Fluids, Heat Transport and the Strength of the San Andreas Fault

Along the San Andreas Fault in California, heat flow observations and inferred stress orientations have been interpreted to indicate that the fault is weak. The strength of plate boundary faults is an important issue in understanding the balance of forces that affect plate tectonics, earthquake mechanics, static stress transfer between faults, rupture propagation and arrest, and aftershock activity. In this context, understanding the strength of seismogenic plate boundary faults like the San Andreas has emerged as a central issue in plate boundary tectonics.

More recent work has questioned the interpretation of the mechanical constraints - suggesting that the San Andreas fault may be strong after all. Critics of the weak fault hypothesis have suggested that the fault actually does generate frictional heat, but that groundwater flow, driven by topographic relief, redistributes the heat. In this scenario, active groundwater flow could hide the heat generated by a strong fault. The idea of heat redistribution by topographically-driven groundwater flow is attractive because there is considerable topography associated with the fault itself - making all of the ingredients available for hiding the thermal signature of a strong fault.

To test the hypothesis that groundwater flow might obscure the heat generated by a strong fault, I (along with colleagues Barbara Bekins and Steve Hickman at the U.S. Geological Survey) use a steady state, 2-D model of coupled heat and fluid flow within cross sections oriented perpendicular to the fault and to the primary regional topography. We studied two areas along the fault - the Parkfield area in Central CA and the Mojave area in Southern CA. Both regions are well studied, with lots of heat flow data to constrain the models. However, the two areas differ in their seismic behavior, geology, and topographic relief.

Research on this topic is ongoing, and we have been funded by NSF to study this problem in more detail.

Contact: Demian Saffer