Revisiting the San Andreas Heat Flow Paradox From the Perspective of Seismic Efficiency and Elastic Power in Southern California.

We investigate the relation between frictional heating on a fault and the resulting conductive surface heat flow anomaly using the fault's long‐term energy budget. Analysis of the surface heat flow surrounding the fault trace leads to a constraint on the frictional power generated on the fault—the mechanism behind the San Andreas fault (SAF) heat flow paradox. We revisit this paradox from a new perspective using an estimate of the long‐term accumulating elastic power in the region surrounding the fault, and analyze the paradox using two parameters: the seismic efficiency and the elastic power. The results show that the constraint on frictional power from the classic interpretation is incompatible with the accumulating elastic power and the radiated power from earthquake catalogs. We then explore four mechanisms that can resolve this extended paradox. First, stochastic fluctuations of surface heat flow could mask the fault‐generated anomaly (we estimate 21% probability). Second, the elastic power accumulating in the region could be overestimated (≥550 MW required). Third, the seismic efficiency—ratio of radiated energy to elastic work—of the SAF could be higher than that of the remaining faults in the region (≥5.8% required). Fourth, the scaled energy—ratio of radiated energy to seismic moment—on the SAF could be lower than on the remaining faults in the region (a factor 5 difference required). In the last three hypotheses, we analyze the interplay of the energy budget on a single fault with the total energy budget of the region. Plain Language Summary: When earthquakes move rock against rock, friction heats the contact surface. If this frictional resistance were like laboratory measurements of typical crustal rock, the heat would cause a considerable heat flow signature ("anomaly") at Earth's surface. For the San Andreas fault (SAF) in Southern California, such a signature has not been observed. One solution to this paradox is that the fault is weak. We approach the paradox from a new angle by using additionally the rate at which elastic energy accumulates in California. This elastic power is incompatible with the radiated power from earthquake catalogs and the maximum rate of frictional heating from the paradox if only simple assumptions are made. We call this conflict the extended heat flow paradox. Four mechanisms could individually resolve the extended paradox: randomness in regional heat flow measurements could conceal the anomaly, the elastic power on the SAF could be overestimated, the seismic efficiency (ratio of radiated energy per input work) on the SAF could be comparatively high, or the scaled energy (ratio of radiated energy per seismic moment) on the SAF could be comparatively low. A combination of multiple effects is possible. Key Points: Heat flow around the San Andreas fault is incompatible with radiated power and elastic input power under simple assumptionsRegionally, a stochastic view on heat flow and/or an overestimated total elastic power can resolve the paradoxLocally, a high seismic efficiency or a low scaled energy (radiated energy/seismicmoment) on the San Andreas fault can resolve the paradox [ABSTRACT FROM AUTHOR]