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<div class=Section1>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Art,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>This is the first time we see these
comparisons:<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>“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 <st1:place
w:st="on"><st1:City w:st="on">Memphis</st1:City></st1:place> area from the
1811-12 earthquakes.”<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>The selected design ground motion should be consistent with the
scientific facts. However, these comparisons seem to be contradictory to your
early statements:<o:p></o:p></span></font></p>
<ol style='margin-top:0in' start=1 type=1>
<li class=MsoNormal style='color:navy;mso-list:l0 level1 lfo1'><font size=3
color=black face="Times New Roman"><span style='font-size:12.0pt;
color:windowtext'>“</span></font><font size=2 color=black
face=Arial><span style='font-size:10.0pt;font-family:Arial;color:windowtext'>In
fact, we release seismic hazard curves (a range of ground motion, from 0.0
to 10g or larger) for a grid of sites across the nation, so that users can
calculate the ground motions at any probability level they choose.” “It
should also be reiterated that the national seismic hazard maps are based
on the average hazard curves from a variety of input models and
attenuation relations; they are not worst-case maps.” </span></font><font
size=2 face=Arial><span style='font-size:10.0pt;font-family:Arial'><o:p></o:p></span></font></li>
<li class=MsoNormal style='color:navy;mso-list:l0 level1 lfo1'><font size=2
color=navy face=Arial><span style='font-size:10.0pt;font-family:Arial'>“It
is not correct to compare the intensity observations from 1811-1812 with
the probabilistic hazard maps that also include the hazard from
earthquakes closer to <st1:City w:st="on"><st1:place w:st="on">St. Louis</st1:place></st1:City>.”
<o:p></o:p></span></font></li>
</ol>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Thanks.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Zhenming<o:p></o:p></span></font></p>
<div>
<div class=MsoNormal align=center style='text-align:center'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>
<hr size=2 width="100%" align=center tabindex=-1>
</span></font></div>
<p class=MsoNormal><b><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma;font-weight:bold'>From:</span></font></b><font size=2
face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'>
ceus-earthquake-hazards-bounces@geohazards.usgs.gov
[mailto:ceus-earthquake-hazards-bounces@geohazards.usgs.gov] <b><span
style='font-weight:bold'>On Behalf Of </span></b>Arthur D Frankel<br>
<b><span style='font-weight:bold'>Sent:</span></b> Thursday, February 14, 2008
3:35 PM<br>
<b><span style='font-weight:bold'>To:</span></b>
ceus-earthquake-hazards@geohazards.usgs.gov<br>
<b><span style='font-weight:bold'>Subject:</span></b> [CEUS-earthquake-hazards]
reply to Joe Tomasello; buildings codes and earthquake hazard</span></font><o:p></o:p></p>
</div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><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 <st1:City
w:st="on"><st1:place w:st="on">Memphis</st1:place></st1:City> 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 (<st1:country-region w:st="on"><st1:place
w:st="on">S.A.</st1:place></st1:country-region>), 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 <st1:City w:st="on">Memphis</st1:City> from a scenario earthquake with
moment magnitude 7.7 located on the portion of the current New Madrid
seismicity trend northwest of <st1:City w:st="on"><st1:place w:st="on">Memphis</st1:place></st1:City>
(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 <st1:City w:st="on"><st1:place w:st="on">Memphis</st1:place></st1:City>
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 <st1:City w:st="on"><st1:place
w:st="on">Memphis</st1:place></st1:City> (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 <st1:City w:st="on"><st1:place w:st="on">Memphis</st1:place></st1:City>
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 <st1:City w:st="on">Memphis</st1:City> area during the
1811-12 earthquakes have been assigned as intensity VIII by Hough et al. (2000)
and as intensity X by <st1:City w:st="on"><st1:place w:st="on">Johnston</st1:place></st1:City>
(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 <st1:City w:st="on"><st1:place
w:st="on">Memphis</st1:place></st1:City> 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 <st1:City w:st="on"><st1:place w:st="on">Memphis</st1:place></st1:City>
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 <st1:City w:st="on"><st1:place
w:st="on">Memphis</st1:place></st1:City> from the 1811-12 earthquakes, based
on intensity data. <br>
<br>
</span></font><font size=2 face=sans-serif><span style='font-size:10.0pt;
font-family:sans-serif'>-Art</span></font> <br>
<br>
<br>
<br>
<font size=2 face=sans-serif><span style='font-size:10.0pt;font-family:sans-serif'>Art
Frankel<br>
<st1:country-region w:st="on"><st1:place w:st="on">U.S.</st1:place></st1:country-region>
Geological Survey<br>
MS 966, Box 25046<br>
DFC<br>
<st1:place w:st="on"><st1:City w:st="on">Denver</st1:City>, <st1:State w:st="on">CO</st1:State>
<st1:PostalCode w:st="on">80225</st1:PostalCode></st1:place><br>
phone: 303-273-8556<br>
fax: 303-273-8600<br>
email: afrankel@usgs.gov</span></font><o:p></o:p></p>
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