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

[Allen Jones]

Speaking for myself, I find these discussions very interesting and informative.  I find think this sort of intellectual debate healthy and provides a perspective missing from publications.  I encourage you to keep the discussion public and appreciate everyone's efforts in taking the time to respond.

Allen Jones
________________________
Allen Jones, PE, PhD
South Dakota State University
Department of Civil Engineering
CEH 148, Box 2219
Brookings, SD 57006

Direct: 605-688-6467
Fax: 605-688-6476

On Fri, 15 Feb 2008, Arthur D Frankel wrote:

> Zhenming,
>
>  I am sure many people on the bulletin board are getting tired of these
> exchanges.
>
> I presented similar comparisons at the Applied Technology Council
> workshop in Memphis in March 2005.  As I recall, you and Joe Tomasello
> were in attendance.   So I think you have seen these comparisons.
>
> Of course, I don't see any contradiction in what I've said.   I think it
> is reasonable to compare code values with the expected ground motions from
> the next 1811-12 type earthquake and with ground motions estimated for the
> 1811-12 earthquakes using intensity observations.
>
>  I think further discussion between us on these issues should be made off
> of the bulletin board.
>
> -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
>
>
>
> "Wang, Zhenming" <zmwang at email.uky.edu>
> 02/15/2008 08:35 AM
>
> To
> Arthur D Frankel <afrankel at usgs.gov>
> cc
> James Cobb <cobb at uky.edu>, "Keifer, John D" <kiefer at email.uky.edu>,
> "ceus-earthquake-hazards at geohazards.usgs.gov"
> <ceus-earthquake-hazards at geohazards.usgs.gov>
> Subject
> RE: [CEUS-earthquake-hazards] reply to Joe Tomasello;   buildings codes
> and earthquake hazard
>
>
>
>
>
>
> Art,
>
> This is the first time we see these comparisons:
>
> ?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.?
>
> The selected design ground motion should be consistent with the scientific
> facts. However, these comparisons seem to be contradictory to your early
> statements:
> 1.      ?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.?
> 2.      ?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 St. Louis.?
>
> Thanks.
>
> Zhenming
>
> 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 3:35 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
>