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

[Wang, Zhenming]

I want to thank Mark and Nico for their clarifications on the USGS national hazard maps. "While USGS hazard maps are the basis for design maps in model building codes, hazard and design maps are not one in the same (for example, due to the capping mentioned in note #3 above).  This was true of the 1996 and 2002 USGS hazard maps and the building code design maps derived from them, and it continues to be true (more so, in part due to notes #1-2 above) for the 2008 USGS hazard maps."  "As far as I know, the USGS hazard maps have never been used directly in building design." These clarifications are important for healthy discussions on seismic hazard assessment and engineering design.

I would like to point out that the hazard data will be the basis, "the new 2008 USGS hazard data will be the basis for the seismic design maps in future editions of the NEHRP Recommended Provisions," not the hazard maps (the ground motions with 500, 1,000, 2,500 years return periods), "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."

Now let's discuss how we can use "the new 2008 USGS hazard data" for our discussions on the design ground motions.

In the central US, the large New Madrid earthquakes are the main concern. So, we can follow the analyses (deterministic or scenario) by Art and Mark.  As shown by Art and Mark, the median PGA, 0.2s and 1.0s SAs are about 0.30g, 0.60g and 0.16g in Memphis, respectively, if the large New Madrid earthquake (M7.7) occurs on the central strand (Petersen, 2004, http://www.uky.edu/KGS/geologichazards/summary_usgskgsmeeting.pdf). If we do not consider risk (probability of building damage in the next 50 years), these ground motion parameters (0.3g PGA, 0.6g 0.2s SA, and 0.16g 1.0s SA) may be appropriate for seismic design in Memphis.

Following the same analyses and assumptions by Art (see Art's email comments to Laurence last year), the risk is about 3.5-5% PE damage in 50 years for a building that is designed to the median ground motion in Memphis.  From the 2008 USGS hazard database, the median PGA, 0.2s and 1.0s SAs are about 0.27, 0.59 and 0.21g in San Francisco, respectively (only the San Andreas earthquake, M7.9 with 350-y recurrence interval, is considered). The risk is about 7% PE in 50 years for a building that is designed to the median ground motion in San Francisco. To achieve the same risk level (7% PE damage in 50 years) in Memphis, the design PGA, 0.2s and 1.0s SAs will be about 0.17-0.21g, 0.35-0.42g, and 0.09-0.11g, respectively (a log standard deviation of 0.7 is assumed).

These deterministic or scenario analyses demonstrate that the recommended design ground motions for the NEHRP Provisions are too conservative in Memphis.

Thanks,

Zhenming
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