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Re: Mistreatment of Seismology Issues

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Interesting topic... Let's analyze it a bit... I should preface my
statements by saying that I'm and engineering geologist/land-use planner and
not a structural engineer and that I'm going to generalize to focus on the
important points.

Richter magnitude doesn't tell you anything about distance to the fault or
epicenter, subsurface conditions, or your structural design parameters.
Thus, were someone to attempt a simple declaration that the potential for a
7.5 M earthquake is seismically unacceptable, you'd have to ask "At what
distance, for what ground conditions, and for what type of structure,
occupancy, and use?" This essentially is the reason that the state codes
(Seismic Hazards Mapping Act, for example) use the term "acceptable level of
risk" to which Reichle refers. The equation for determining acceptable level
of risk involves the natural environment, occupancy and planned use of the
structure, structural and site design (mitigation) measures, and, to a
considerable degree, political will. To illustrate the latter, some
communities want to focus on economic viability of the community after an
earthquake and are willing to require the builder to spend more to achieve
that level of safety, while others, to minimize cost of construction, want
the minimum level of safety (life safety) required by the state. [There may
even be some agencies who are politically willing to violate state minimums,
but we'll leave that part of the discussion for another time and place.]
Thus, the Seismic Hazards Mapping Act simply requires mitigation to a level
of risk that is acceptable to the permitting agency.

  -- Ted Smith

'Yank2002(--nospam--at)aol.com' wrote:

On October 13, 1998 I sent the following question addressed to California
Department of Conservation, Seismology Division:

How do you define "acceptable level of seismic hazard" in terms of Richter
scale magnitude?... [snip]

Reichle, Michael responded
> There is no "acceptable level of seismic hazard,"  because we have no
> control over the shaking.  We generally use the term "acceptable level of
> risk,"  which refers to how buildings and other structures respond to the
> shaking.  What the "acceptable level" actually is depends on your
> structure.  For example, buildings built according to the Uniform Building
> Code are built to life safety standards.  They should not collapse, but
> could be severely damaged.  Schools and hospitals in California are built
> to remain operational following an earthquake.  
> 
> Future building codes will incorporate "performance-based design" that
> will permit the owner to determine the performance level (or risk level)
> above the life safety standard that he may wish for his building.  The
> Federal Emergency Management Agency has some publications on
> performance-based design, if you wish to pursue that subject further.

[snip] 

Yank2002 reponded:

[snip]
The so-called "life safety standards" you mentioned in your E-Mail, defined
by
the state (and federal) people are misleading as they inform the public that
certain special structures (hospitals and schools) are life-safe. In case of
school structures the "life safety standards" statement misinforms the
parents
by suggesting that they should not worry about their children's safety while
they are in school. The reality, however, was and is different. 

[snip]

As a result of all of it: If an individual officially or unofficially
inquires
geologists or structural engineers about the state of seismic affairs in
California, he will be not necessarily deliberately but certainly knowingly
misinformed and his question will never be truthfully answered only because
the "acceptable level of risk" is a meaningless concept. Your comments would
be appreciated.

[snip]

Before I receive the response from the Conservation people, I would like to
put the issue on the conference table of structural engineers for possible
discussion. I realize that "structural engineering officialdom" would want
to
shoot me in the back ... but they don't know where I live. So, for the time
being I am safe.




Ted Smith <tsmith(--nospam--at)consrv.ca.gov>
   Seismic Hazards Mapping Outreach Coordinator
   California Department of Conservation
   Division of Mines and Geology
See http://www.consrv.ca.gov/dmg/shezp/ for seismic hazard zone maps and
related information.