Re: [Paddlewise] Puget-sound area paddlers...

----- Original Message -----
From: "Dave Uebele" <daveu_at_sptddog.com>
Subject: Re: [Paddlewise] Puget-sound area paddlers...


> As someone that was in both the Loma Prieta quake and the recent
> quake centered between Tacoma and Olympia. The recent quake didn't seem
> as intense. The period of really intense shaking wasn't as long or
> quite as intense.  PNW quakes seem to have a different
> quality, there is usually a much longer duration gentle rolling quality
> to the last part of the quake.
>
> Anyone with a knowledge of the different types of plates between
> the two areas care to comment (off list) about how the nature of
> the faults is different?
>

The quake yesterday (which my house in Seattle survived with zero damage
except for scaring my cat into a fern bush) was a deep inter-plate quake on
the boundary of the North American continental plate and the Juan de Fuca
oceanic plate. As you all should know from geology 101 (grin), the Juan de
Fuca plate is subducting under the NA plate, which means that is diving
under and eventually melting into oblivion into the Mantle. Some of the
melted plate rises upward and forms the multiple stratovolcanos in Cascadia
(e.g. Mt. Rainier, Mt. St. Helens, Mt. Shasta, etc).  It's a shallow angle
subduction zone, so the quake was 100 miles inland from the coast and 30
miles deep. It's the depth of the quake that limited the impact, because
much of the high frequency shaking energy was attenuated by travelling 30
miles underground to reach the surface. There is nothing special about
quakes in the PNW that makes them less mild, except for the location of this
particular one. There are shallow faults in the PNW, such as the Seattle
fault which has a potential for a magnitude 7 tremblor. This would be very
similar in character to the Loma Prieta quake. Or the quake could occur at a
shallow portion of the subducting plate towards the coast, also giving rise
to much more destructive energy. In fact, this is a very scary thought,
because that kind of quake would likely be of magnitude 8-9 causing awesome
destruction not to mention severe tsunamis. The last time this happened was
in 1700 according to tree-ring records and the recording of a correlated
tsunami in Japan. The period of such quakes is about 300 years on average
(*gulp*), with variations between 200 to 600 years so don't hold your breath
waiting for it.

Technically, the quake yesterday was on a thrust-fault while the Loma Prieta
was on a strike-slip fault. But the major difference destructive energy was
caused by the depth of the epicenter. Both quakes occur on inter-plate
boundaries. The San Andreas fault is the boundary between the NA continental
plate and the Pacific oceanic plate -- these plates are just sliding by each
other. The Juan de Fuca plate is sandwiched between the NA and Pacific
plates, and is being subducted under the NA plate.

Cheers,
kevin

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