<br><font size=3 face="Times New Roman">Joe,</font>
<br>
<br><font size=3 face="Times New Roman">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.
</font>
<br>
<br><font size=3 face="Times New Roman">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. </font>
<br>
<br><font size=3 face="Times New Roman">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. </font>
<br>
<br><font size=3 face="Times New Roman">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. </font>
<br>
<br><font size=3 face="Times New Roman">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.
</font>
<br>
<br><font size=3 face="Times New Roman">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). </font>
<br>
<br><font size=3 face="Times New Roman">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.
</font>
<br>
<br><font size=3 face="Times New Roman">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.</font>
<br>
<br><font size=3 face="Times New Roman"> 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.
</font>
<br>
<br><font size=3 face="Times New Roman"> 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 </font>
<br>
<br><font size=3 face="Times New Roman">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. </font>
<br>
<br><font size=3 face="Times New Roman">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.</font>
<br>
<br><font size=2 face="sans-serif">-Art</font>
<br>
<br>
<br>
<br><font size=2 face="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</font>