An important constraint on the size and destructive potential of earthquakes is the depth extent of rupture. Laboratory studies of the transition from unstable to stable sliding, along with ob served relationships between surface heat flow and the thickness of the seismogenic crust, provide strong evidence for the significance of temperature in determining the maximum nucleation depth of large earthquakes. The June 28, 1992, Mw 7.3 Landers earthquake ruptured fault segments within 20 km of 11 pre-existing heat flow measurements, and shallowing of the base of aftershock seismicity along strike correlates with an increase in heat flow. Crustal geotherms estimated from these measurements place the base of seismicity along the 250??C isotherm. This temperature is consistent with predictions from laboratory studies of the factional stability of Westerly granite, but estimated temperatures for the seismic-aseismic transition along other faults within the San Andreas fault system are in the range of 350 to 400??C. Variations in country rock and fault gouge composition, together with higher slip rates, may account for this difference, although part of the Landers seismogenic crust might remain unruptured. Copyright 1996 by the American Geophysical Union.