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Re: [Seismic Zone 4 vs Richter Magnitude Number]

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seaint-return(--nospam--at) wrote:
If I am not mistaken, neither Blue Book nor Uniform Building Code (among
others) do not give a "translation" of  vertical / horizontal design load
requirements for (say) Seismic Zone 4 into "Richter-scale-magnitude number".
I know that this information is not needed to perform required structural
calculations  ... but it would be nice to know where I am at.

Your help would be appreciated ..

Dear Karen,

There is no correlation between seismic zones and Richter scales, and in fact
seismologists use a different scale, moment magnitude, to describe a faults

To determine how a fault will effect a given area equations were developed to
calculate the peak ground acceleration at a site or a region.  These equations
are called attenuation equations and there are many of them.  They consider
the specific soil types to 'translate' energy to acceleration.

Different faults have different potentials.  This is determined by the length
of fault and by the measured slip rate of a fault.  Large magnitude
earthquakes are more rare than lower magnitude earthquakes.  All earthquake
magnitudes can be defined by their rate of occurrence or their 'return rate'. 
The code for example refers to the 475 (often called the 500 year event) year
event.  If you assume that building's life is roughly 50 years there is only a
10% chance of a greater earthquake in any 50 year span, often called a 10%
chance of exeedance.

In California, most regions are effected by more than one fault.  The pga at
any site for the 500 year event is determined by what they call probabilistic
approach.  This basically means they statistically analyze all the pga's of
all the faults and since say .4g acceleration can occur on two faults for that
site or that region, the return rate of that pga would be reduced.  The effect
is that for the 500 year event the pga's are higher.

Please note that a building in Salt Lake City when compared to a Building In
Los Angeles can have the same or higher seismic forces in its life.  Just that
the probability is that the building in Los Angeles will have the higher

This is my final point.  We as Structural Engineers Design buildings.  Over
the last few years there appears to be a greater reliance on the numbers
coming out of the calculator.  Seismic zone 4 roughly (very roughly) refers to
zones with a pga of .4g.  Most buildings are designed to .183g.  Since .4g
would create stresses greater than the strength of the material, something
else must be working to hold the buildings together.  This the detailing, and
inherent ductility of our construction techniques.  This not taught in school.

I am not an expert on this stuff, but if I can answer any questions you might
have Email me at ktarlow(--nospam--at)


Ken Tarlow

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