Coulomb theory Hypothesis: Faults interact by the transfer of stress WINCH SPRING BRICK EARTHQUAKE! (only if stick-slip) Force Balance – Brick will not move until: Force on springForce resisting motion (its length change x its stiffness) (the weight of the brick x friction on surface) Modeled after Ross Stein’s Coulomb Training I -Lisa Walsh
Coulomb theory Modeled after Ross Stein’s Coulomb Training I Source fault Receiver fault WINCH SPRING BRICK SPRING BRICK EARTHQUAKE! EARTHQUAKE! • Add another spring & brick • If you start cranking winch, PURPLEwill move first. • Then tension on spring will move GREEN. + ΔCFS = closer to failure - ΔCFS = farther from failure Coulomb stress calculation Coulomb stress change = shear stress change + (coefficient of friction x normal stress change) ΔCFS = Δτs+μ' Δσn -Lisa Walsh
Key concepts: • Source faults • Receiver faults • Optimally oriented faults • Assume receiver faults are close to failure • Triggering lag time is a problem
Change of coulomb stress on faults of specified orientation Can change spatially Remote: Sremote Induced: Sinduced Total:Sremote+Sinduced Can change spatially
Stress changes are permanent but seismicity is not from Todal et al (JGR, 2005)
Southern California – San Andreas Stress Shadow Landers M = 7.3 (1st) Big Bear M = 6.5 (2nd - 3 hrs later) Los Angeles Stress trigger zone First 3 hr of Landers aftershocks plotted from Stein (Nature, 2003) 1992 - Landers earthquake triggered Big Bear earthquake 3 hrs later!
Hector Mine Los Angeles First 7 yr of aftershocks plotted from Stein (Nature, 2003) …and promotes the M=7.1 Hector Mine shock 7 years later (1999).