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Re: Northridge Earthquake a Major "Event"

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Thanks to Mr. Hamburger, Mr. Turner, Mr. Greenlaw and Mr. Valley for
your responses to my posting. 

I basically agree with most of what was said by these people.  However,
:)  my point I think is still valid.  And perhaps I made no attempt to
make a point, other than a very broad one.

Richter scale magnitude is a measure of the total energy released from
an earthquake.  It has no direct correlation to ground motion. 
Obviously, a 9.0 earthquake will see more ground motion than a 1.0. 
However, the difference can not be as clear when discussing the
difference in ground motion from a 6.7 to a 7.3 earthquake.

The depth of the epicenter, the soils strata a building is sitting on,
as well as MANY other factors are important in predicting the actual
ground motion a building will see during a design level event. 

There were in fact buildings close to the epicenter that experienced
STRONG ground motion that performed up to a level of what I would call
life safety.  From my recent readings, it appears life safety has been
defined differently by various groups.  

If I took that actual spectral curves from actual data during the
Northridge earthquake and plugged them into the computer analysis
instead of the UBC curves, guess what, virtually none of the buildings
close to the epicenter that experienced strong ground motion would
figure as being able to perform adequately during this event.  Some did
indeed failed.  And there are very valuable lessons to be learned from
that.  But the lessons that I think are being missed are the ones that
tell us why some of these buildings that were subjected to very high
ground accelerations were able to come through this event successfully
when judged from a life safety perspective.  

What I was responding to was a comment that the Northridge Earthquake
was not a big enough earthquake to really test these buildings.  My
position is that there were many areas that did sustain very large
ground accelerations, and if it wasn't a Code level earthquake, it was
very close to one.  For areas that were subjected to these strong
motions, you cannot dismiss the evidence that many buildings performed
adequately due to the fact that the earthquake was only "moderate".    

Yes, we can all visualize and find earthquakes that have happened that
are larger and produced higher ground motion with a longer duration. 
But do these very large events fall into the 10% probability of
exceedence in 50 years.  For some areas they probably do, but you have
to be very close to the San Andreas Fault.  I know that for our area
(Santa Barbara) an 8.0 earthquake that last 2 minutes is considered to
have a very low chance of occurrence.

Again, thanks for all the great responses.

Lynn