Area or a specific site identified by the most recent USGS data (see United States Geological Survey (USGS) Peak Ground Acceleration (PGAPGAPeak Ground Acceleration as determined by the United States Geological Survey.) Calculator Tutorial) as having a PGAPGAPeak Ground Acceleration as determined by the United States Geological Survey. equal to or greater than 0.15g (i.e., 15% of the acceleration of gravity (g) using a 10% probability of exceedance in a 50 year period).