“Is The Northwest Next For A Major Earthquake And Tsunami?”

California just had its 11th most powerful earthquake in recorded history. It was a 7.1 earthquake on Friday evening July 5, 2019. The epicenter was about 150 miles north of Los Angeles.  

This is a great time to ask the question: what do Oregon and Washington currently face when it comes to earthquake and tsunami threats?

For the answer, we go to the man I always call first to learn about earthquakes, tsunamis, and northwest mudslides.

You’re looking at him here–in his rock room. It’s former Portland State Geology Professor (and former Geology Department Chair) Scott Burns. 

Here’s part one of my interview with retired Professor Scott Burns which I did after the Japan megaquake in 2011. The assessments he shared still stand.

• Lots of us in Oregon & Washington are watching the Japan quake and tsunami coverage and wondering: “Is the northwest next for a major quake and tsunami?”
  That is a huge question.  Many people are asking that right now and one geologist with no data is saying that the quakes are working around the Ring of Fire from Chile to New Zealand to Japan to the Pacific Northwest.  There is no data to support this. In 1960 we had similar large quakes, but the Pacific Northwest had no large quake.  We are at the plate boundary between the Juan de Fuca Plate and the North American Plate – the Pacific Plate is not touching us so there is no major concern.  Could “the big one” happen tomorrow – yes, but that is not connected to the other quakes.  I am not “extra worried”.
• So you’re not extra worried but you still believe we’ll see ‘the big one’ off the Oregon Coast someday in the Cascadia Subduction zone. Is this overdue? And how big with this mega-quake be?
  We have had the “Big One” on the average of every 500 years for many, many years. The Big One is a subduction quake which will happen because the Juan de Fuca Plate is passing under the North American plate and the earthquake will be generated at the boundary.  These are on the magnitude of 8.6-9.2.    It will happen again. The last one was on January 26, 1700 at 9:30 PM.  It broke from California to British Columbia. It could happen tomorrow. It could be later. Chris Goldfinger, a great professor at Oregon State University in geology, has studied this problem really well.  He also finds that the southern edge of the Cascadia margin goes off more often – like every 300 years.  We are long overdue for that one which will be smaller than the 8.5+ whole margin quake.  It might be only 8.0 and break from northern California to southern Oregon.  It would be a big one and would really shake us here in Portland.  I think that “southern Big One” is much more likely in our lives.  We need to be prepared.  The quake in Japan two days ago, the Good Friday quake of  1964, the Chile quake of last year, the Indonesian quake of 2004 are all examples of subduction quakes.  They are our equivalent “Big Ones”.  We see those – that is what will happen.
• We know from history that quake will likely cause coastal slides and tsunamis. How long might it take a tsunami to reach the coast?
  The rule of thumb that we tell people when we have a subduction quake along our coast is that the tsunami will arrive (if one is produced – not all past subduction quakes have produced tsunamis) in 20 to 30 minutes after the shaking stops. We tell people if they are knocked to the ground by a large quake on the coast and it lasts 2-3 minutes, then it is a subduction quake and one needs to get to high ground (over 50 feet elevation) in 20 minutes.  Sometimes the high area is the top floor of your building if you cannot get to an official “tsunami evacuation route”.Part 2 of my interview with Professor Scott Burns is tomorrow. I’ll be asking him about the impact to the I-5 Corridor from our next subduction zone quake (the “Big One”) off the Oregon and Washington Coast. Plus, remember the Spring Break Quake? Are we due for a repeat? That’s in tomorrow’s post.