A new relationship between modified Mercalli intensity (MMI) and peak ground velocity (PGV) is developed by comparing instrumentally recorded and inferred historical PGV values to observed MMI for 18 significant earthquakes from eastern North America (ENA), of moment magnitude 3.6-7.25. This PGV-MMI relationship is implemented in ShakeMaps in Ontario to estimate ground-shaking intensity (instrumentally derived MMI), given instrumental recordings of PGV. Corresponding relationships for 5% damped pseudoacceleration (PSA) at frequencies of 1, 5, and 10 Hz are also developed. We compare the results with previous relationships between MMI and ground motion parameters developed for California by Wald et al. (1999a) and Atkinson and Sonley (2000). We conclude that empirical relationships between MMI and ground motion are significantly different in eastern North America than in California. Thus, such relationships should be assessed for each region where ShakeMaps are implemented.