<br><font size=2 face="sans-serif">Joe,</font>
<br>
<br><font size=2 face="sans-serif"> I feel I need to reply to you
on the bulletin board, since you seem to want to keep this discussion going
and you directly ask me questions. My apologies to Paul Segal and others
who are fed up with this.</font>
<br>
<br><font size=2 face="sans-serif"> I guess you missed one of the
key points of my previous email. I said that the IBC values (2/3
times the ground motions with 2% probability of exceedance in 50 years
[2%/50]) were probably experienced in Memphis during the 1811-12 earthquakes,
based on intensity data from those earthquakes. </font>
<br>
<br><font size=2 face="sans-serif"> You have this mistaken
notion that IBC values (2/3 times the 2%/50 motions) are only observed
once in a millenium. You have the mistaken notion that 2%/50 values
are only observed once in 2500 years. This is flat out wrong. Some
locations will experience 2/3 times the 2%/50 ground motions during the
next 1811-12 type earthquake. Some locations will experience the
2%/50 ground motions during the next 1811-12 type earthquake. This
is a simple consequence of the observed variability of earthquake ground
motions. </font>
<br>
<br><font size=2 face="sans-serif"> As far as the magnitude
issue you bring up, the USGS uses the range of magnitudes that various
seismologists have determined for the 1811-12 earthquakes from the
intensity data. The central values of these moment magnitude determinations
range from 7.4-7.5 (Hough et al., 2000 in JGR) to 7.8 (Bakun and
Hopper, 2004 in BSSA) to 8.0-8.1 (Johnston, 1996 in Geophysical Journal).
In the national maps we use a logic tree to express this range, with
a value of 7.7 given the highest weight. I used M7.7 in the scenario
in my previous email, because it is in the center of the range of magnitudes
determined for the 1811-12 earthquakes. I also gave results for a
M7.4 earthquake.</font>
<br><font size=2 face="sans-serif"> </font>
<br><font size=2 face="sans-serif"> In the example in my previous
email, I placed the scenario earthquake where the current seismicity
trend for the New Madrid seismic zone is located. The closest distance
to downtown Memphis is about 60 km.</font>
<br>
<br><font size=2 face="sans-serif">-Art</font>
<br>
<br><font size=2 face="sans-serif"> </font>
<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>
<br>
<br>
<br>
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<td width=40%><font size=1 face="sans-serif"><b>"Joe Tomasello"
<JT@ReavesFirm.com></b> </font>
<p><font size=1 face="sans-serif">02/15/2008 09:32 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">"'Arthur D Frankel'" <afrankel@usgs.gov>,
<ceus-earthquake-hazards@geohazards.usgs.gov></font>
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<td>
<div align=right><font size=1 face="sans-serif">cc</font></div>
<td>
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<td>
<div align=right><font size=1 face="sans-serif">Subject</font></div>
<td><font size=1 face="sans-serif">RE: [CEUS-earthquake-hazards] reply
to Joe Tomasello; buildings codes and
earthquake hazard</font></table>
<br>
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<td>
<td></table>
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<br>
<br>
<br><font size=2 color=#1f497d face="Times New Roman">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? </font>
<br><font size=2 color=#1f497d face="Times New Roman"> </font>
<br><font size=2 color=#1f497d face="Times New Roman">Extending your reasoning</font><font size=3 color=#1f497d face="Times New Roman">
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.</font>
<br><font size=3 color=#1f497d face="Times New Roman"> </font>
<br><font size=3 color=#1f497d face="Times New Roman">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>
</font>
<br><font size=2 color=#1f497d face="Times New Roman"> </font>
<br><font size=3 color=#1f497d face="Arial"><b>Joseph Tomasello, PE</b></font>
<br><font size=3 color=#1f497d face="Arial"><b>5880 Ridge Bend Rd.</b></font>
<br><font size=3 color=#1f497d face="Arial"><b>Memphis, TN 38120</b></font>
<br><font size=3 color=#1f497d face="Times New Roman"> </font>
<br><font size=3 color=#1f497d face="Arial"><b>Phone:</b></font>
<br><font size=3 color=#1f497d face="Arial"><b>(901) 761-2016 office</b></font>
<br><font size=3 color=#1f497d face="Arial"><b>(901) 821-4968 direct</b></font>
<br><font size=3 color=#1f497d face="Arial"><b>(901) 412-8217 mobile</b></font>
<br><font size=2 face="Tahoma"><b>From:</b> ceus-earthquake-hazards-bounces@geohazards.usgs.gov
[mailto:ceus-earthquake-hazards-bounces@geohazards.usgs.gov] <b>On Behalf
Of </b>Arthur D Frankel<b><br>
Sent:</b> Thursday, February 14, 2008 2:35 PM<b><br>
To:</b> ceus-earthquake-hazards@geohazards.usgs.gov<b><br>
Subject:</b> [CEUS-earthquake-hazards] reply to Joe Tomasello; buildings
codes and earthquake hazard</font>
<br><font size=3 face="Times New Roman"> </font>
<br><font size=3 face="Times New Roman"><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>
</font><font size=2 face="Arial"><br>
-Art</font><font size=3 face="Times New Roman"> <br>
<br>
<br>
</font><font size=2 face="Arial"><br>
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>
<br>