1. SLICING UP THE BAY AREA: Insights from regional block modeling of GPS U.S. Department of the Interior U.S. Geological Survey Matthew A. d’Alessio U. S. Geological Survey Roland Bürgmann U. C. Berkeley

2. ROAD MAP Goal of Study Determine fault slip-rates for seismic hazard. Block Modeling Advantages Disadvantages JGR Paper in Press … Want Pre-prints? dalessio@usgs.gov

3. Known: Surface Deformation Unknown: Fault Slip Rates

4. DEFORMATION MODELING Elastic Models Dislocation theory Visco-elastic Viscous Sheet Planar faults extend to infinite depth. Locked near the surface. Slip at constant rate below transition depth. Independent Dislocations or Block Model

5. 3-D BLOCK MODEL Elastic Strain Accumulation at Boundaries Rigid Block Offset Locked Block Boundary Shallow Aseismic Creep + + Based on Code by Brendan Meade

6. 3-D BLOCK MODEL Locked Block Boundary Solve for block rotation most consistent with GPS 3 free parameters per block. Pole of Rotation Lat. & Lon. Rate of rotation Fault slip rates Resolve block motion onto fault orientation. No additional model parameters for complicated fault geometry.

7. GLOBAL VIEW For Pacific Plate, global GPS sites constrain total PA- NA motion. Dislocation-only models limited to few stations on land west of SAF. (North American Reference Frame)

8. BAY AREA BLOCK GEOMETRY?

9. 2-D FAULT GEOMETRY Mt. Diablo Berkeley

10. 3-D FAULT GEOMETRY

11. Faults Intersect Here 3-D Gap Bottom edge of Locked Dislocations

12. CONSTRAINING LOCKING DEPTH Locking Depth Strong trade-off between SLIP RATE and LOCKING DEPTH Solving for both can produce erratic slip rates. Locking depth related to brittle-ductile transition. Can be constrained from depth of seismicity!

13. LOCKING DEPTHS Geysers (shallow D 95 ) Great Valley (deep D 95 ) D 95 Surface shows maximum* Depth of seismicity (*95%, or D 95 )

14. LOCKING DEPTHS D 95 - 2.5 km

15. LOCKING DEPTHS D 95 - 5.0 km

16. LOCKING DEPTHS D 95 - 2.5 km

17. LOCKING DEPTHS D 95

18. LOCKING DEPTHS D 95 D 95 + 2.5 km

19. LOCKING DEPTHS D 95 D 95 + 5.0 km

20. LOCKING DEPTHS D 95 D 95 + 2.5 km

21. LOCKING DEPTHS D 95 Physical Constraints from Seismicity. Relative brittle-ductile transition depths remain constant. Absolute transition depth unknown. Shift D95 surface up and down. Solve for best fit to GPS data.

22. SLIP RATES San Andreas Valley Margin Mt. Diablo: 3.9±0.5 Thrust 4.2±0.5 RL Calaveras San Gregorio

23. SUMMARY & RESULTS Block Model Advantages “ Realistic ” geometries. Global GPS constrains total block motion. Slip rates on adjacent segments consistent with overall block motion. Block Model Limitations Uniform Elastic medium (our model) 3-D geometric issues poorly constrained, physically unrealistic representations) Locking Depth v. Slip rate Trade-Off Constrain using depth of seismicity Slip Rate Highlights Most within 1-sigma of existing geologic estimates, BUT some important differences. JGR paper in press -- dalessio@usgs.gov