difference between mce and pga

huangyhg

difference between mce and pga
what exactly is the difference between mce (maximum credible earthquake) and pga (peak ground acceleration)? it's not clear to me from their definitions. how're their magnitudes determined?
also, why is it that, generally, buildings are designed for pga (eg. ubc recommneds effective pga) where as mce values are used in bridge designs? is it just a matter of choice or there is any technical rationality?
-thanks,
rao.
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the mce is the largest credible earthquake that appears to be possible along a recognised fault or within a geographically defined tectonic province under the presently known or presumed tectonic framework.  thus no matter how long a return period you consider, the mce represents the best guess at what would be the largest earthquake likely to occur.
the peak ground acceleration varies depending on the return period of the earthquake event.  that is, the pga of a 1:100 event is smaller than the pga of a 1:500 earthquake.  
the pga is got either from the accelerogram of an actual event = absolute peak value measured off the accelerogram or from the acceleration response spectrum for period = 0 secs.  this is because the short period spectra represents the response of a very stiff single degree of freedom system and thus will follow the ground motion.
important facilities such as major bridges/power stations etc are usually designed to survive the mce because of their public utility and need to be operative after major earthquakes.  hence the seismic loadings codes for these sorts of structures talk about mce.
general buildings are usually designed to survive with damage an earhquake with a return period of around 1:500 years. in areas of the world that are not seismically active, a mce could have a return period less than this and so that area's seismic loadings code should reflect this and only require design to a lesser event.