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<p class=MsoNormal><span style='font-size:11.0pt;color:#1F497D'>What I’ve
heard at commission meetings and read here is that you only recommend a map
that shows the ground motion from a 7.7 close to Memphis. Any other choice
would not meet with your (USGS) approval because a building wouldn’t be designed
for those forces. Which is precisely the point. Why do we need to design
for forces that the building will not likely see for a millennia or more? How
many buildings do we design to last a millennia or more? <o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;color:#1F497D'>Extending your
reasoning</span><span style='color:#1F497D'> it would seem that we should
design for 120 mph or more winds (it’s possible in this region) because
if the building experiences them they are larger than accounted for in the
design. This is contrary to the intent of building codes.<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>From your response it’s
reasonable to assume that USGS policy is that the only acceptable hazard maps
in the New Madrid Seismic Zone are those that show ground motion from an M7.7.
Is this correct? <br>
<br>
</span><span style='font-size:11.0pt;color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><b><span style='font-family:"Arial","sans-serif";color:#1F497D'>Joseph
Tomasello, PE</span></b><span style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-family:"Arial","sans-serif";color:#1F497D'>5880
Ridge Bend Rd.</span></b><span style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-family:"Arial","sans-serif";color:#1F497D'>Memphis,
TN 38120</span></b><span style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'> <o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-family:"Arial","sans-serif";color:#1F497D'>Phone:</span></b><span
style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-family:"Arial","sans-serif";color:#1F497D'>(901)
761-2016 office</span></b><span style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-family:"Arial","sans-serif";color:#1F497D'>(901)
821-4968 direct</span></b><span style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-family:"Arial","sans-serif";color:#1F497D'>(901)
412-8217 mobile</span></b><span style='font-size:11.0pt;color:#1F497D'><o:p></o:p></span></p>
<div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'>
<p class=MsoNormal style='margin-left:.5in'><b><span style='font-size:10.0pt;
font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;
font-family:"Tahoma","sans-serif"'>
ceus-earthquake-hazards-bounces@geohazards.usgs.gov
[mailto:ceus-earthquake-hazards-bounces@geohazards.usgs.gov] <b>On Behalf Of </b>Arthur
D Frankel<br>
<b>Sent:</b> Thursday, February 14, 2008 2:35 PM<br>
<b>To:</b> ceus-earthquake-hazards@geohazards.usgs.gov<br>
<b>Subject:</b> [CEUS-earthquake-hazards] reply to Joe Tomasello; buildings
codes and earthquake hazard<o:p></o:p></span></p>
</div>
<p class=MsoNormal style='margin-left:.5in'><o:p> </o:p></p>
<p class=MsoNormal style='margin-left:.5in'><br>
Joe, <br>
<br>
The USGS policy is to support the process of the Building Seismic Safety
Council (BSSC) establishing probability levels and design procedures for the
national model building codes, such as the International Building Code.
The BSSC membership consists of a large group of engineers and
stakeholders. The BSSC is a council of the National Institute of Building
Sciences. The code development process of the BSSC is funded by FEMA. The
design procedures are published in the NEHRP Recommended Provisions for the
Development of Seismic Regulations for New Buildings, which is written by the
BSSC/NIBS and published by FEMA. <br>
<br>
I think a key responsibility of the USGS is to provide the best scientific
information to decision makers. Part of this scientific information is
assessment of the ground motions from the 1811-12 earthquakes and estimation of
the ground motions for the next 1811-12 type earthquake. <br>
<br>
When I talk to code committees and other groups, I compare the relative level
of protection that designing to different probability levels of ground shaking
will provide to buildings. This can be assessed by comparing the
ground-motion values for the probability levels in the building codes to the
median ground motions expected when the next 1811-12 type New Madrid earthquake
occurs and by comparing code values to intensities observed in the Memphis area
from the 1811-12 earthquakes. <br>
<br>
For example, the value of ground motions around 1 Hz with a 10% probability of
exceedance in 50 years (10%/50) is substantially lower than the median 1 Hz
ground motion expected for the next 1811-12 type earthquake. The new
Memphis code adopted in 2006 uses the 10%/50 year ground motions from the 1996
vintage of the national maps (the 2002 maps are higher). Here I am
considering 1 Hz spectral accelerations (S.A.), which are used for the design
of buildings with about 10 stories. For a site in Memphis (35.15 N; 90.05
W), the 10%/50 value of 1 Hz spectral acceleration is 0.16g (from the
1996 maps and using an amplification factor of 2.4 for class D stiff-soil site
relative to firm-rock site from the NEHRP amplification factors). This is
much lower than the median 1 Hz S.A. of 0.36g expected in Memphis from a
scenario earthquake with moment magnitude 7.7 located on the portion of the
current New Madrid seismicity trend northwest of Memphis (using the stiff-soil
amplification factor from the NEHRP factors). This calculation of the expected
spectral acceleration is based on the average of the five attenuation relations
used in the 2002 national maps. If the next large New Madrid earthquake
was a moment magnitude 7.4, the calculated median 1 Hz S.A. at Memphis would be
0.29g for a stiff-soil site, still much higher than the 10%/50 value (0.16g)
from the 1996 maps. <br>
<br>
The 10%/50 values for 1 Hz S.A. from the 2002 hazard maps would still be
significantly lower than the scenario ground motions. For 5 Hz S.A., the
expected values of the median ground motions for a M7.7 earthquake are more
sensitive to assumptions on the nonlinearity and attenuation of sediments in
the Mississippi Embayment. <br>
<br>
The International Building Code (IBC) uses spectral accelerations that are 2/3
times the values with a 2% probability of exceedance in 50 years for most of
the nation (there are some areas where IBC uses the median deterministic ground
motions, depending on the level of the probabilistic motions). For 1 Hz S.A.,
the 2006 IBC specifies a value of 0.42g for Memphis (stiff-soil site). This is
similar to the median value of 0.36g expected for a M7.7 earthquake (see
above). <br>
<br>
In my presentations, I also compare the code values to the ground motions
estimated from intensity reports in the Memphis area from past earthquakes.
Here I use peak ground accelerations (PGA) rather than spectral
accelerations, because intensities are generally correlated in the literature
with PGA’s or peak ground velocities. <br>
<br>
Intensities in the Memphis area during the 1811-12 earthquakes have been
assigned as intensity VIII by Hough et al. (2000) and as intensity X by
Johnston (1996), depending on their interpretation of earthquake effects.
Intensity VIII corresponds to a peak ground acceleration between about
0.34 and 0.65g, based on the work David Wald did for Shakemap. This range
is consistent with the calculated median PGA at Memphis of 0.39g for a M7.7
earthquake determined from the average of 5 attenuation relations used in the
2002 hazard maps (using the NEHRP amplification factors). The calculated PGA
for a M7.4 earthquake is 0.32g, close to the range of the PGA’s estimated
for intensity VIII. <br>
<br>
The new Memphis code procedure of using the 10%/50 values from the 1996
maps results in a PGA of 0.23g (for a stiff soil site), which is substantially
lower than the range of ground motions estimated from the intensities reported
in Memphis during the 1811-12 earthquake sequence (0.34-0.65g for intensity
VIII). A similar value of PGA (0.22g) is found by taking the 5 Hz S.A.
with 10%/50 and dividing by 2.0, which is the factor relating PGA to 5 Hz S.A.
derived for M7.4-7.7 earthquakes from the average of the five attenuation
relations. <br>
<br>
Using a PGA that is 2/3 times the PGA with 2% probability of exceedance
in 50 years (2%/50), which corresponds to the procedure used for spectral
accelerations in the 2006 International Building Code for the Memphis area,
gives a PGA value of 0.50g for Memphis (stiff soil site), which is in the range
of the values estimated from the 1811-12 intensities. This is similar to
the PGA value of 0.47g derived from the 5 Hz S.A.in the IBC divided by a factor
of 2.0 to convert to PGA. So, there is evidence from intensity data that
the ground motions specified in the IBC have been experienced in Memphis during
the 1811-12 earthquakes <br>
<br>
In summary, the 1 Hz spectral accelerations with a 10% probability of
exceedance in 50 years, as used in the current Memphis code, are substantially
lower than the median 1 Hz spectral accelerations expected for the next 1811-12
type earthquake. The 10%/50 value of PGA is probably lower than the
ground shaking experienced in Memphis during the 1811-12 earthquakes, based on
intensity data. <br>
<br>
The 1 Hz spectral accelerations specified in the International Building Code
(2/3 times the motions with a 2% probability of exceedance in 50 years) are
similar to the median 1 Hz spectral accelerations expected
for the next 1811-12 type earthquake. The PGA with 2/3 times the value with
2%/50 is probably comparable to the PGA experienced in Memphis from the 1811-12
earthquakes, based on intensity data. <br>
<br>
<span style='font-size:10.0pt;font-family:"Arial","sans-serif"'>-Art</span> <br>
<br>
<br>
<br>
<span style='font-size:10.0pt;font-family:"Arial","sans-serif"'>Art Frankel<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</span><o:p></o:p></p>
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