Threat of natural disasters: earthquakes (Portland, St. Helens: houses, construction, tornadoes)

[Samoi137]

Thinking of Portland in terms of a second home, eventual retirement home. I'm from L.A., so I'm not worrying about it in terms of Portland, but my wife (from Brazil) does worry about quakes (well, volcanic activity, as well, but she knows that could occur hundreds if not thousands of years in the future). For people in Portland, how often do you feel a quake? I see records of primarily 2-3.5 on the Richter Scale (rarely above but they do occur), which to me seems nothing.

[PNW-type-gal]

Here is a quick guide to earthquake history in the Portland area:

Historic earthquakes


The big risk is from the Pacific Subduction zone which can (and regularly does) generate a 9.0-level earthquake. The interval is roughly every 300-600 years - the last one was in 1700 (on the nose, as documented from coastal records and the tsunami it caused in Japan). Exactly where the next earthquake in the zone will be centered - the zone goes from Northern California all the way up into British Columbia - can't be known, nor can the magnitude. So there is the risk of a large earthquake being felt in the Portland area.

[Beenhere4ever]

Originally Posted by PNW-type-gal

Here is a quick guide to earthquake history in the Portland area:

Historic earthquakes


The big risk is from the Pacific Subduction zone which can (and regularly does) generate a 9.0-level earthquake. The interval is roughly every 300-600 years - the last one was in 1700 (on the nose, as documented from coastal records and the tsunami it caused in Japan). Exactly where the next earthquake in the zone will be centered - the zone goes from Northern California all the way up into British Columbia - can't be known, nor can the magnitude. So there is the risk of a large earthquake being felt in the Portland area.

Is Mt St Helens related to that activity in any way?

[PNW-type-gal]

Originally Posted by Beenhere4ever

Is Mt St Helens related to that activity in any way?

Yes and no - Mt St Helens is an active volcano, part of the Cascades chain. The volcanoes wouldn't be there if the subduction zone, where the Pacific plate slides under the continent, wasn't there. Earthquake activity at Mt St Helens (or Mt Rainer, Mt Hood, Mt Adams and the other active volcanoes) tends to reflect what is happening in the magma plume directly under the volcano rather than a larger regional issue. In other words, there is no known link between what is going on at the individual volcanoes and timing predictions for a big Cascadia Subduction Zone (CSZ) quake.


This link shows recent earthquake activity, which is pretty much a daily occurrence - it's just that they are all very small
Earthquake Map | Pacific Northwest Seismic Network

CVO has a good website with lots of graphs and pics:
USGS: Volcano Hazards Program Cascades Volcano Observatory

[Nell Plotts]

I have lived in Portland almost 60 years and have felt earthquakes, minor ones, maybe three times. That does not mean that a large earthquake won't happen tomorrow.

If avoiding the impact of an earthquake is a concern then locate away from the coast line, Puget Sound, rivers and hills, build a house to SF earthquake standards and you will be fine. There are locations in the US where earthquakes historically are fewer but they have other risks, like tornadoes and hurricanes.

[wanderbygrace]

I wouldn't worry about it terribly much, especially if moving into a newer home, as they are generally very earthquake-sound. I'd definitely be worried about some of those older brick masonry houses, as that type of construction doesn't hold up very well against quakes, unless retrofitted.

Personally, I think PDX would hold up better against an earthquake than San Fran and to both an earthquake & volcanic hazard than Seattle. The other two cities & metro areas have been built on vast stretches of soil that are known for crumbling pretty quickly in an earthquake. Portland's igneous rock tends to hold up better. Also, the Portland area isn't as close to other plate boundaries like Seatle is, so not as much shaking, I'd imagine.

As far as volcanic eruptions, I'd be FAR more concerned about living in somewhere like Tacoma or SE Seattle if Rainier erupted than anywhere in PDX if Adams, Hood or St. Helens erupted. Why? Because much of Tacoma & SE suburban Seattle is built on lahar plains - past mudflows deposited directly from Rainier. PDX doesn't have that. And if Rainier could do it back then, it can do it again. If you get the chance, google the Puyallup area of Seattle and turn the Google map into terrain view. Note the very low land that places like Puyallup, Sumner, and the port of Tacoma sit on, then look where most of Federal Way sits. Note the latter sits above the former like an island. The lower lands are the lahar plain. I wouldn't live there if you paid me.

In the event Hood erupts, the #1 concern would be ashfall, and since most winds in the area move west to east, most ashfall would be carried away from PDX. Lahars, lava flows, and pyroclastic flows wouldn't be an issue to metro Portland, due to the multiple ridges in between us & Hood protecting us. Not sure I'd want to be in Hood River or Sandy, though, and certainly no closer to Hood than Sandy, since Government Camp is built on Hood's last lahar.

As a final note, I wouldn't recommend a place on a hillside, especially a steep one. Not just because of the earthquake factor, but because landslides aren't uncommon here. Some of the homes in western Portland hills would just terrify me to live in (seems to me that if you need stilts to build your house, that's not a good sign). Oh, and make sure you check if your property is in a flood plain - due to the amount of rain here, creeks/rivers overflowing their banks is pretty much the norm.

[Beenhere4ever]

There are all these predictions of massive earthquakes or eruptions. In the Yellowstone area. In the vicinity of Madrid Missouri. If you spent much time worrying, you'd find yourself old before you know it. Face it, the earth doesn't have a "safe zone". There are massive tectonic forces everywhere and the whole thing is shifting all the time. And if that isn't enough, we have all those meteors flying through space.

[Nell Plotts]

The State of Oregon Geologists office in Portland LOVES visitors. When I was there last I was looking at conditions surrounding a specific piece of property and they explained the site conditions. Go see them as they will also comment on slide conditions.

[BayAreaHillbilly]

Not if but when the next mega thrust quake hits it will be legendary. Granted, the Pac NW is not Indonesia in terms of either shoreline profile or weak infrastructure but the Boxing Day Quake and Tsunami was a hint of some of the immediate and delayed impacts of such an event. Immense accelerations are possible. Liquifaction of water logged, uncompacted soils and any areas of fill is a given. The resulting tsunami will wreak havoc not only in the Pac NW but in several other places around the North Pacific. Be prepared!

[Beenhere4ever]

I saw a documentary about how one seacoast town in Oregon is preparing for an inevitable tsunami with evacuation drills and places in town high enough to be above the wave. I think it was Tillamook, but that might be wrong.