By Jake Ellison
The devastating megathrust earthquake that struck Alaska 50 years ago 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.
And we all thought the 6.8 Nisqually earthquake of 2001 was big … well look at these photos from the 9.2 magnitude Alaskan megathrust quake:
Damage from the Great Alaska Earthquake of 1964:
Luckily these megathrust, magnitude-9 quakes happen only every few hundred years, so Alaska should be fine for many more decades. Unlucky for this part of the Northwest, however: The last Cascadia megathrust was a few hundred years ago … so we’re due.
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 Alaska megathrust “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 (in 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 or fiords.
The 1964 Alaska tsunami was the second-largest ever recorded, again following only the one caused by the 1960 Chile earthquake (4 meters, more than 12 feet, at Sitka). Of the 119 deaths attributable to the effects of the ocean, about one-third were due to the open-ocean tsunami: four at Newport Beach, Ore.; 12 at Crescent City, Calif.; 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. …
Interslab event. Pacific plate is sliding down beneath our feet. It bends and breaks sort of like when you would bend a Snickers bar and get cracks on the top of it. We could potentially have unto a magnitude 7 of that type of earthquake. The Nisqually earthquake that occurred in Washington 13 years ago was that type of quake.
– Peter Haeussler, research geologist and Alaska coordinator for earthquake hazards, U.S. Geological Survey
And the second response was by Michael West, the state seismologist and director of the Alaska Earthquake Center at the University of Alaska at Fairbanks:
A magnitude-9 earthquake is inconceivably large. These are very, very rare but massive events. In Alaska, because we have one of these in our history, … because we have this, we tend to relegate everything else to the status of lesser earthquakes, and I think that’s really dangerous.
Other places in the world, a magnitude 6, a magnitude 7 does tremendous devastation and that can certainly happen here.
(We can have earthquakes like the Nisqually quake of 2001) that had, on the grand scheme of things, modest impacts for, really, a phenomenally large earthquake. The reason nothing catastrophic happened during that earthquake or during the magnitude-7.5 earthquake last year in the southeast (of Alaska), the biggest earthquake in a decade in the state, the reason these didn’t have catastrophic impacts is merely one of chance.
They just didn’t happen to line up with the population, but the danger in this is we begin to think, ‘Oh, you know we’re Alaskans. We’re tough. We can handle a magnitude-7 earthquake.’ And, I think that is a very dangerous way of thinking.
NW earthquake photos:
Risks to Seattle?
The city of Seattle is in the process now of ameliorating risks to our older buildings. The city’s Department of Planning is mid-way through a two-year process of identifying the buildings – about 800 — at risk in the city and drafting ordinances that will require they be retrofitted to better survive a quake.
Here are some details from the city’s FAQ:
Unreinforced masonry buildings have proven over the years and around the world to be the most vulnerable buildings in an earthquake. …
URMs are the brick buildings commonly seen in Seattle’s older neighborhood commercial cores, such as in Pioneer Square, Chinatown/International District, Columbia City, Capitol Hill and Ballard. Most of the URMs constructed in Seattle were built before 1940 when seismic reinforcement was not required by the building code. These buildings were originally built without steel reinforcement and with inadequate ties and connections between building elements.
The Department of Planning and Development (DPD) estimates there are around 800 URM structures in Seattle based on a “sidewalk survey” of buildings in the city. Of these we estimate that 10-15% may have been retrofitted to some degree but may not meet the proposed retrofit standard. Citywide field observation efforts included both public and private buildings. Single-family residences were not included in this observation effort.
And, in October we wrote … Study: 8,000 Seattle buildings face landslide threat in big quake:
… the Bulletin of the Seismological Society of America … found that damage from earthquake-triggered landslides will be worse and more widespread in Seattle than previously thought.
The study focused on the Seattle Fault, a 30-mile fracture that runs east-west through Seattle, under CenturyLink Field and over to Issaquah. Capable of inflicting mass damage, it’s due for another rupture, but no one knows when.
“A major quake along the Seattle Fault is among the worst case scenarios for the area since the fault runs just south of downtown,” said Kate Alltstadt, a University of Washington doctoral student and co-author of the study.