[CEUS-earthquake-hazards] reply to Joe Tomasello; buildings codes and earthquake hazard
Arthur D Frankel
afrankel at usgs.gov
Fri Feb 15 11:50:00 MST 2008
Joe,
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.
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.
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.
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.
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.
-Art
Art Frankel
U.S. Geological Survey
MS 966, Box 25046
DFC
Denver, CO 80225
phone: 303-273-8556
fax: 303-273-8600
email: afrankel at usgs.gov
"Joe Tomasello" <JT at ReavesFirm.com>
02/15/2008 09:32 AM
To
"'Arthur D Frankel'" <afrankel at usgs.gov>,
<ceus-earthquake-hazards at geohazards.usgs.gov>
cc
Subject
RE: [CEUS-earthquake-hazards] reply to Joe Tomasello; buildings codes
and earthquake hazard
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?
Extending your reasoning 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.
>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?
Joseph Tomasello, PE
5880 Ridge Bend Rd.
Memphis, TN 38120
Phone:
(901) 761-2016 office
(901) 821-4968 direct
(901) 412-8217 mobile
From: ceus-earthquake-hazards-bounces at geohazards.usgs.gov
[mailto:ceus-earthquake-hazards-bounces at geohazards.usgs.gov] On Behalf Of
Arthur D Frankel
Sent: Thursday, February 14, 2008 2:35 PM
To: ceus-earthquake-hazards at geohazards.usgs.gov
Subject: [CEUS-earthquake-hazards] reply to Joe Tomasello; buildings codes
and earthquake hazard
Joe,
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.
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.
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.
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.
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.
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).
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.
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.
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.
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
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.
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.
-Art
Art Frankel
U.S. Geological Survey
MS 966, Box 25046
DFC
Denver, CO 80225
phone: 303-273-8556
fax: 303-273-8600
email: afrankel at usgs.gov
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