Canada.

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2 Responses to “The BIG “Big One””

  1. Joan Says:

    That’s food for much thought. Have a nice day? RIGHT!

  2. listguy Says:

    Here’s an article on the latest research on Cascadia using data from the recent earthquake in Japan:
    http://mynorthwest.com/11/631122/Experts-evaluate-earthquake-potential-off-coast