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Re: Northridge Earthquake a Major "Event"

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Lynn - You've identified some confusing aspects with current semantics
describing earthquakes. There was strong ground shaking with spectral values
higher than code spectra in Northridge. On the other hand, short durations
in moderate events like Northridge will tend to limit overall damage.

Recorded accelerations can be quite high in relatively small earthquakes.
This implies that accelerations are really not the ideal way to correlate
design parameters with all types of damage or performance. Particularly,
high peak ground accelerations are often associated with such high
frequencies that they have limited use to structural engineers.

Spectral accelerations seem better suited than PGA for most structures , but
they aren't too meaningful for structures more sensitive to displacement or
velocity or duration, such as structures with highly nonlinear or degrading

Engineers and scientists have been studying other earthquake parameters to
see if there are more straightforward ways to correlate response with
recorded ground motion parameters. No single measure appears relevant for
all types of structures. Accelerations don't increase proportionately with
increases in the size of earthquakes. In other words, the worst
accelerations in major earthquakes are not going to be a great deal higher
than in some moderate earthquakes, but they will tend to have longer
durations and more long-period content.

To make matters more fuzzy, different measures are used by other

Definitions from the geoscience community are:

Great Earthquakes are Moment Magnitude Mw 8 or greater. 0.7 events per years

Major or large earthquakes are Mw 7 or greater. 15.5 events per year

Moderate earthquakes are Mw 6 or greater. 195 events per year worldwide.

Moment Magnitude has taken the place of Richter Magnitude as a measure of
preference for energy release by seismologists. The media still insists on
referring to Richter Magnitude even though it has long been surpassed in the
scientific community.

So, here's my attempted answer to your question: Strong ground motion
(acceleration) occurs in moderate earthquakes. But this doesn't mean that
structures which survived Northridge will perform in a similar fashion in
major earthquakes. It also doesn't mean that Northridge confirmed the
adequacy of code requirements. The Seismic Safety Commission concluded in
1995: "It is reasonable to assume that either a larger magnitude earthquake
or one of similar strength but longer duration (than Northridge) will
subject similar structures to a substantially more strenuous test."

Hope this helps.

Fred Turner, Staff Structural Engineer, California Seismic Safety
Commission, 1755 Creekside Oaks Drive Suite 100, Sacramento, CA 95833
916-263-0582 Work Phone, 916-263-0594 Fax fturner(--nospam--at)

-----Original Message-----
From: Lynn <lhoward(--nospam--at)>
To: seaint(--nospam--at) <seaint(--nospam--at)>
Date: Monday, October 04, 1999 9:28 AM
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
>The 1994 Northridge earthquake was in fact a Code design level