The high-quality databases for California seismicity (from the Southern and Northern California Earthquake Centers) and an extensive compilation of thermal measurements in California are used to quantify the concept of temperature as a fundamental parameter for determining the thickness of the seismogenic zone. The base of this zone, below which only a small percentage of crustal earthquakes occur, is termed the “cutout depth,” and it is at or near the brittle-ductile transition in the crust. Based on laboratory deformation studies, this transition should be temperature, strain rate, lithology, and stress-state dependent. In this study, qualitative comparisons between the heat flow in California and earthquake hypocentral distributions show first that, as expected, earthquake cutout depths are inversely related to heat flow. Second, the epicentral distributions tend to parallel thermal transitions. This correlation is probably related to stress concentrations in these locations. An ancillary observation is that the cooler western Mojave block in southern California appears to be behaving similarly to the Tibetan indenter block in Asia, as faults tend to go around it into areas of higher heat flow where the seismogenic zone is thinner and the crust may be weaker, and it is pushed toward (and under) the southern cold Sierra Nevada block. Third, to quantitatively compare the data sets, temperatures for the seismicity cutout depth in California were calculated using the steady-state, 1D heat conduction equation, with the variables based on published values for heat flow, heat generation, and thermal conductivity. The analysis was restricted to profiles along which the heat flow and earthquake databases were constrained, which allowed the error in the temperature calculation to be determined using Monte Carlo simulations. The results show that two distinct ranges of temperatures (dependent upon location) describe the cutout depth (D99%) of seismicity for California: 450 ± 50°C and 260 ± 40°C. The 450 ± 50°C cutout depth temperature is most widespread geographically, occurs within many provinces, and is higher than the frequently referenced temperature of 300 ± 50°C for the brittle-ductile transition. The lower temperature (260°C) seems to be restricted to provinces where the heat flow is near or below 50 mW m-2, such as the Mojave block and Sierra Nevada. The differences in these cutout temperatures suggest that additional factors, such as strain rate, stress regime, and/or lithology, may contribute to the seismicity cutout depth. Fourth, along a profile with significant seismic activity and a 450°C cutout temperature, the envelope for the maximum energy released by earthquakes falls at or below the 300°C isotherm. Detailed characterization of the heat flow and earthquake synergy in this manner furthers the understanding of the earthquake process and can aid in the estimation of the maximum depth of rupture for great earthquakes, particularly in areas of low seismicity, thus reducing uncertainties in hazard calculations.