Obviously, I have no idea on what you seen previously, so it is hard to answer you...I just try to give you few ideas.
- There is an update of the codes/documents based on 2008 USGS hazard data versus codes/documents which makes use of 2002 USGS hazard data.
I'm not sure this is the root of your concern because the differences are not so big.
USGS was not explaining the differences; they rather say "High values on these maps do not necessarily indicate high ground motion values, but simply an increase in ground motion values from the 2002 update".
Moreover, I think 2010 CALIFORNIA BUILDING CODE is based on IBC-09 and ASCE 7-05, even some reference to 2008 USGS maps can be found there; anyway I'm not sure about this point.
I think the best for you is to establish which set of USGS map you need to consider- if I understand well, 2008 USGS is linked to IBC 2012 and ASCE 7 2010; 2002 USGS is the basis of IBC 2009 and ASCE 7 2005.
- There is an USGS software on USGS site that gives you spectral accelerations on locations in USA and allows you to switch between USGS maps versions.
See
http://geohazards.usgs.gov/designmaps/us/application.php The report generated by that application constructs both MCE Spectrum and Design Spectrum.
Maybe your concern is related to values of MCE Response Spectrum that are 1.5 larger than the "design" values?
See attached two reports generated by USGS application for the same location.
Just hope this help.
My best regards.