Load-strengthening versus load-weakening faulting

Abstract

Increases in shear stress (τ) along a fault during loading to failure cannot generally occur without changes in the normal stress across the fault (σ n). The fault loading parameter ( ∂δ′ n/ ∂τ = ∂δ n/ ∂τ − ∂P f/ ∂τ) distinguishes situations of load-strengthening ( ∂δ′ n/ ∂τ > 0), where the frictional shear strength of faults increases as tectonic shear stress rises, from load-weakening environments ( ∂δ′ n/ t6 τ < 0) where it decreases. Compressional faulting in tectonic regimes with δ v = δ 3 is always load-strengthening unless fluid pressure is rapidly increasing. Extensional faulting in regimes where δ v = δ 1 is load-weakening unless fluid pressure is dropping rapidly. Strike-slip faulting in terrains where δ v = δ 2 can be either load-weakening or load-strengthening. The particular case where ∂δ′ n/ ∂τ = 0, so that frictional shear strength stays constant during fault loading, is a very special situation corresponding to direct shear. Load-strengthening strike-slip faulting appears to correlate with tectonic transpression and load-weakening with transtension. Differing loading characteristics of faults in different tectonic regimes must induce varying patterns of cyclic fluid redistribution accompanying the seismic cycle, with implications for earthquake recurrence and precursory groundwater phenomena.