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

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

3. When should correlation imply causation?
Normal Stress Change
Stress transfer by the 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

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

5. One End Member - Static Load ChangeDt
failure threshold Dt
shear stress
time

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

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

8. The Picture Post-Loma Prieta:

9. A wide spectrum of triggers between the end members:

10. The Picture Post-Loma Prieta:

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

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

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

14. Other Slowly-Developing, Permanent Load Changes:

15. 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

16. 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

17. Oscillatory, continuous loads:

18. 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

19. 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

20. ‘Dynamic’ Triggering (by seismic waves).

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

22. A Few More Outstanding Questions…

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

24. 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

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

26. 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

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

28. “There are known knowns. These are things we know that we know
“There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know.”
Thank You!