We develop and use a spectral empirical Green’s function approach to estimate the relative source amplitudes of earthquakes near San Juan Bautista, California. We isolate the source amplitudes from path effects by comparing the recorded spectra of pairs of events with similar location and focal mechanism, without computing the path effect. With this method, we estimate the relative moments of 1600 M 1.5–4 local earthquakes, and we use these moments to recalibrate the duration magnitude scale in this region. The estimated moments of these small earthquakes increase with catalog magnitude MD roughly proportionally to 101.1MD, slightly more slowly than a moment‐magnitude scaling of 101.5Mw. This more accurate magnitude scaling can be used in analyses of the local earthquakes, such as comparisons between the seismic moments and geodetic observations.

Electronic Supplement:Additional description and figures showing estimated source amplitudes obtained with different parameters, and observed amplitudes of some earthquakes with large misfits.

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