Heat Flow on the Creeping Section of the San Andreas Fault: A Localized Transient Perspective

Most analyses of predicted frictional heat flow along the San Andreas fault have been carried out in terms of line source models. This is clearly not an adequate approximation in the frictional heat source is very localized. In addition most studies have focused on the heat flow expected if the source has been active for a very long time,--near steady state. There is no strong evidence for this assumption. One explanation for the lack of frictional heat generation along the seismically active (with large earthquakes), non-creeping, sections of the fault is dynamic fault weakening, resulting in essentially all of the available elastic energy being radiated seismically (little energy going into frictional heat). Then a puzzle remains as to why there is no heat flow anomaly over the creeping section of the San Andreas fault between San Juan Bautista and Parkfield, where, according to the standard model, an approximate line source has been active for a very long time. However, consideration of the possible localized transient nature of the creeping section suggests that there really may not be a puzzle, and that there actually may be some evidence for frictional heat generation, e.g., a localized asymmetric high heat flow anomaly on the northeast side of the fault near the center of the creeping section. If the creeping section is a localized transient phenomenon, there is no guarantee that it will not have a large earthquake in the future. Conditions for creep along a major fault such as the San Andreas fault are apparently very rare and unique. Thus very special conditions may exist along the creeping section at this time. One logical possibility is that creep requires the juxtaposition of peculiar rocks on both sides of the fault. If this is so, then, at a slip rate of 3.5 cm/yr, this condition may have existed for less than 2 ma, perhaps for considerably less. If most of the heat generation is at a depth below 5 km, then the corresponding anomaly (not corresponding to a line source) may not have reached the surface yet, and at any rate might not be large enough to be discernable above the noise. Another logical possibility is that creep requires a peculiar type of rock along only one side of the fault, very likely the northeast side of the fault where unique Franciscan rocks are present. In this case the appropriate model is a warming slider moving southeast along the northeast side of the fault, continuously being juxtaposed against cooler material on the southwest side. We would not expect a heat flow anomaly centered over the fault, but rather an asymmetric anomaly gradient from the cool southwest side to the warm northeast side. As the warm slider moved past the rocks on the cool side, they would be warmed for less than 2 ma, and, depending on the depth of heat generation, the corresponding anomaly might not be obvious at the surface, and certainly wouldn't be observable at the leading edge of the slider. Examination of presently available data suggests that indeed there may be a asymmetric anomaly near the center of the creeping zone. At any rate the data do not preclude this possibility. This consideration suggests that more heat flow measurements should be made on the northeast side of the fault in the creeping section.