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Re: Northridge Earthquake a Major "Event"

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Lynn:

I think the correct answer on this point is rather tricky.  Yes, 
at some sites the recorded accelerations (and corresponding response 
spectra) were at a higher level than the zone 4 design values.  
However, the design ground motion has been that level of shaking 
that has a 10 percent chance of exceedence in 50 years.  
Seismologists indicate that this corresponds to something like a 
magnitude 7.5 event in the LA area.  As you pointed out, the 
Northridge earthquake was considerably smaller (6.7).  So, I 
believe that we can learn (or relearn) a few important points from 
the Northridge earthquake.

1) The design earthquake in LA (if there is such a thing) is 
expected to be much larger than the Northridge event.  (About 16 
times the energy to go from magnitude 6.7 to 7.5.)

2) Local ground accelerations can be significantly higher than the 
code-level design accelerations (even for events smaller than the 
design earthquake).

Taking points 1 and 2 together, the factor by which measured 
accelerations exceed design accelerations may be larger than that 
measured in Northridge.  The size of the "local" area with 
accelerations larger than the design values is expected to be larger 
in the design event than was experienced in the Northridge event.  
And , the duration of strong shaking in the design event will be 
longer.  All of the above items emphasize point 3.

3) The level of shaking expected for earthquakes of a given 
magnitude is highly uncertain and variable.

In earthquake engineering we should remember that our 
characterization of the basic information (demands and capacities) 
are essentially rough estimates.  Ground motions can easily vary by 
50% to 100% and ductility capacities can be nearly as variable.  We 
don't know it all.

-Mike

> Date:          Mon, 04 Oct 1999 09:24:34 -0700
> From:          Lynn <lhoward(--nospam--at)silcom.com>
> Subject:       Northridge Earthquake a Major "Event"

> There was some discussion a while back that lead me to believe that some
> engineers think that the Northridge Earthquake was not a Code design
> level earthquake, but rather a "moderate" earthquake.
> 
> After attending the SEAOC seminar, I was reading through the 1999 Blue
> book and found the following:
> 
> "Strong ground shaking was recorded during the magnitude Mw=6.7 1994
> Northridge earthquake at sites located close to the fault.  At these
> near-source sites, ground motion response spectra were often
> significantly larger than the Seismic Zone 4 design spectra"
> 
> This was taken from the Appendix F, page 309 of the 1999 SEAOC blue
> book.  Appendix "F" is a section that discusses the Northridge
> earthquake. In the appendix they actually have graphs of some recorded
> spectras.  
> 
> The 1994 Northridge earthquake was in fact a Code design level
> earthquake.

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Michael Valley                                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:        -1201