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Wednesday, August 11, 2010

Not what you want to see

Earthquakes of an odd type, somehow tied to the Cascadia subduction zone. Scientists don't have a good explanation. Let's hope these tremors turn out to reduce, not increase, danger.

Comments (11)

It's around the anniversary of our purchase so I just renewed the earthquake and flood policies on the old homestead -- Dang that hurt! Earthquake alone is up to half a mortgage payment with a 10% deductible! But we are so overdue for a big one in this region that it would be hard for me to go without.

I'm not so worried about the flooding actually getting us directly - I buy flood insurance because it's the only way you can get insured for sewer backups that fill the house with finless brown trout and a stench worse than a Merritt Paulson deal.

I've been learning a lot about this Cascadia fault, and it's a real trip. When they say the last mega-quake happened in 1700, it sounds like an estimate - but it isn't. it was on January 26th and they know the date from reports of when the "orphan" tsunamis hit Japan. This name refers to tsunamis arriving with no shaking of the earth first - meaning the event was a long way off.
Then there's the part about these quakes being 9's on the Richter Scale and lasting for 5 minutes because of the length of time the entire fault needs to slip. Let me repeat that: 5 MINUTES.

Finally, there's the Ghost Forest of Washington State - a bunch of trees that all died as a result of the quake of 1700 -not from any sudden shock, but from being submerged in salt water after the land they were growing on dropped 8 feet.
Let me repeat that: The land on this part of the coast dropped 8 FEET.

When you start hearing terms like "orphan tsunamis" and "ghost forest", you know you're way up the list of natural disasters. Humans get very poetic and profound when they're facing something this big.

Bill -

Ghost forest ...trees dead from land drop into salt water innundation - Siletz bay, Oregon coast... you can see it on the left (east) side of 101 as you head south from Lincoln City...

Do you know how old those trees are? Neskowin Beach in Oregon has dead upright trees from 1,700 to 2,000 years old - presumably from other earthquakes.
The ghost forest by Washington State's Copalis River is significant because it's from the last big Cascadia event. They can look at the tree ring pattern and tell that all these trees made their last ring in 1699. It's really quite fascinating.

Another part of the story is the exposed layers of ground where you can see evidence of the last 4 or 5 quakes. The 1700 Quake created a very stark contrast. There was a dark layer of peat from the forest, followed by a white layer of sand, deposited very quickly as the big waves - going hundreds of miles-an-hour - hit the shore.
One positive: The gaps between quakes changes, but if I remember correctly from one video about this, the bigger time spans were further back. As I recall, the recent ones had more of the 300-year gaps between events.
Right now, we're sitting on 310 years, 6 months, and just over 2 weeks.
We're due.

I have to wonder if very slow temblor propagation has anything to do with underground magma intrusion. The really big cascade volcanoes are pre-ice-age, but 10,000 years is an eyeblink in geologic time.

Here's an earthquake candidate running for mayor in Tigard.


She says we're in big trouble.

Don't know anything about her but reccomend support anyway because the current mayor thinks light rail on Barbur and H99 would reduce congestion and spur developoment.

I can't imagine the madness of having all that infrasructure seen on North Interstate stuck on Barbur/H99.

Wow, kudos to her. Of course, she's suggesting that the politicians lift their heads from the laps of the developers they're "partnering with" long enough to give a rip about the hoi polloi, but hey, everyone can dream.

When I worked for the Bureau of Land Management in SW back in the late Nineties, I was honestly surprised at the lack of any kind of earthquake disaster plan. You go to Florida, and the hurricane evacuation routes are mapped out and ready to go, but nothing on an active fault zone on what to do when the earth starts shaking. I talked with a couple of geologists at the BLM about this, and they told me that not only was there no plan, but the developers had seizures every time someone started talking about constructing one. Combine that with the reasonable possibility of Mount Hood going active in our lifetimes, and I got scared.

Now, what makes things even more interesting from a Cascadia point of view is that the oceanic plate slipping underneath North America is scraping off the layers of mud above it while doing so. It's much like scraping mud off your shoe. One of the reasons why the Portland area hasn't seen catastrophic earthquakes since 1700 is that this mud acts as an insulator, allowing the oceanic plate to soften and bend instead of stress and crack. Occasionally, chunks of plate snap, as in 1700, but otherwise you're seeing the plate bend and subduct into the mantle. Said subduction and the release of carbon dioxide as the rocks are melted in the mantle leads to the volcanic activity seen along the West Coast, but that's a completely different dangerous vision.

Earthquakes of an odd type -- I keep hearing "rodents of unusual size" when I read that phrase.

Rainer is overdue, too, I think.

A lot of people see Mt. Hood as peaceful but it had a volcanic event in the early 1900s. It could wake up anytime and if Rainer blows, we'll all be drinking lattes with lava in them.

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