Control of the ~13 km range of the maximum depth of earthquakes in southern California has been attributed previously to lateral variations in either crustal temperature or crustal rock composition. Here I compare heat flow (a proxy for crustal temperature), local maximum depths of earthquakes (d95), and best-guess crustal rock compositions. Plotting heat flow versus d95 produces distinct trends for each rock composition, with each trend representing an isotherm marking the onset of plastic behavior. Offsets between the trends measure the differences in the plastic behavior temperatures of each rock composition; these offsets are comparable to the d95 variations with heat flow. Thus, crustal temperature and composition contribute about equally to the control of the maximum depths of earthquakes. The observed isotherm slopes imply vertical variations in crustal composition in addition to the lateral variations.