On shear stresses, temperatures, and the maximum magnitudes of earthquakes at convergent plate boundaries

Apparent coefficients of friction, urn:x-wiley:jgrb:media:jgrb52909:jgrb52909-math-0001, on nine subduction interfaces for which surface heat flux constraints exist fall in the range urn:x-wiley:jgrb:media:jgrb52909:jgrb52909-math-0002. Surface heat flux above the maximum depth of thrust‐faulting earthquakes adjusted for radiogenic heat production, QT, falls in a narrow range, implying that the transition from seismic to aseismic slip on the interface takes place at a temperature, TT, that depends upon its depth, zT. For the mean value of QT∼40 mW/m2, TT∼zT×12°C/km. The maximum depth of thrust‐faulting earthquakes, determined from a further 82 profiles of seismicity across subduction zones, exhibits a systematic dependence on convergence rate and the age of the descending plate, which is consistent with a narrow range in QT for these zones also. An analytical expression links urn:x-wiley:jgrb:media:jgrb52909:jgrb52909-math-0003 to zT, QT, and the subduction parameters. Application of this relation to the 82 profiles gives urn:x-wiley:jgrb:media:jgrb52909:jgrb52909-math-0004. The narrowness of that range can be explained if the primary source of heat on the subduction interface is frictional heating during earthquakes with shear stresses limited by thermal pressurization of pore fluids. Apparent coefficients of friction exhibit insignificant variation with the fraction of the relative motion that is expressed in earthquakes. Either the apparent coefficients of friction for seismic and aseismic slip are indistinguishable or zones with historically low rates of seismicity are prone to large earthquakes that have not been recorded