PSN-L Email List Message

Subject: Re: Gorda Locked
From: John Hernlund hernlund@............
Date: Sun, 11 Apr 2004 13:01:28 -0700

Yes, this is a common scenario in several subduction zones, where the 
largest earthquakes are generated. The Juan de Fuca and Gorda plates 
are a bit of an oddity though, since the ridge is so close to the 
trench, making the plate size and age smaller. The young age means the 
ocean lithosphere is thin and the whole thing is more buoyant than old 
oceanic lithosphere (it resists being pushed under). A similar plate 
scenario exists in southwestern Mexico (Oaxaca region) and parts of the 
west coast of South America (parts of the Japan subduction zone also 
get locked up and can generate giant EQs, although the age of 
subducting plate is different in the north and south). JdF and Gorda 
are dangerous, since the recurrence interval is so long compared to 
those in other places and if you are around for one, things might not 
be so good. Perhaps a more important measure is the recurrence interval 
times the plate convergence speed, which gives you the average length 
of convergence (offset) per earthquake, which in turn figures into the 
magnitude of the EQ that can be generated (offset times fault area 
times shear modulus).

Another good practical measure is a person's average lifespan divided 
by the recurrence interval in the area they live in...which in the case 
of the Pac NW turns out to be nice and small compared to Oaxaca (which 
is probably the most dangerous of all when coupled with population, 
economic and engineering factors).  A similar recurrence interval to 
the Pac NW exists for the giant thrust fault system that cuts across LA.


On Sunday, April 11, 2004, at 07:10 AM, Bob Shannon wrote:

> I have been thinking about Gorda and its locked position for a long 
> time
> now...but this AM I awoke with a question I don't believe I have ever
> asked....Is there any other scenario in the world, similar to Gorda
> where a small plate is locked and could break loose????
>  Bob Shannon
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