We describe an efficient method to determine moment magnitude and stress parameter in near real time, in the immediate aftermath of a small‐to‐moderate earthquake (M∼3–6), from ShakeMap ground‐motion parameters (5% damped pseudospectral acceleration) at 1 Hz, 0.33 Hz, 10 Hz and/or peak ground acceleration. The methodology is based on relating ShakeMap parameters to source and attenuation parameters within the context of a generic stochastic point‐source model, to provide an event‐specific ground‐motion prediction equation (GMPE) that will reliably predict amplitudes. It is an approach that has been optimized for applications in regions with sparse networks; an example is provided for southern Ontario. In brief, we initially develop a simulation‐guided generic GMPE model based on the available database in the region. We derive the applicable regional anelastic attenuation, stress parameter, and site amplification models. Then, using the estimated regional models, we show how to invert ShakeMaps parameters to estimate moment magnitude and stress parameter in near real time, to define an event‐specific GMPE for an event that just happened. The event‐specific GMPE can be used to provide robust, calibrated ShakeMaps that are fully consistent with ground‐motion observations.