Healy, David

Meredith, Philip

Mitchell, Tom

Rizzo, Roberto

Wong, Teng-Fong

English

Heriot Watt Pure

Geophysical Research AbstractsVol. 21, EGU2019-3959, 2019EGU General Assembly 2019© Author(s) 2019. CC Attribution 4.0 license.Coulomb Failure Stress and triggered seismicity: The consequences offault zone damageDavid Healy (1), Tom Mitchell (2), Philip Meredith (2), Teng-fong Wong (3), and Roberto Rizzo (1)(1) University of Aberdeen, Dept of Geology, Aberdeen, United Kingdom (d.healy@abdn.ac.uk), (2) Department of EarthSciences, University College London, London, WC1E 6BT United Kingdom , (3) Faculty of Science, The Chinese Universityof Hong Kong, Shatin, NT, Hong KongPredictive models for the stress triggering of earthquakes have been built on the concept of Coulomb FailureStress (CFS). CFS is the change in proximity of a given ‘target’ fault to the Mohr-Coulomb shear failure criteriondue to changes in stress induced by slip on a neighbouring ‘source’ fault. Stress triggering models, often based onOkada’s elastic dislocation solution for a fault in an isotropic elastic half-space, form the basis of many seismichazard assessments, e.g. the Coulomb software package from the USGS.A common assumption in CFS models of stress triggering is that of constant apparent friction. Apparentfriction is a term used to combine frictional strength and effective stress contributions to the localised stresschanges at the target fault. Beeler et al. (2000) and Cocco & Rice (2002) have both explored the underlyingmechanics of apparent friction, and considered isotropic and anisotropic poroelastic effects in fault zones. Weknow that fault zones are inherently anisotropic in their physical properties, in part due to arrays of sub-parallelcracks and fractures in their damage zones (Faulkner et al., 2006; Mitchell & Faulkner, 2009). We can model themechanical influence of these crack patterns using anisotropic poroelasticity (Sayers & Kachanov, 1995; Wong,2017). For short-term effects, we can consider the target faults responding as ‘undrained’ poroelastic materials(constant fluid mass, varying pore fluid pressure), and for longer-term deformation we can use the ‘drained’ case(constant pore fluid pressure, varying fluid mass). We have built a new open source software tool called CFSape(Coulomb Failure Stress anisotropic poroelasticity), using MATLAB. The tool includes a GUI to collect sourceand target fault parameters, select among alternative poroelastic boundary conditions, and specify the requiredoutputs.Calculations using CFSape show that the possible consequences of ignoring anisotropic poroelasticity atthe target faults are highly significant. Faults which appear stable (locked) under either of the typical assumptionsof constant apparent friction or isotropic poroelasticity are predicted to fail when anisotropic poroelasticity isincorporated. The converse is also true: apparently ‘risky’ faults are predicted to be stable. The model predictionsconfirm the theoretical finding of Cocco & Rice (2002) that in the extreme case of perfectly fault-parallel cracksat the target fault, an assumption of constant apparent friction is generally valid. However, this begs the criticalquestion: what is the correct or best value of the Skempton coefficient (isotropic case), or the Skempton tensor(anisotropic case), for these models? We urgently require more and better experimental data on the anisotropicporoelastic response of rocks, measured under likely crustal conditions of true triaxial stress. We need data forboth the Skempton (undrained) and Biot (drained) tensors to apply better constraints for short- and long-termseismic hazard assessments due to stress triggering of earthquakes.

Coulomb Failure Stress and triggered seismicity:The consequences of fault zone damage

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Coulomb Failure Stress and triggered seismicity:The consequences of fault zone damage

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