The Evolution of Regional Seismicity Between Large Earthquakes David D. Bowman

1. The Evolution of Regional Seismicity Between Large Earthquakes David D. Bowman California State University, Fullerton Geoffrey C. P. King Institut de Physique du Globe de Paris Charles G. Sammis University of Southern California

2. All California Earthquakes M≥6.51950-1995

3. Stress Change From Loading a Locked Patch on a Simple Fault Slipping Fault Slipping Fault Seismic Slip Seismic Slip Creep at Depth Calculate from motion on all adjacent faults plus creep at depth Future Earthquake

4. Where are pre-earthquake stresses? Future Earthquake Creep at Depth = Negative Slip

5. Accelerating Seismicity in Stress Accumulation Regions

6. Static stress (Coulomb)changes during the earthquake cycle Earthquake

7. Current Stress Field Stress Field From Previous History of EQs Stress Field From Loading = +

8. Characteristics of the Background Stress Field(for a simple model) • Stress must be low along the strike of the fault (or rupture would continue) • Stress must be low along the strike of the fault (or rupture would continue) • Stress must be low along the strike of the fault (or rupture would continue) • Stress must be low along the strike of the fault (or rupture would continue) • Stress distant from the fault must approach, but not exceed, the failure stress • Stress distant from the fault must approach, but not exceed, the failure stress • Stress distant from the fault must approach, but not exceed, the failure stress • Coulomb field + background field can not exceed the failure stress (except locally) • Coulomb field + background field can not exceed the failure stress (except locally) - Positive Coulomb stresses must have a corresponding negative value in the background field Coulomb field immediately after the event Coulomb field immediately before the event Background stress field

9. Stress Change vs. Stress Through the Earthquake Cycle 25 bars 0 -25 failure stress 0 bars -50 The start of the earthquake cycle The end of the Earthquake cycle Earthquake Stress Change Tectonic Memory Stress Earthquake Stress Level Relative to the Failure Stress

10. Rough Stress field - Eqs move in Failure Stress Stress A B Fault Stress A B Fault An earthquake occurs when stress rises above the failure level

11. Creating a Synthetic Catalog Magnitude of the event depends on the size of the stress concentration This allows the calculation of the Frequency-Magnitude relation Failure Stress

12. Stress and Seismicity Through the Seismic Cycle failure stress bars -50 0 Immediately after the earthquake 25% of the cycle 50% of the cycle 75% of the cycle Immediately before the earthquake

13. Seismicity in the Earthquake Cycle The end of the Earthquake cycle Start of the Earthquake Cycle Approaching the Earthquake Immediately before the Earthquake Immediately after the Earthquake 75% of the Earthquake Aftershocks Stress shadows Earthquake

14. Implications of Regional Stress Accumulation Model • Off-fault aftershocks occur in regions of elevated static stress change due to the earthquake • Main fault is seismically quiet for most of the seismic cycle • “Mogi doughnuts” • Accelerating Moment Release over a broad spatial region before large EQs • Region size scales with size of the “predicted” earthquake • Evolution of the frequency-magnitude statistics (Gutenberg-Richter relation) • Stationary (time-independent) b-value • a-value increase before a large event and decreases after the event

15. California Seismicity 1912-2001 M>3.5

16. Build-up to the Earthquake Earthquake Immediately before the earthquake Immediately after the earthquake Late in the Earthquake cycle

17. Which cumulative moment release curve is fora REAL seismicity sequence? Model San Fernando Earthquake Cumulative Benioff Strain Time Time

18. AMR Model vs. Observed Accelerating Seismicity

19. Build-up to the Earthquake Earthquake Immediately before the earthquake Immediately after the earthquake Late in the Earthquake cycle

20. Evolution of Gutenberg-Richter Scaling Beforethe 1987 Superstition Hills Earthquake

21. Seismicity in the Pacific Northwest Two large nearby events show accelerating moment release The regions overlap, approximating the evolution of the seismicity over 2.5 cycles

22. Pacific Northwest Seismicity Statistics Pacific Northwest Seismicity

23. Seismicity in the model & Pacific Northwest

24. Benioff Strain in the model & PNW Cumulative Benioff Strain

25. Evolution of the frequency-magnitude statistics

26. Frequency-magnitude stats in model & PNW Evolution of the frequency-magnitude statistics

27. Looking Forward: • Better calculation of the “noise” functions - incorporate stress transfer? • More complex fault geometries - simulate real fault networks • Additional Testing on real data - Test on earthquakes from other regions (Greece, Turkey, China, etc) - False alarm rate? • Relationship to Time-Dependent Hazard Analysis Pre/re-prints available at: http://geology.fullerton.edu/faculty/dbowman