Coulomb stress transfer is an interaction criterion that promises a deeper understanding of earthquake occurrence, and a better description of probabilistic hazard.
When an earthquake reduces the average value of the shear stress on the fault that slipped, shear stress rises at sites in addition to the fault tips. This discovery lay in waiting for 20 years, when lobes of off-fault aftershocks were seen to correspond to small calculated increases in shear or Coulomb stress.
In its simplest form, the Coulomb failure stress change, Δσf (also written ΔCFS or ΔCFF) is:
Δσf = Δτ+ μ*(ΔP + Δσn)
where Δτ is the shear stress change on a fault (reckoned positive in the direction of fault slip), Δσn is the normal stress change (positive if the fault is unclamped), ΔP is the pore pressure change in the fault zone (positive in compression), and μ is the friction coefficient (with range 0-1). Failure is encouraged if Δσf is positive and discouraged if negative; both increased shear and unclamping of faults promote failure. The tendency of ΔP to counteract Δσn is often incorporated into (1) by a reduced ‘effective’ friction coefficient, μeff’.
The calculated off-fault stress increases are rarely more than a few bars (1 bar = 0.1 MPa ~ atmospheric pressure at sea level), or just a few percent of the mean earthquake stress drop. In addition, the proximity to failure at any site is presumably variable but in any event unknown. It is unclear why aftershocks concentrate at the site of such small stress increases. Studies by US and international teams find a surprisingly strong influence of stress change on seismicity, explaining it in terms of rupture nucleation phenomena observed in the laboratory.
Over the past years, it has become generally accepted that small co-seismic stress perturbutions can influence the location and timing of future events. Stress changes in the crust due to an earthquake can hasten the failure of neighboring faults and induce earthquake sequences.
Read more about Coulomb Stress Transfer: Examples of Earthquake Sequences
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