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We analyze two sets of repeating earthquakes on the Calaveras fault to estimate in‐situ rates of fault strengthening (healing). Earthquake recurrence intervals t_r range from 3 to 803 days. Variations in relative moment and duration are combined to study changes in stress drop, rupture dimension, rupture velocity, and particle velocity as a function of t_r. Healing rates and source variations are compared with predictions of laboratory‐derived friction laws. Two interpretations of event duration τ are used: one in which τ is given by the ratio of slip to particle velocity and one in which it scales as rupture dimension divided by rupture velocity. Our data indicate that faults strengthen during the interseismic period. We infer that source dimension decreases with t_r due to aseismic creep within the region surrounding the repeating events. Stress drop increases 1–3MPa per decade increase in t_r, which represents an increase of a …