earthquakes in Portland? (Medford, St. Helens, Rainier: HOA, house, buying)

[Slowink]

My wife and I are retired and contemplating a move to the Portland area. We were thinking about Medford, but after reading the forums in that area, no thanks. We did notice that alot of the people said move to Portland. That's all well and good, but what about earthquakes. Is there an area where one can buy a couple acres and build a foam and steel home? We have lots of realatives in Washington State, but I had enough of that state. Can anyone give me a heads up?

[PNW-type-gal]

Originally Posted by Slowink

My wife and I are retired and contemplating a move to the Portland area. We were thinking about Medford, but after reading the forums in that area, no thanks. We did notice that alot of the people said move to Portland. That's all well and good, but what about earthquakes. Is there an area where one can buy a couple acres and build a foam and steel home? We have lots of realatives in Washington State, but I had enough of that state. Can anyone give me a heads up?

You can build a foam and steel home pretty much anywhere in Oregon - it's a well-documented technology. The only exception might be a benighted HOA with out-of-date rules that wouldn't permit it. Buying a couple of acres gets harder as you get closer to Portland proper, easier as you get away from the metro areas.

As far as earthquakes -everywhere on the west coast (read as: this side of the Rockies to the sea, Mexico to Alaska) is considered earthquake country, with local faults, regional faults and then the possibility of the larger subduction zone quake. In general, the further you are from the guesstimated epicenter of the big subduction quake the less the potential damage, based on local conditions, of course.

[Nell Plotts]

New construction in most areas west of the Cascades must meet California earthquake standards. Your home is more likely to be at risk from supersaturated soils that slide.

If you are nervous about earthquakes don't visit San Francisco metro, Los Angeles, St. Louis, Hawaii, Seattle or Portland... or anyplace in-between. Ahh... don't drive or cross the street either.

Always take judicial risks but in many ways life is a throw of dice.

[philwithbeard]

The City of Portland is part of the Boring Lava Field. A bunch of ancient volcano vents from several million years ago. The biggest is Mt. Tabor, what is left of a 'cone-shapped' volcano, and the only one inside the city limits of any any City in USA. BTW: Boring is only the name, not an editorial comment on how fast you will fall asleep reading about it.
CVO Website - Boring Lava Field, Portland, Oregon

and Wikipedia:
Boring Lava Field - Wikipedia, the free encyclopedia

With that said, about and hour, or hour 1/2 away from Portland is Mt. Hood. Mt. Hood is of course an ACTIVE volcano. Mt. St. Helens is a sister mountain to the North of Portland, and further north is Mt. Rainier outside of Seattle Washington another active volcano. Read somewhere that the next eruption isn't due for something like 70 years, and the massive pyroclastic ash and blast (like Mt. St. Helens) will be directed due south of Mt. Hood.
Mount Hood--History and Hazards of Oregon's Most Recently Active Volcano, Volcano | USGS Fact Sheet 060-00

Yes, Portland will have mild earthquakes when Mt. Hood unleashes, but you should be more concerned with very hot dust, ash, and such fouling the air in 70 or so years (give or take a decade or two.) Stock up on painter's dust masks from Home Improvement Big Box store.

*****************

Several miles off the Oregon Coast is the Cascadia Subduction Zone. Do you remember the great Thailand Tsunami back in December 2004? Yes, that kind of earthquake.
Cascadia Megathrust Earthquakes (http://www.pnsn.org/HAZARDS/CASCADIA/cascadia_zone.html - broken link)
Google on your own about the risk of Earthquake vs flooding rolling up the Columbia River from the Tsunami induced ocean wave.

I have a child like belief that the Cascadia Subduction Zone won't have a major event which will affect Portland for several hundred years. (Seattle and Southern Oregon coast could be another story!)

***********************

The soil in and around Portland is several feet of dust and ash from volcano activity over the last million years. There is some threat of loose soil erosion in specific areas around Portland, which can cause home damage.
Landslide hazards in Oregon

This is of course very rare. Check out the interactive landslide map on the link at this site:
Introduction - SLIDO - Statewide Landslide Information Database for Oregon (SLIDO) - Oregon Department of Geology and Mineral Industries


I know, I know, way too much information.

Phil

[hamellr]

New geological research shows that the Portland area gets a 9.0+ Earthquake about every 130-150 years. The last one was about ~140 years ago.

If that hits, Portland will no longer be an Utopia for anyone.

[PNW-type-gal]

Originally Posted by hamellr

New geological research shows that the Portland area gets a 9.0+ Earthquake about every 130-150 years. The last one was about ~140 years ago.

Do you have a source for that? Because it's news to me and I actually worked on research into past earthquakes. If you go up on to the DOGAMI (Oregon Dept of Geology and Mineral Industries) you'll see a map of Oregon's historic earthquakes, from 1842 to 2002 and there is nothing even close to a 9 - and remember that the Richter scale is a more-or-less log scale, not a linear scale, so once you get up there in Richter numbers, there is a HUGE difference in the energies involved between, say, a 6 and an 8.

If you mean the Cascadia Subduction zone quake, the last one was in 1700 and 10,000 years' worth of record show that they are between 300-500 years apart, on average.

Here's a general fault map in Oregon, showing the major known fault zones:
http://www.oregongeology.com/sub/pub...gms/gms100.pdf

Also, the biggest risk from Mt. Hood is not really a St. Helens-style blowout (though that's very remotely possible), it's debris flows, much the same as the general hazards involved in Mt. Rainier near Seattle.

[Larry Caldwell]

You can still see traces of the January 26, 1700 Cascadia quake at my place. It hit in the winter when the soils were saturated, and the whole hillside across from my house slid into the valley and dammed the creek. Everything upstream is flat as a pancake lake bottom for about a mile. It was still a swamp when the first settlers arrived. They blew a creek channel through the dam to drain the swamp for farm ground.

My house is built on the alluvial fan from the slide, but that's OK, because there is nothing left up there to slide. In 311 years, there has been no significant recovery of the topsoil layer on the slope.

When the next Cascadia quake hits, there is going to be a lot of view property sitting at the bottom of the hills.

[hamellr]

Originally Posted by PNW-type-gal

Do you have a source for that? Because it's news to me and I actually worked on research into past earthquakes.

I heard it on NPR driving to work a few days ago.


*Edit - I tried to Google for it and am not finding it now. I'd be interested in reading more in-depth as I too have an interest in Geology, especially Oregon's.

[jjpop]

Originally Posted by PNW-type-gal

If you mean the Cascadia Subduction zone quake, the last one was in 1700 and 10,000 years' worth of record show that they are between 300-500 years apart, on average.

I heard a DOGAMI geologist at a talk recently and she indicated that a Subduction Zone earthquake could potentially be in the 9.0 range at the epicenter, but that attenuation of the shaking away from the subduction zone would make it feel like about a 6.0 here in Portland. Ceratinly still a potentially devastating earthquake for Portland, but cataclysmic for coastal communities (between the quake and the resulting tsunami). This would be the United States' version of the Banda Aceh, Indonesia quake/tsunami of 2004, except with spruce and shore pine being knocked over instead of palm trees.

[PNW-type-gal]

Originally Posted by jjpop

I heard a DOGAMI geologist at a talk recently and she indicated that a Subduction Zone earthquake could potentially be in the 9.0 range at the epicenter, but that attenuation of the shaking away from the subduction zone would make it feel like about a 6.0 here in Portland. Ceratinly still a potentially devastating earthquake for Portland, but cataclysmic for coastal communities (between the quake and the resulting tsunami). This would be the United States' version of the Banda Aceh, Indonesia quake/tsunami of 2004, except with spruce and shore pine being knocked over instead of palm trees.

Although the "subduction zone" is offshore (about 100 miles through most of Oregon), the shape of the zone is a wedge that goes right under the coast line. That wedge takes a sharp downward drop right about the point of the line of Cascades volcanoes (that drop accelerates the crust melting and is what supplies the Cascades with magma, in part). The question is where along that wedge the epicenter of a quake is. It's actually unlikely for it to be right at the surface expression of the subduction zone and more likely to be somewhere toward inland.

Distance from the epicenter is important, but so is terrain and geology - at a point close to the epicenter, the combination of conditions might attenuate damage where a further area, much more prone to, say, liquefaction will have much more widespread damage.

The coastal subsidence is less from the earthquake shaking and more from the freeing of stresses - as the ocean plate slides under the continental shelf, the continental plate sort of sticks and begins to pile up or bow up. When that stress and strain is released in a quake, it springs back to flat again, usually dropping down quite a bit and burying marshes.