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Re: Seattle Earthquake a few mintues ago



On Wed, 28 Feb 2001, Paul Middents wrote:

I am across the Puget Sound from Bill in central Kitsap County. I was on the
ground floor near the main entrance of a large training center on the
Trident Submarine Base. The ex-Californians in the crowd all immediately
estimated the magnitude as 6 plus. I can confirm the rolling sensation Bill
describes. Minor structural damage on the second floor of the building
resulted.

People in different locations noted a strange continuing "after sway".
I thought it was just psychological, like stepping onto land after being
on a ferry. Trees and streetlights kept swaying for a good long time, so
perhaps they were still being driven. News says that the epicenter was
30mi deep in Puget sound between Tacoma and Olympia.

A shallow quake a few years ago in Everett WA acted more like a gunshot or
a bomb going off.

I wonder if most quakes are actually brief impulses lasting less than a
second, but the signal is "chirped" by the nonlinear medium so it becomes
ripples spread over time. If so, then the quake would last longer for
sites at a greater distance from the origin.

Scary thought: the power output seems sort of arbitrary. Why wasn't that
quake an order of magnitude worse? Why wasn't it TEN orders of magnitude
worse? Forces and breaking strength of materials must luckily program the
typical energy output to be what it is. Also luckily there don't seem to
be many natural "lenses" which bring the waves to a tight focus.

In the Pacific Northwest we also have another earthquake hazard:
triggered landslides from the volcanoes. The nortwest volcaones are like
compacted powder rather than like the Hawaiian lava. There are maps which
show the ancient landslides around Mt Ranier, and some of them extend many
tens of KM from the base of the mountain and are many KM wide. This is a
strange physics problem, since the moving mass shouldn't go so far. It
apparantly is "liquified" by shear, and flows like a river. Even so, I
think the extreme length of the horizontal travel is not well explained.
A large number of towns in Washington state are built on the flat regions
which are the ancient landslide pathways many tens of KM from the
mountain.


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