50th anniversary warning: A megathrust quake like Alaska’s 9.2 will hit Seattle
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50th anniversary warning: A megathrust quake like Alaska’s 9.2 will hit Seattle

50th anniversary warning: A megathrust quake like Alaska’s 9.2 will hit Seattle

Posted on March 27, 2014 | By Jake Ellison

The devastating megathrust earthquake that struck Alaska 50 years ago today (Thursday) is a pretty good indication of what’s in store for Seattle and the upper regions of the West Coast when the Cascadia Subduction Zone lets one fly.

Luckily these megathrust, magnitude 9 quakes happen only every few hundred years, so Alaska should be fine for a many more decades. Unlucky for this part of the NW, however, the last Cascadia megathrust was a few hundred years ago … so we’re due.

THE CASCADIA SUBDUCTION ZONE: The geography of northern California, Oregon, Washington, and southern British Columbia is shaped by the Cascadia subduction zone, where the North American Plate collides with a number of smaller plates: the largest of these is the Juan de Fuca Plate, flanked by the Explorer Plate to the north and the Gorda plate to the south. These smaller plates “subduct” (descend) beneath the North American Plate as they converge along a 700-mile long (1,130 km) boundary. A large portion of the boundary between the subducting and overriding plates resists the convergent motion, until this part of the boundary breaks in a great earthquake. Above: Schematic view of the source area for the largest Cascadia earthquakes. (Image adapted from U.S. Geological Survey Professional Paper 1707 (page 8), Atwater et al., http://pubs.usgs.gov/pp/pp1707/)

(Click for larger version) THE CASCADIA SUBDUCTION ZONE: The North American Plate collides with a number of smaller plates: the largest of these is the Juan de Fuca Plate, flanked by the Explorer Plate to the north and the Gorda plate to the south. These smaller plates “subduct” (descend) beneath the North American Plate as they converge along a 700-mile long (1,130 km) boundary. A large portion of the boundary between the subducting and overriding plates resists the convergent motion, until this part of the boundary breaks in a great earthquake.
(Image adapted from U.S. Geological Survey Professional Paper 1707 (page 8), Atwater et al., http://pubs.usgs.gov/pp/pp1707/)

On the 314 year anniversary of the last one, the Cascadia Region Earthquake Workgroup published an updated scenario document for what that magnitude of quake would do to us now. The group said in a news release:

“Cascadia’s last great earthquake occurred on January 26, 1700 and ‹stresses have been building on the fault ever since. While the full extent of the earthquake hazard was not realized until the 1980s, the Cascadia subduction zone is now one of the most closely studied and monitored regions in the world.”

And no wonder.

As we wrote in June, the Cascadia Subduction Zone running the length of the coast from northern Vancouver Island down to California last slipped and shook the surface of the Earth 300 years ago, and that was just the latest of 22 such quakes in the past 11,000 years.

The latest megathrust to hit this part of the world “lasted approximately 4.5 minutes and is the most powerful recorded earthquake in U.S. history. It is also the second largest earthquake ever recorded, next to the M9.5 earthquake in Chile in 1960,” reports the USGS.

That agency has put together a bunch of information around that quake under the title: The Great Alaska Earthquake and Tsunami of March 27, 1964.

That quake’s damage was severe:

  • The area where there was significant damage covered about 130,000 square kilometers (about 50 square miles). The area in which it was felt was about 1,300,000 square kilometers (502,000 square miles or all of Alaska, parts of Canada, and south to Washington).
  • The four minute duration of shaking triggered many landslides and avalanches. Major structural damage occurred in many of the major cities in Alaska. The damage totaled 300-400 million dollars (1964 dollars).
  • The number of deaths from the earthquake totaled 131 — 115 in Alaska and 16 in Oregon and California.
  • The death toll was extremely small for a quake of this magnitude due to low population density, the time of day and the fact that it was a holiday, and the type of material used to construct many buildings (wood).
  • Much of the damage and most of the lives lost were due to the effects of water waves. These were mainly of two kinds: the tsunami of open-ocean sea wave, generated by large-scale motion of the sea floor; and the local wave, generated by underwater landslides in bays of fiords.
  • The 1964 Alaska tsunami was the second largest ever recorded, again following only the one caused by the 1960 Chile earthquake (4 meters at Sitka). Of the 119 deaths attributable to the effects of the ocean, about one-third were due to the open-ocean tsunami: 4 at Newport Beach, Oregon; 12 at Crescent City, California; and about 21 in Alaska.

Video: This USGS recount of the quake is great and will give you all you need to know. (There is a longer version here and a fact sheet pdf has been added below.)

At a news conference held Tuesday at the U.S. Geological Survey Alaska Science Center in Anchorage, two officials answered our question with their lens mostly focused on Alaska. We asked, How do you describe the risk of another megathrust earthquake?

(In Alaska, (these quakes on average come) 600 years apart, 330 years is the shortest time.) So we think it probably will be awhile before the next really big one here in south central Alaska.

That said, there are other potentially significant earthquakes that we don’t know about. …

via 50th anniversary warning: A megathrust quake like Alaska’s 9.2 will hit Seattle – The Big Science Blog.

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