[Rockhounds] 6.4 quake off Oregon coast

[Axel Emmermann]

I went to a lecture by Professor Dr. Manuel Sintubin (University of Leuven,
Belgium) about Earthquakes and tsunamis two years ago.
http://geo.kuleuven.be/medewerkers/index.htm?sap=11680
What I learned of it was this (mind you, reproduction of lengthy talk by
non-scientist ;-):
Apparently the slab of crust that is subducting "hangs" down into the
earth's mantle for a few thousand kilometers.
http://gsa.confex.com/gsa/2003CD/finalprogram/abstract_50530.htm 
Talking about large sails! Actually this appears to be causing the crust to
literally sail on convection flows in the mantle.
Subduction itself would rather (than just underlying flows)  be caused by
the difference in specific gravity between basaltic rock of the crust, which
is heavier, and granitic rock from the mantle which is lighter. Basaltic
eruptions that stem from deep within the earth deposit heavy basalts on the
surface. The upper mantle is lighter so tectonic plates tend to sink,
dragged under by the weight of the many kilometers of subducting sheet.
(there are also other forces in play like the convection cells and such...)

The convecting cells have an independent movement relative to each other.
The overall movement of the mantle however seems to be west to east (wheels
turning within a wheel). As I understood it, this causes a drag on the slabs
of plate that hang in the subduction zones.
When a subducting plate creeps under another plate, the direction of it's
movement is crucial for the intensity of the quakes that are resulting from
it. When a slab dives to the east it has a "backwind" in it's "sail". The
overall mantle flow (aside from local convection cells) pushes it firmly
against the continent under which it is subducting. High pressure, high
friction, super quakes of 7.5 on the Richter scale and upwards. Logically,
the west coast of continents underneath which subduction takes place are hit
by severe earthquakes. Think of Chili or in fact practically all of the
South American west coast. If the plate dives "into" the flow rather than
"with" it, the subducting plate would be slightly pushed away from the
collision and this would result in less and less powerful quakes.


I'm sure I could have reproduced this more precisely but I just wanted to
convey the possible reason why I think that Oregon suffered "only" a 6.4
quake while the really powerful ones seem to be reserved for Chili, Alaska,
Aleutian islands, Ocean floor off West coast of Indonesian islands
(Sumatra?). 

I'm also sure there is some debate in this ;-)))
I read that there is no absolute consensus among scientists about the cause
and mechanics of mantle convection cells.

Cheers
Axel




> -----Oorspronkelijk bericht-----
> Van: rockhounds-bounces at lists.drizzle.com 
> [mailto:rockhounds-bounces at lists.drizzle.com] Namens Tim Fisher
> Verzonden: donderdag 10 januari 2008 7:50
> Aan: Rockhounds at drizzle.com: A mailing list for rock and gem 
> collectors
> Onderwerp: Re: [Rockhounds] 6.4 quake off Oregon coast
> 
> Dec 19 - Aleutians, Alaska 7.3
> Jan 5 - 6.7 Queen Charlottes, BC
> Jan 9 - 6.4 OR
> 
> Looks like the Alaska-Aleutian & Cascadia Subduction Zones 
> are blowing off some steam :)
> 
> Good article: http://www.msnbc.msn.com/id/6786109/
> 
> At 08:05 PM 1/9/2008, you wrote:
> ><http://www.foxnews.com/urgent_queue/index.html#446ef0d0,2008-01-09>
> >
> >BK
> >
> >--
> >"I know I shall be castigated by a large group of people 
> today, but I 
> >was trained to assume that art related to the elusive 
> quality of beauty 
> >and that the purpose of art was concerned with the elevation 
> of the spirit"
> >Ansel Adams
> >
> >
> >J Bryan Kramer
> >North Florida, USA
> >photos at:
> >http://pbase.com/photoburner
> 
> Tim Fisher
> Ore-ROCK-On!
> Email address at http://OreRockOn.com  
> 
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