A Post-Loma Prieta Progress Report on Earthquake Triggering by a Continuum of Deformations Presented By Joan Gomberg

A pleasingly simple correlation…. Response of Regional Seismicity to Static Stress Change Produced by the Loma Prieta Earthquake. Reasenberg & Simpson, 1992

When should correlation imply causation? Normal Stress Change Stress transfer by the 1988-1989 M=5.3 and 5.4 Lake Elsman foreshocks to the Loma Prieta fault: Unclamping at the site of peak mainshock slip. Perfettini et al., 1999 Right-lateral Stress Change Reverse Stress Change Coulomb Stress Change

One End Member - Static Load Change shear stress failure threshold time Dt

One End Member - Static Load Change Dt failure threshold Dt shear stress time

The Other End Member - Dynamic Load Change shear stress failure threshold time

Static Triggering -> Load or Strength Change Dynamic Triggering -> Strength Change failure threshold Dt shear stress time

The Picture Post-Loma Prieta:

A wide spectrum of triggers between the end members:

The Picture Post-Loma Prieta:

A Popular Physical Model: The ‘Dieterich (1994)’ or ‘Clock-Advance’ Model

Stress -> Rate Change Highly Non-linear. The ‘Dieterich (1994)’ or ‘Clock-Advance’ Model

Frictional Model Rate Change Predicted for Stress Step perturbed Rate Change background Afterslip and Aftershocks in the Rate-state Friction Law. Helmstetter & Shaw, 2009

Other Slowly-Developing, Permanent Load Changes:

Frictional Model Rate Change Predicted for Stress Step & Afterslip perturbed Rate Change background Afterslip and Aftershocks in the Rate-state Friction Law. Helmstetter & Shaw, 2009

Measured Afterslip Deformation & Aftershock Rate -> Linear Relationship Cumulative # of Aftershocks Post-seismic Deformation Time after Landers (yr) Modeling Afterslip and Aftershocks Following the 1992 Landers Earthquake Perfettini & Avouac, 2007

Oscillatory, continuous loads:

Rainfall Modulated Seismicity Earthquake Rate Geodetically Measured Displacement River Elevation Seasonal Variations of Seismicity and Geodetic Strain in the Himalaya Induced by Surface Hydrology Bettinelli et al., 2008

Lack of tidal modulation constrains nucleation time. Earthquake Rate +2-4 kPa Geodetically Measured Displacement River Elevation Seasonal Variations of Seismicity and Geodetic Strain in the Himalaya Induced by Surface Hydrology Bettinelli et al., 2008

‘Dynamic’ Triggering (by seismic waves).

The ‘Dieterich (1994)’ or ‘Clock-Advance’ Model The Failure of Earthquake Failure Models. Gomberg, 2001

A Few More Outstanding Questions…

Measured Linear Aftershock Densities Aftershock Scale Independence -> Large & Small Earthquakes Triggered Identically Measured Linear Aftershock Densities Felzer & Brodsky, 2006

Foreshock Scale Dependence -> Large & Small Earthquakes Begin Differently M3 source dimension M4 source dimension Evidence for Mogi Doughnut Behavior in Seismicity Preceding Small Earthquakes in Southern California Shearer & Lin, 2009

Complex Aftershock Behaviors: Rate Increases Precede Quiescences background rate quiescence background rate quiescence background rate quiescence background rate quiescence 1998 1999 2000 2001 2002 1998 1999 2000 2001 2002 The Response of Seismicity to Coulomb Stress Triggers & Shadows of the 1999 Mw=7.6 ChiChi Earthquake. Ma et al., 2005

Complex Aftershock Behaviors Reflect Stress Heterogeneity seismicity rate change background rate time (ta units) Can Coseismic Stress Variability Suppress Seismicity Shadows? Insights from a Rate-State Friction Model Marsan, 2006

Complex Aftershock Behaviors Reflect Fault Patch Depletion A Frictional Population Model of Seismicity Rate Change Gomberg et al., 2005

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