[CEUS-earthquake-hazards] Comment for the CEUS web discussion

[Oliver Boyd]

>From Mark Petersen:

 

-----------------------------------------------

Seth Stein recently sent a comment discussing a statement on the USGS

website:

 

“Readers may be interested in the latest proposal for seismic building

design criteria. In it "the 2008 USGS hazard maps should not be substituted for the model building code design maps nor should they be used with ASCE/SEI 41 or 31 for seismic rehabilitation or evaluation." 

 

Specifically, there are a set of scale (risk) factors to apply to the 2008 USGS hazard maps to produce design maps: "Resulting risk coefficients are generally 0.85-1.15, but as low as 0.7 near New Madrid and Charleston".

 

I would like to follow up and expand on Nicolas Luco’s response. Seth

Stein’s comment could be interpreted as implying that the USGS and the

engineering community are changing their view and backing off the USGS

hazard maps and that these maps are not useful for design. Nothing could be further from the truth. As far as I know, the USGS hazard maps have never been used directly in building design. During the recent code revisions there was always a separate effort to develop design maps. Over the last decade there were two separate efforts by the Building Seismic Safety Council (BSSC) to produce design maps from the USGS hazard maps (Project 97 and Project 07). Since 1997 the engineers have used the USGS hazard maps as the basis for the design maps and this has not changed in development of the 2008 design maps. The reason we included the disclaimer cited above by Seth Stein is that we did not want engineers to be confused and use the USGS hazard maps directly for designing buildings since this is not their intended purpose.

 

For your interest I tried to quickly pull together some of the comparisons of the new proposed 2008 design values with the different building codes for a site in Memphis, please feel free to check these numbers if you are interested so we can be sure that we all agree with these values. The 2008 building code values of NEHRP are proposed values since they have not been fully adopted by the BSSC. For a NEHRP D-class soil site at 35.15 N, 90.05 W, the new 2008 design ground motions are 0.4 g at 1 s SA and 0.74 g at 0.2 s SA. These new proposed 2008 design ground motions are similar to the 2006 IBC ground motions. The 2006 IBC values for the site are 0.44 g at 1 s SA and 0.92 g at 0.2 s SA; these values are 10% and 24% higher, respectively, than proposed 2008 values. The Standard Building Code values for Memphis (that was used until 2006 for Memphis) are 0.29g for 1 s SA and 0.5 g for 0.2 s SA, which are 28% and 32% lower than the 2008 design values. The current Memphis/Shelby County building code design values for non-essential structures are considerably lower than the new 2008 proposed values. The Memphis code for non-essential structures currently recommends 0.16 g for 1 s SA and 0.45 g for 0.2 s at this same site, which are 60% and 39% lower than the new 2008 proposed design values. This was also discussed by Art

Frankel about a year ago.

 

 

In my examinations of historical intensities and ground motions in the CEUS and other regions I have concluded that if New Madrid were to experience a repeat of the 1811-1812 earthquake sequence that many sites in the Memphis region (located 30-50 km from the events) would exceed the 0.16g at 1 s SA.

 

Mark Petersen

 

U.S. Geological Survey

phone: (303) 273-8546

fax (303) 273-8600

e-mail: mpetersen at usgs.gov

 

mailing address: USGS, Denver Federal Center,

MS 966, Box 25046, Denver, CO 80225

 

USGS office (for overnight deliveries):

1711 Illinois Street, Golden, CO 80401

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://geohazards.usgs.gov/pipermail/ceus-earthquake-hazards/attachments/20090123/fed3fa7e/attachment.html