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SLICING UP THE BAY AREA: Insights from regional block modeling of GPS

SLICING UP THE BAY AREA: Insights from regional block modeling of GPS. Matthew A. d’Alessio U. S. Geological Survey Roland B ürgmann U. C. Berkeley. U.S. Department of the Interior U.S. Geological Survey. ROAD MAP. Goal of Study Determine fault slip-rates for seismic hazard.

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SLICING UP THE BAY AREA: Insights from regional block modeling of GPS

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

  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 Locked near the surface. Independent Dislocations or Block Model Planar faults extend to infinite depth. Slip at constant rate below transition depth.

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

  6. 3-D BLOCK MODEL • 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. Locked Block Boundary

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

  12. CONSTRAINING 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! Locking Depth

  13. LOCKING DEPTHS Geysers (shallow D95) Great Valley (deep D95) D95 Surface shows maximum* Depth of seismicity (*95%, or D95)

  14. LOCKING DEPTHS D95 - 2.5 km

  15. LOCKING DEPTHS D95 - 5.0 km

  16. LOCKING DEPTHS D95 - 2.5 km

  17. LOCKING DEPTHS D95

  18. LOCKING DEPTHS D95 D95 + 2.5 km

  19. LOCKING DEPTHS D95 D95 + 5.0 km

  20. LOCKING DEPTHS D95 D95 + 2.5 km

  21. LOCKING DEPTHS • 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. D95

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

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