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Re: Mistreatment of Seismology Issues

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Since general public knows very well what kind of damage can be expected
(say) 6.4 earthquake strikes but has absolutely no idea as to what kind of
damage can result when "life-safety standard"-compliant structure is hit by
the same magnitude earthquake


Beyond my geotech class, and associated material I learned or have read
about relating to seismic design, I am not all that hip on the actual
mechanics of earthquakes.  That being said, I would have to question (in a
somewhat uninformed manner) the validity of the above statement.

I would seem that there are at least 5 parameters that would help in
determining the damage that can be expected from an earthquake:

1) Richter magnitude

2) Duration

3) Frequency content

4) The characteristics of the structures being excited.

5) Accelerations

To say that an area experienced a "6.4" earthquake does not, IMO, give a
person enough information to even really guess the magnitude of damage that
should be expected.

Case in point:

Compare the Guam earthquake (Richter magnitude 8.2) to the Northridge

Duration of strong motion on Guam - about 40 seconds

Duration of strong motion for Northridge - about 15 seconds.

VERY different frequency contents (Guam EQ caused by a subduction fault,
located very deep, while Northridge I think was a strike slip fault EQ). 
Guam EQ was as I understand it a much longer period earthquake than NR.

Very high accelerations for NR, estimated accelerations from Guam EQ are
not nearly as high.

Two major structural failures on Guam, a bunch more for NR (although you
guys have a heck of a lot more buildings than we do!).

No deaths on Guam.  More than that for NR.

OK.  My point is that boiling down an earthquake to one single parameter
can, IMO, be very misleading.  Using this method, the amount of damage,
collapsed buildings, and deaths that resulted from the 8.2 on Guam should
have far surpassed the results of the "smaller" NR earthquake.  But that
did not happen.  

I don't wish to be just a naysayer, but at this time I don't feel that I
have covered all the bases as far as relevant earthquake parameters are
concerned.  Anyone care to add (or modify, or delete) from the above list? 
It might be informative (for me, at least!)


T. Eric Gillham PE