Cascadia Subduction Zone

Fast facts about the Cascadia Subduction Zone:

• The Cascadia Subduction Zone runs from the northern most part of California into southern British Columbia, Canada
• It has a history of rupturing along the entire subduction zone, resulting in massive M9+ events all along the Oregon and Washington coasts
• The last such Cascadia megathrust earthquake was on January 26, 1700, at approximately 9pm, and was a M9+ event
• It caused tsunamis in Japan and North America, and submerged & flooded coastal forests in Oregon and Washington
• The Cascadia Subduction Zone is nearly a twin of the Sumatra Subduction Zone (which caused the devastating 2004 Indian Ocean tsunami, produced a measurable wobble of the Earth, and left a gravity scar in the earth.)
• A Cascadia megathrust earthquake could last up to five minutes
• The subduction is causing the land between the coast and the Cascade mountains to bow upward; when a great quake hits, inland areas (like Portland) could lose as much as 6 feet elevation in less than five minutes, as the bow is released
• During these Cascadia earthquakes, the whole fault is in motion; it isn't a pinpoint event like most crustal earthquakes; all western Oregon and Washington will experience terrific shaking
• Aftershocks could themselves be major earthquakes, capable of substantial damage, further compounding the destruction; a large quake can produce hundreds of aftershocks, most of which occur in the weeks immediately following the event, often starting just a few hours after the initial event
• Although the average between Cascadia earthquakes is 500 years, that number is misleading
• Cascadia earthquakes come in clusters of typically three to five M9+ earthquakes, spaced approximately 300 years apart
• A long quiescent period between clusters typically lasts from 500 to a little over a 1000 years
• The 1700 earthquake was about the fourth event in its cluster; we're either overdue for the next one, or in a quiescent period
• New evidence suggests the southern half of the Cascadia subduction zone also produces M8+ events even during quiescent periods between the M9+ clusters; these quakes will shake Portland, but Seattle not as much
• If we include the smaller M8+ Oregon Cascadia earthquakes, the average between quakes shrinks to only 250 years
• The duration of our current quiet period has already exceeded 75% of the quiet periods between quakes in Cascadia's history
• Chris Goldfinger of OSU, who has studied Cascadia extensively, now says he expects either a M8+ or M9+ quake before 2060
• Portland's building codes are designed for M7 "major" quakes
• A M9 "great" quake is over 1000 times more energetic than a M7 quake
• A M8 "great" quake is about 32 times more energetic than a M7 quake
• Western Oregon's seismic codes aren't as strong as California's, which in turn aren't as strong as Japan's or Chile's
• A M9+ Cascadia earthquake will damage cities and towns between the Cascades and the coast, from Canada to California
• It's doubtful that FEMA is prepared for such an event
• A 72-hour kit isn't going to be enough after a Cascadia earthquake
• Earthquakes often liquify landfill or alluvial soils; most of the damage and loss of life from the M7.1 Loma Prieta (aka “World Series”) earthquake occurred on such soils, and it only lasted 15 seconds
• Earthquakes don’t kill many people; it’s the structures people are in, on, or around which kill most people
• Portland’s weak seismic codes will increase the death toll
• Cascadia will continue to kill people days, weeks and perhaps months after the initial event by depriving them of shelter, food, water, medicine, heat, etc.
• Many people get seriously injured because they don't know what to do (or not do) during an earthquake; advance training is your best protection during an earthquake, and for surviving the aftermath
• Briefly… during an earthquake: running is likely to result in serious injury; leaving a building may be fatal; sheltering in doorways may actually be dangerous (and no longer recommended). Get CERT/NET training so you know what to do.

The Japanese Seismic Analog to Cascadia

We can expect more damage and destruction in the Pacific Northwest from a large (total rupture) Cascadia megathrust earthquake than Japan experienced in their 2011 Sendai (Honshu) quake for two reasons: Cascadia routinely ruptures along a much longer fault, and our seismic building codes are significantly weaker than Japan’s (our building codes only anticipate a M7 quake, whereas Japan’s codes anticipate M9 quakes).

The Nova program “Japan's Killer Quake” documents the awesome power and destructiveness of a M9 earthquake. It also contains a short discussion about the Cascadia subduction zone and our region. The show is well worth watching (and you are encouraged to follow the link); it's a wake-up call to the Pacific Northwest.

While much of the graphic devastation in Japan was due to the tsunami, it’s important to remember that Japan’s much stronger seismic codes protected their inland areas in ways our inland areas won’t be. Their buildings are far more resilient and earthquake resistant than ours. A Cascadia quake is likely to do a great deal more damage inland in our region than the Sendai quake did to inland Japan.

Clearly our coastal areas will experience large tsunamis every bit as powerful as Japan’s. What is unclear is how far up the Columbia River a tsunami wave might travel, and how much water it might carry. Also unclear is whether Columbia River damns might breach. Portland has enough to worry about even if we don’t ponder breaching damns and monster inland tsunamis (scenarios which could occur, but hopefully won’t).

We already know that Cascadia quakes of the past — such as the 1700 quake — have caused costal areas to subside and flood. Japan experienced this in the 2011 Sendai quake (it was a factor in some of the tsunami wall breaches). We would be fools not to consider the Sendai quake as a preview of what to expect in the Pacific NW. We need to prepare, become better educated about hazards and how to respond, and we need to build stronger, more resilient buildings and structures.

How The Earth Was Made

An episode of the History Channel’s How The Earth Was Made TV series, titled Tsunami, has an in-depth discussion of the Cascadia subduction zone, an account of the history of the initial research into its geology, and graphically illustrates the magnitude of threat it poses to our area. Follow the link to watch the full episode.

[Embedded on the right is a segment of a Nova episode: Deadliest Earthquakes. Follow the link to watch the full episode.]