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[Fwd: 7.0 Hector Mine EQ was[RE: 7.0 Landers Aftershock]

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--- Begin Message --- Roger Turk  Sat, 16 Oct 1999 14:32:47 wrote


After this quake and the previous Landers EQ that caused buildings in Phoenix
to sway and creak and caused the strong motion instrument at the Palo Verde
Nuclear Generating Plant to trigger, don't you think that it is time to
eliminate Seismic Zone 1 from *all* parts of Arizona?  Paul, can you check
with the VA Hospital in Phoenix to see if their strong motion instrument
triggered?  You may have to do some digging to find someone that knows that
they have one.  (Is there someone from Menlo Park on the list that could
answer this question?)

While the EQ physically woke you and your wife up (and prompted a very
germane question from her), I would hope that it also woke up all Structural
Engineers in Arizona to the absurdity of having Seismic Zone 1 in Arizona.

It is well known that there is a geologic "duct" that causes earthquakes east
of the coast mountains in California to be felt in Phoenix, but not so
readily in Tucson.

*  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *

Your reference to geologic "duct" is seismologically referred to as crustal wave guide phenomena or focusing within the earth's crust, perhaps even basin effects.  When the frequency content of incoming seismic waves matches the natural frequency of a sediment filled basin, there is maximum response; and the energy ricochets around in the basin for a while, extending the duration of shaking effects.  These "crustal seismic effects", together with 3D basin modeling, are shifting the paradigm on engineering considerations that formerly mostly relied on distance from the source.

Is there a Seismology Committee of structural engineers in Arizona that deals with Seismic Zone issues?  I believe back in 91 or 92 there was a proposal to upgrade the Seismic Zone in Arizona (that I think was withdrawn).  What do the new NEHRP maps mean for seismic sources and seismic design values in Arizona?

There is a telling story.  Back in the 1960s, I believe, when Degenkolb engineers were designing the Bank of California (high rise) here in Portland; Henry Degenkolb looked at the Building Code Seismic Zone map and said to his design team:  "I used to live in Portland, and I can't believe with all those volcanoes that they don't have earthquakes.  Design it for seismic!"  

Here are some websites I know of: (fill out felt shaking effects form)

James Bela
Oregon Earthquake AwarenessTM  /  The Quake Northwest
"We Have Nothing To Fear But Shear Itself"  /  "We're All Subducting In This Together"
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