<br><font size=2 face="sans-serif">To Laurence Wright,</font>
<br>
<br><font size=2 face="sans-serif"> You ask an excellent question
that deserves an answer (I hope you'll get many answers). Certainly,
facilities of critical importance, such as a regional blood bank, need
to be designed so that they will still be operational after the next moderate
or large earthquake in the vicinity, if there is a significant probability
of that earthquake occurring during the lifetime of the building.
</font>
<br>
<br><font size=2 face="sans-serif"> One of the major reasons for
using terminology such as a "2% chance of exceedance in 50 years"
is to express the probability that a building will experience ground motions
larger than a certain value over its approximate lifetime. If
there were no factor of safety engineered into a building, designing to
ground motions with a 2% chance of being exceeded in 50 years would imply
that there is a 2% chance of building damage or failure over 50 years.
Here I am assuming that having ground motions larger than the design
value will cause damage or failure of the building and I am ignoring the
variability in construction. This 2% probability takes into account the
probability of having a moderate or large earthquake over 50 years and
the probability that such an earthquake would produce a certain level of
ground shaking or larger if it occurs. </font>
<br>
<br><font size=2 face="sans-serif"> It would be up to you and the
Red Cross to decide if 2% (in this example) is an acceptable risk
over the lifetime of the building, keeping in mind the factor of safety
engineered into the structure. Structural engineers would
provide you with information on this factor of safety. Structural engineers
could also tell you the expected consequences to the building for
different levels of shaking.</font>
<br><font size=2 face="sans-serif">I am not making a statement on the appropriateness
of using 2% chance of exceedance in 50 years as a design probability
level for your building.</font>
<br>
<br><font size=2 face="sans-serif"> A useful check on this decision
of what probability level to use in design is to compare the design motions
to the ground motions expected for the next 1811-12 type New Madrid earthquake,
which the USGS estimates to have a 7-10% chance of occurring in the next
50 years (see previous emails). Do you want to design for the median
ground motions expected for such an earthquake? Of course, these
ground motions would be exceeded during the next 1811-12 type earthquake
at 50% of the locations, on average. This is caused by random variability;
we don't know if one of these locations with higher ground shaking would
be where your blood bank is built. This 50% chance of having ground
motions larger than the design value during the next earthquake might not
be acceptable to you. It turns out that ground motions in Memphis
with a 5% chance of being exceeded in 50 years approximately equal the
median ground motions expected for the next 1811-12 type earthquake. You
could design to higher ground motions that have lower probabilities of
being exceeded in 50 years that correspond to, say, the median plus
one standard deviation ground motions that will be exceeded at only
16% of the locations during the next 1811-12 type earthquake.
You could also do this comparison using the ground motions expected for
a nearby magnitude 6 earthquake. </font>
<br>
<br><font size=2 face="sans-serif"> Joe Tomasello in a recent email
suggested that you use a design with 10% probability of exceedance in 50
years for your facility. As I pointed out in a previous email, this
design value would be far less than the median ground motions expected
for the next 1811-12 type earthquake. It might also have the consequence
that your regional blood bank in Memphis would have a higher probability
of failure from earthquake shaking over its lifetime than, say, a WalMart
in Nashville that was built with design ground motions of 2/3 times the
value with 2% probability of exceedance in 50 years according to the International
Building Code.</font>
<br>
<br><font size=2 face="sans-serif"> As a seismologist, I am
not qualified to give you a specific probability level for design. I
can provide our best estimates of the ground motions with various probability
levels of occurring during the lifetime of your building. I can also
tell you the range of ground motions expected during the next moderate
or large earthquake and the range of estimates of the probability of that
earthquake occurring over the lifetime of your building. These are scientific
issues that are addressed through analysis of data on earthquake recurrence
and ground motions.</font>
<br>
<br><font size=2 face="sans-serif"> I hope this reply is helpful.
I would be happy to talk to you in more detail about the scientific
aspects of this issue. </font>
<br>
<br><font size=2 face="sans-serif">-Art</font>
<br>
<br>
<br><font size=2 face="sans-serif">Art Frankel</font>
<br><font size=2 face="sans-serif">Coordinator for Earthquake Effects Research<br>
U.S. Geological Survey<br>
MS 966, Box 25046<br>
DFC<br>
Denver, CO 80225<br>
phone: 303-273-8556<br>
fax: 303-273-8600<br>
email: afrankel@usgs.gov</font>
<br>
<br>
<br>
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<td width=40%><font size=1 face="sans-serif"><b><WrightLa@usa.redcross.org></b>
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<br><font size=1 face="sans-serif">Sent by: ceus-earthquake-hazards-bounces@geohazards.usgs.gov</font>
<p><font size=1 face="sans-serif">02/28/2008 07:40 AM</font>
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<div align=right><font size=1 face="sans-serif">To</font></div>
<td><font size=1 face="sans-serif"><WrightLa@usa.redcross.org>, <ceus-earthquake-hazards@geohazards.usgs.gov></font>
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<div align=right><font size=1 face="sans-serif">cc</font></div>
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<div align=right><font size=1 face="sans-serif">Subject</font></div>
<td><font size=1 face="sans-serif">Re: [CEUS-earthquake-hazards] Question-</font></table>
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<br><font size=2><tt>I find it intriguing that nobody on the list has actually
answered the<br>
questions I asked, which were-<br>
<br>
"Assume that you are designing a blood processing center for the Memphis<br>
Tennessee area which will serve approximately 10% of the US population.<br>
You are being asked to provide design standard requirements for this<br>
facility on which 10% of the United States population will depend for<br>
blood products.<br>
The base facility is being designed for a 45 year useful life.<br>
<br>
What seismic standard will you use?<br>
<br>
Why?"<br>
<br>
So why is it that everyone is willing to speak in generalities, but is<br>
apparently not willing to state whether they'd use 2%/50 as the MCE, or<br>
something else? I am not asking for someone to do any design, or go<br>
through the various evolutions of site evaluation, I just want to know<br>
what standard<br>
you, if you were the designer of record, would use as the basis of<br>
design. And why. I'm not going to criticize anyone, and
this is a<br>
hypothetical situation, but I am really interested in knowing what<br>
standard the experts here would use, and why they'd use that standard.<br>
I think most of us would find that enlightening.<br>
<br>
Laurence M. Wright<br>
Senior Project Engineer<br>
Manufacturing Engineering<br>
202-303-4457 (o)<br>
202-549-5843 (c)<br>
<br>
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</tt></font>
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