[CEUS-earthquake-hazards] reply to Joe Tomasello; buildings codes and earthquake hazard

[Arthur D Frankel]

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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