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Providence · 09-10-2008, 04:08 AM

Hi all – This post address volcanoesm, tsunamis and quakes, oh my!

Thanks everyone for some great posts! I love being part of this Ground Crew – I don’t feel so alone or nuts anymore. However, I have to admit that this is distracting me during the weekdays! Perhaps I need to start a new thread for those like me to learn moderation – this website is ADDICTIVE! After this post tonight I am only allowing myself to post on the weekends….!

Being a Geologist, I thought I would respond to some concerns voiced about earthquakes, subduction zones, tsunamis and volcanoes here in the PNW. Indeed! - All of these phenomena are related and need to be considered. However…

As most of us know, we live near a subduction zone. Basically, this means that we live on the leading edge of a continental plate (the North American Plate to be specific), which is slowly colliding with an oceanic plate (the Juan De Fuca plate to be exact). When our advancing continental shelf runs up and over the oceanic plate, forcing the oceanic plate deep down into the earth, all of that “good earth stuff” that comprised the oceanic plate melts. The melt eventually makes its way back up topside and spews out lovely melted magma from our dearly departed oceanic crust (very simplified mind you and its mixed with other stuff of course). The result? The everlasting beauty of our Cascade Mountain Range, of course!

We have all been warned about the mega thrust earthquakes that occur out here every 500 years or so. And yes we are about due for another one. A large subduction earthquake would be in the range of 7.0 – 8.0 on the Richter Scale, which is a logarithmic scale; i.e. for every increase in whole number, the energy released by the earthquake increases by a factor of 10. So, A 7.0 would release 10 x the energy of a 6.0. A 9.0 would really be a whopper!!

So….. a few thoughts:

1. Not all earthquakes will result in widespread devastation. Depending on the type of energy released, the epicenter of the quake, and the construction of buildings, the devastation can be a little or a lot. Most of our earthquakes in the PNW happen off the coast, within the subduction zone area. Quakes also occur very frequently (in the 2.0 to 3.0 range around most all of our volcanoes. For those of you in Oregon, please check out DOGAMIs website for excellent information on earthquake destruction zones, maps, tsunami information. Here is the link:

http://www.oregongeology.com/sub/default.htm

2. Not all earthquakes release tsunamis. It depends where the earthquake is centered, its depth, and the general direction of the energy expended. Sometimes people in an earthquake can tell the direction from which the earthquake originated. As a Geologist, it is hard to believe that even a 9.0 earthquake occurring off the coast of Oregon would create a tsunami large enough and with enough volume of water to reach Portland, which is approximately 70 miles inland and upstream of Astoria. Even with a 100 foot tsunami wave, the vertical (over the coast range) and horizontal (the areal mass it must cover) hurdles would be immense, likely reducing the tsunami effect to near nothing.

3. In the event of a volcanic eruption, Seattle and Portland would not be wiped out. But…depending on wind direction either city could be affected by ash. Also, large debris flows could move into the Duwamish River. But Seattle’s biggest impact would come from debris being washed into Elliot Bay. Would Seattle be wiped out by a volcanic eruption? NO. As for Portland, it could be affected by light ash falls from an erupting Mt. Hood but it would not be affected by lava flows, lahars or anything like that. Would Portland be wiped out by a volcanic eruption? No.

There are some good websites that you can go to where you can find well researched, science-based information on the potential for natural disasters to occur in these areas. But I must warn you these fact-based websites based on years of scientific study so they won’t be as much fun to read or get excited as the multitude of disinformation out there.

I figure that my best chances for making good decisions about the safety me and my family and now all of us in our Ground Crew is to do my best to try to separate the speculation and fear-mongering from the truth and reality. I try to cross-check what I read on alternative webpages to make sure what I am reading makes sense to me…. What is great about our newly formed e-group is that if I do come across something that distresses me (economically, politically, or naturally) I can post it here for comment!

09-10-2008, 04:08 AM	#4
Providence Avalon Senior Member Join Date: Sep 2008 Location: Pacific Northwest Posts: 33	Re: Pacific Northwest Ground Crew Hi all – This post address volcanoesm, tsunamis and quakes, oh my! Thanks everyone for some great posts! I love being part of this Ground Crew – I don’t feel so alone or nuts anymore. However, I have to admit that this is distracting me during the weekdays! Perhaps I need to start a new thread for those like me to learn moderation – this website is ADDICTIVE! After this post tonight I am only allowing myself to post on the weekends….! Being a Geologist, I thought I would respond to some concerns voiced about earthquakes, subduction zones, tsunamis and volcanoes here in the PNW. Indeed! - All of these phenomena are related and need to be considered. However… As most of us know, we live near a subduction zone. Basically, this means that we live on the leading edge of a continental plate (the North American Plate to be specific), which is slowly colliding with an oceanic plate (the Juan De Fuca plate to be exact). When our advancing continental shelf runs up and over the oceanic plate, forcing the oceanic plate deep down into the earth, all of that “good earth stuff” that comprised the oceanic plate melts. The melt eventually makes its way back up topside and spews out lovely melted magma from our dearly departed oceanic crust (very simplified mind you and its mixed with other stuff of course). The result? The everlasting beauty of our Cascade Mountain Range, of course! We have all been warned about the mega thrust earthquakes that occur out here every 500 years or so. And yes we are about due for another one. A large subduction earthquake would be in the range of 7.0 – 8.0 on the Richter Scale, which is a logarithmic scale; i.e. for every increase in whole number, the energy released by the earthquake increases by a factor of 10. So, A 7.0 would release 10 x the energy of a 6.0. A 9.0 would really be a whopper!! So….. a few thoughts: 1. Not all earthquakes will result in widespread devastation. Depending on the type of energy released, the epicenter of the quake, and the construction of buildings, the devastation can be a little or a lot. Most of our earthquakes in the PNW happen off the coast, within the subduction zone area. Quakes also occur very frequently (in the 2.0 to 3.0 range around most all of our volcanoes. For those of you in Oregon, please check out DOGAMIs website for excellent information on earthquake destruction zones, maps, tsunami information. Here is the link: http://www.oregongeology.com/sub/default.htm 2. Not all earthquakes release tsunamis. It depends where the earthquake is centered, its depth, and the general direction of the energy expended. Sometimes people in an earthquake can tell the direction from which the earthquake originated. As a Geologist, it is hard to believe that even a 9.0 earthquake occurring off the coast of Oregon would create a tsunami large enough and with enough volume of water to reach Portland, which is approximately 70 miles inland and upstream of Astoria. Even with a 100 foot tsunami wave, the vertical (over the coast range) and horizontal (the areal mass it must cover) hurdles would be immense, likely reducing the tsunami effect to near nothing. 3. In the event of a volcanic eruption, Seattle and Portland would not be wiped out. But…depending on wind direction either city could be affected by ash. Also, large debris flows could move into the Duwamish River. But Seattle’s biggest impact would come from debris being washed into Elliot Bay. Would Seattle be wiped out by a volcanic eruption? NO. As for Portland, it could be affected by light ash falls from an erupting Mt. Hood but it would not be affected by lava flows, lahars or anything like that. Would Portland be wiped out by a volcanic eruption? No. There are some good websites that you can go to where you can find well researched, science-based information on the potential for natural disasters to occur in these areas. But I must warn you these fact-based websites based on years of scientific study so they won’t be as much fun to read or get excited as the multitude of disinformation out there. I figure that my best chances for making good decisions about the safety me and my family and now all of us in our Ground Crew is to do my best to try to separate the speculation and fear-mongering from the truth and reality. I try to cross-check what I read on alternative webpages to make sure what I am reading makes sense to me…. What is great about our newly formed e-group is that if I do come across something that distresses me (economically, politically, or naturally) I can post it here for comment!