But the time history and magnitude of past earthquakes have been used to determine a statistical probability. One of the US building code design standards is the statistical estimate of earthquake parameters having a 2 percent probability of exceedance in 50 years (typical design life of a building). That 2%/50 event correlates mathematically to a 2,500-year return interval. For the Salt Lake area, those parameters are roughly Magnitude 7.2 and a peak horizontal ground acceleration of about 0.5g, or half the acceleration of gravity.

http://earthquake.usgs.gov/hazards/?source=sitenav