Although the use of GPS data to infer or constrain earthquake occurrence rates was ultimately not adopted, the investigation resulted in two useful outcomes: a compilation of global studies in which crustal strain rates were compared to seismicity rates, and a summary of physical theories for aseismic deformation. Using the Kostrov (1974) formula relating strain rate to seismic moment rate (and then to seismicity rate), direct comparisons can be made. The ratio of seismic moment rate from seismicity to that from crustal strain is termed seismic efficiency. Physical mechanisms that allow elastic strain to be released aseismically include afterslip, poro-elastic rebound, and visco-elastic deformation (which are three types of post-seismic deformation, or PSD), fault creep, metamorphic processes, post-glacial rebound, magma movement, water table changes, and ductile behavior of salt deposits. PSD is known to "contaminate" non-secular crustal strain signals, and its effect scales with magnitude, time and distance for large events. A literature search yielded about 25 regional studies worldwide in which geodetic data was compared to seismicity rates, and in some cases paleoseismic rates. Seismic efficiencies from these studies were tabulated, and showed a wide range of values. Statistical fits to the data yielded a range of 0.15 to 1.0 with a peak at 0.63 for a triangular distribution, and a similar range with a peak at 0.68 for a logistic distribution.