Relation between stress drop, fault friction, and crustal strength in shallow focus strike slip faulting

Abstract

The rather small seismic stress drop (10 to 102 bars) measured on shallow strike slip faults has been taken to be a measure of the shear strength of the earth's crust; this assumption is reexamined here. The mechanics of faulting on a vertical strike slip fault, the faces of which are lined with breccia, is reviewed. It is noted that faulting will begin at the point on the fault where the applied tectonic stress field first overcomes the strength of the soil-like material (breccia) lining the fault faces and that the new zone of faulting, once formed, will serve as a sharp crack with very large stresses at its boundaries and will propagate across the entire fault plane. The ‘stress drop’ occurring on the fault plane is just the difference between the static and kinetic soil mechanical strength of the breccia at the point on the fault plane where the new episode of faulting began (i.e., the zone from which the crack began). Assuming that the fault breccia has properties similar to those of other soil-like materials, each episode of faulting will begin at the surface of the earth and the stress drop of seismic faulting will be just equal to the intrinsic cohesive shear strength of the fault-face breccia at the surface of the earth; the stress drop is neither a measure of mean crustal strength nor even a measure of mean breccial strength. The apparent discrepancy between Chinnery's small values of stress drop (10 to 102 bars) and Jeffreys' large estimates (1.5 × 103 bars) of mean crustal strength seems to have been resolved because the one quantity is not related to the other.