TEMPERATURE AND THE SEISMIC ASEISMIC TRANSITION - OBSERVATIONS FROM THE 1992 LANDERS EARTHQUAKE/

An important constraint on the size and destructive potential of earth quakes is the depth extent of rupture. Laboratory studies of the trans ition from unstable to stable sliding, along with observed relationshi ps between surface heat flow and the thickness of the seismogenic crus t, provide strong evidence for the significance of temperature in dete rmining the maximum nucleation depth of large earthquakes. The June 28 , 1992, M(w) 7.3 Landers earthquake ruptured fault segments within 20 km of 11 pre-existing heat flow measurements, and shallowing of the ba se of aftershock seismicity along strike correlates with an increase i n heat flow. Crustal geotherms estimated from these measurements place the base of seismicity along the 250 degrees C isotherm. This tempera ture is consistent with predictions from laboratory studies of the fri ctional stability of Westerly granite, but estimated temperatures for the seismic-aseismic transition along other faults within the San Andr eas fault system are in the range of 350 to 400 degrees 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.