same values in GA from the many times i've looked at both (i always use the calculator and my supervisor uses the lat/lon hazard values as a check). huh, i was under the impression they were the same but apparently i was wrong. if i had to guess, i would say it is some areas outside the conterminous u.s. and/or in high seismic areas such as "spots" in the new madrid. i suspect the new madrid since the codes wouldn't want to make it so completely expensive to build something that they hurt a particular economy while the usgs has the duty to report it as calculated. again, THAT IS PURE SPECULATION ON MY PART! i'll do some digging to try and find the answer now that i'm curious.

by the way, be sure to keep up to date with the new maps coming out. if you look at the preliminary maps available, there are a couple maps that show the difference from the current maps to the new maps. some areas will see another round of lowered values similar to that from the '96 to '02 maps.

perhaps this helps narrow down where the error message is referring to. i haven't checked for comparison but it seems to go along with my speculation about IBC "cutting off" the numbers at the high end values on the maps for high seismic areas. i'll keep looking for a more definitive answer to your (and now my) question.

http://www.seismosoc.org/publications/SRL/SRL_78/srl_78-4_op.html

here's the particular paragraph:
The seismic design loads in the International Building Code (IBC) are based on the site-specific hazard analysis carried out by the California Geological Survey (CGS) and the U.S. Geological Survey (USGS). The 475- year return-period site-specific hazard analysis did not capture the effects of rare but large earthquakes, which pose significant aggregate risk in the New Madrid seismic zone, the Pacific Northwest, and the Wasatch region. Therefore, the Building Seismic Safety Council decided to increase the return period of the ground motions in the IBC to 2,475 years but require the design of buildings for only two-thirds of the 2,475-year return-period ground motions. The two-thirds of the 2,475-year return-period ground motions were “too high” in the western United States due to the high occurrence rate of earthquakes. Therefore, the 2,475-year return-period ground motions were truncated by the “deterministic limit” to obtain the MCE (maximum considered earthquake) ground motions. As a result of these arbitrary adjustments and truncations, the seismic design according to the IBC is difficult to articulate.

"...it has come to our attention that it was being used by some to obtain ground motion values for building design."

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And what did the USGS think it was being used for?  Come on.  This is a no brainer.

Mike McCann
McCann Engineering

well, i believe the usgs maps provide a "realistic" scientific evaluation of the seismic motions. building code has to play more of a political game with smoothing over areas with very large spikes (such as immediately over faults perhaps) so as not to create dead development areas.
again, i have not found the specific locations that the ibc usgs maps differ from the usgs maps. but this was the concensus among a few seismic people i talked with. i'm sure usgs would prefer their original maps be used since they are based on scientific data and not political motivation.

Thanks for the enlightenment.  I love the way $$$ drive the numbers.

Mike McCann
McCann Engineering

bad thing is that, if that assumption is true, someone would probably try to start a class action lawsuit against ibc for doing that in the event a large earthquake hit that particular location. and the sad thing is that our screwed up court system would probably rule against the building codes. not that i necessarily agree with the basis of smoothing over numbers in such a case, but the building code is not meant to be "save all" designer. they simply try to establish minimum standards to give some assurance that the public will be safe for whatever the subject matter is. my guess is that site specific hazard analysis would be required if these locations are actually in high seismic areas anyway.

i checked a few spots along the new madrid but couldn't find a variance. however i was just using zip code searches. maybe it's at very specific locations or something. i'd be interested to see where the differences are and the magnitude of the differences.

here's maps showing the changes coming to the usgs maps for the ceus (and likely the next ibc i would presume). the first link is the 1 sec 2%/50yr map and the second is the .2 sec 2%/50yr map.

http://earthquake.usgs.gov/research/hazmaps/products_data/2007/maps/ceus/ceus.1hzratio.2pc50.jpg
http://earthquake.usgs.gov/research/hazmaps/products_data/2007/maps/ceus/ceus.5hzratio.2pc50.jpg

and note to my last post, those maps are showing the changes to the current maps. the links are not the actual proposed Ss and S1. so the links will show a multiplication factor of sorts with the 2002 usgs maps being the denominator. a "1.0" means no change to the previous maps(that's my interpretation of them).

here are the actual proposed maps.
http://earthquake.usgs.gov/research/hazmaps/products_data/2007/maps/ceus/ceus.1hz.2pc50.jpg
http://earthquake.usgs.gov/research/hazmaps/products_data/2007/maps/ceus/ceus.5hz.2pc50.jpg

here's one more link of possible use. it's an interactive map with the new values. you can zoom in to your area for a closer look.
http://gldims.cr.usgs.gov/nshmp2007/

ah, i finally found a more official explanation. it is along the lines of what i suspected. to explain it a little better, essentially, the usgs maps are probablistic mce maps. however, for high seismic areas, the values were so high due to the possible magnitude and occurance of earthquakes that a deterministic method based more on historical documentation (plus it looks like a safety factor was tacted on) was used to put somewhat of a cap on the values. so in other words, ibc & asce7 maps "smoothed over" the maps to take out the biggest spikes for high seismic areas such as the new madrid and the western u.s. coastal states plus a few other places. here's a good place to find this and some other information
http://www.bssconline.org/FEMA451B/451Bchapters.htm
in particular see "topic 5a notes" (around slide 75-80).

Thanks, msucog, for all your input on this thread.