Body waveform modeling is used to determine the source parameters of two earthquakes that occurred along the northern San Jacinto fault system in 1918 and 1923. The 1918 earthquake parameters are consistent with a 20- to 25-km rupture along the Claremont fault at a depth of 7 ± 5 km. Limited data for the 1923 event are more consistent with rupture on the San Jacinto fault than rupture along the San Andreas fault or a buried cross fault, although the greatest damage associated with the earthquake occurred closer to the San Andreas fault. Using the results of this study and previous studies of historic earthquakes along the southern San Jacinto fault, the relationships between rupture length and magnitude, focal depth and heat flow, and other parameters are examined in an effort to estimate the source properties of future events that are expected along currently quiescent portions of the fault zone. One important observation is that rupture lengths along the San Jacinto fault are shorter for an equivalent moment magnitude than those along the Imperial Valley-Mexicali fault system. The higher heat flow of the Imperial Valley region may be responsible for this difference.