Heat flow, strong near‐fault seismic waves, and near‐fault tectonics on the central San Andreas Fault

The main San Andreas Fault strikes subparallel to compressional folds and thrust faults. Its fault‐normal traction is on average a factor of γ = 1 + 2 μ thr ( 1 + μ thr 2+ μ thr ) , whereμ thris the coefficient of friction for thrust faults, times the effective lithostatic pressure. A useful upper limit forμ throf 0.6 (where γ is 3.12) is obtained from the lack of heat flow anomalies by considering off‐fault convergence at a rate of 1 mm/yr for 10 km across strike. If the fault‐normal traction is in fact this high, the well‐known heat flow constraint of average stresses of 10–20 MPa during strike slip on the main fault becomes more severe. Only a few percent of the total slip during earthquakes can occur at the peak stress before dynamic mechanisms weaken the fault. The spatial dimension of the high‐stress rupture‐tip zone is ∼10 m for γ  = 3.12 and, for comparison, ∼100 m for γ  = 1. High dynamic stresses during shaking occur within these distances of the fault plane. In terms of scalars, fine‐scale tectonic stresses cannot exceed the difference between failure stress and dynamic stress. Plate‐scale slip causes stresses to build up near geometrical irregularities of the fault plane. Strong dynamic stresses near the rupture tip facilitate anelastic deformation with the net effects of relaxing the local deviatoric tectonic stress and accommodating deformation around the irregularities. There also is a mild tendency for near‐fault material to extrude upward. Slip on minor thrust faults causes the normal traction on the main fault to be spatially variable.