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James Bela wrote:


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withdrawn).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<<


Thanks for clarifying the effect that I called "ducting."  (As an old Navy 
Radarman, I understand "wave guide.")  This was explained to me some years 
ago by a geologist at the University of Arizona and I remembered the gist of 
what he said, and not the exact words.

The Tucson Chapter, SEAOA, did get the seismic zoning of Southern Arizona 
changed to seismic zone 2A in about 1990 or '91.  I went to the code 
development committee hearings in Indianapolis to present our case to them, 
which they accepted.

There is a great divergence of opinion, particularly with respect to seismic 
zonation, between structural engineers in the Tucson area and structural 
engineers in the Phoenix area.  At one time, there was a state seismic 
committee, which I was on, but it was disbanded, as all state committees 
were, because of divergence of opinions.  At the present time, there are no 
state SEAOA committees.

As far as the NEHRP maps go, I have not studied them in detail (and they 
seem to change with the season) since Tucson and Pima County are remaining 
with the 1994 UBC, but in keeping with the current zonation, it is my belief 
and also a colleague's belief with whom I am serving on the Tucson/Pima 
County Building Code Review Committee, that regardless of what the maps show, 
the minimum seismic design force, if and when the IBC 2000 is adopted, should 
be 7.5 percent g.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona