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

[Hatheway Dr. Allen W.]

Bravo!

Ellis has finally spoken, and so again we have the fundamental rule  
of seismic preparedness
for public safety on the table before us!

May we strive for the honesty derived from keeping at least one eye  
on that shining caveat ....


Allen W. Hatheway
School of Mines & Metallurgy, University of Missouri - Retired
Rolla, Missouri

On Feb 15, 2008, at 12:12 PM, Krinitzsky, Ellis L ERDC-GSL-MS  
Emeritus wrote:

>
> Dear All,
>
> Wonderful things can be done with statistics, when there is plenty  
> of data.
> Demographic numbers are available as data sets of millions of  
> people and they
> can tell insurers exactly how many will die at any age. However, those
> statistics cannot tell you at what age any individual will die. Those
> statistics, like earthquake statistics, are not site specific.  
> Worse, for
> earthquakes there is hardly any data.  Every new earthquake changes  
> the local
> data base enormously and that changes the Gutenberg-Richter  
> probability
> projection. Additionally, the projected b-line is represented  
> through time by
> no data at all. Dozens of studies show that earthquake recurrences  
> are not
> regular and that the linear-projected b-line is not valid. So, what  
> does a
> 2-percent exceedance in x-years really mean?
>
> You need to go back to the basics and ask some hard questions.
>
> Ellis
>
> -----Original Message-----
> From: ceus-earthquake-hazards-bounces at geohazards.usgs.gov
> [mailto:ceus-earthquake-hazards-bounces at geohazards.usgs.gov] On  
> Behalf Of
> Wang, Zhenming
> Sent: Friday, February 15, 2008 9:35 AM
> To: Arthur D Frankel
> Cc: ceus-earthquake-hazards at geohazards.usgs.gov; James Cobb;  
> Keifer, John D
> 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
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