Interpretation of a stress profile across the San Andreas fault

The results of recent stress measurements along the San Andreas fault near Palmdale, California, which show a rapid decrease in shear stress in the proximity of the fault, may be due to the stress drop connected with the great 1857 earthquake, superimposed onto a preseismic ground deformation. This suggestion is made on the basis of a fault model where a preseismic phase of stable sliding at depth is responsible, through stress concentration, for the occurrence of seismic ruptures in the upper lithosphere.