We used a comparison of source time function pulse widths to show that a group of earthquakes on the San Andreas fault near Parkfield have a constant duration over a magnitude range of 1.4–3.7. Earthquakes on secondary faults have an increase in duration with magnitude, which is the expected relationship for the usual observation of constant stress drop. The constant duration suggests that fault area is the same regardless of magnitude and that variations in stress drop are due entirely to variations in slip. Calculated stress-drop values on secondary faults range from 0.31 to 14 MPa, and stress-drop values on the San Andreas fault range from 0.18 to 63 MPa. The observation of constant duration on the San Andreas fault is consistent with a model of a locked asperity in a creeping fault. The differences in durations between the events on the San Andreas fault and on secondary faults suggest that earthquakes on the San Andreas fault are inherently different. We speculate that faults with more cumulative displacement have earthquakes that may rupture differently. Furthermore, the differences in source properties between the two populations might be explained by differences in fault surface roughness.