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San Andreas fault and eastern California shear zone

Spatial versus temporal variation in San Andreas fault and eastern California shear zone slip rates. Rick Bennett Dept of Geosciences University of Arizona with contributions from Josh Spinler, Noah Fay, Sigr ú n Hreinsd ó ttir, and Megan Anderson.

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San Andreas fault and eastern California shear zone

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  1. Spatial versus temporal variation in San Andreas fault and eastern California shear zone slip rates Rick BennettDept of GeosciencesUniversity of Arizonawith contributions from Josh Spinler, Noah Fay, Sigrún Hreinsdóttir, and Megan Anderson

  2. San Andreas fault and eastern California shear zone

  3. Southern California slip rates

  4. Conclusions (preliminary) • SAF slip rate varies appreciably along strike • Along strike variability reconciles (all?) differences among geological and geodetic estimates • Slip rate variation facilitated by transrotation across the ETR

  5. The Joshua Tree GPS network San Bernardino GPS network (CSUSB and UofA) Joshua Tree GPS network (UofA)

  6. Joshua Tree GPS network

  7. Joshua Tree GPS network West East

  8. Southern California velocity field UofA SCEC v3

  9. Strain rate field, So Cal – Observed– Predicted

  10. Strain rate field, ETR – Observed– Predicted “belt” of high strain rate

  11. Seismicity and faulting

  12. Previous models – Observed– Predicted Johnson et al., 1994 Also see Bennett et al., 1996; Meade and Hager, 2005; Becker et al., 2005

  13. San Andreas fault and eastern California shear zone Large Offset (km’s) Small Offset (meters)

  14. Respect the mapped geology Rationale: If perfectly good faults already exist (e.g., Pinto Mt, Blue Cut, others), why create new ones? Can we reject this hypothesis? That is, is the observed strain rate field really inconsistent with strain accumulation on these mapped faults? What is the role of the Pinto Mountain and Blue Cut faults?

  15. Block model for interseismic strain accumulation

  16. A block model for deformation: crustal velocity w.r.t. SBR

  17. A block model for deformation: rigid block motion SBR fixed CW Translating blocks Rotating JT Block

  18. A block model for deformation: strain accumulation due to locking

  19. Residual velocities 

  20. Residual velocities Small JT residuals Large SJFZ residuals

  21. Kinematic results 5 5 JT rotation 14º/Myr clockwise -3 -5 -7 -2 5 5 -1 -9 12 -6 -9 23 2 R-L strike slip (mm/yr) Shortening (mm/yr)

  22. Kinematic results • Block rotation: Paleomag 30-40º sometime in past 10 Myr (Carter et al. 1987). If 14º/Myr => rotating since ~2-3 Ma. • San Andreas fault slip rate ranges between 5 to 23 mm/yr. ~12 mm/yr just north of Biskra Palms. In general agreement with geology. • Pinto Mt and Blue cut left-extensional. Rates as high as 3-6 mm/yr. Total offset 10-20 km (?) consistent with age of 2-3 Ma.

  23. Strain rates reconsidered Observed Modeled

  24. Strain rates reconsidered Observed Modeled at GPS points only

  25. Conclusions Geodesy consistent with the known geology; no “new” faults required Slip rates on ETR faults are important San Andreas slip rate varies appreciably along strike Block rotation rates are fast Dangerous to base geometry of fault models on strain rate inference alone Respect the mapped geology

  26. Future work Continue GPS measurements!! Improve model fault geometry!! Model elastic strain associated with dip-slip motion!! Thoroughly test alternative models!!

  27. Future work

  28. Conceptual issue What is “present-day” slip rate? Zero! • APPLES AND ORANGES ? • Geology sees actual fault displacement • Geodesy (primarily) sees transient strain accumulation

  29. Non-uniqueness – Observed– Predicted

  30. Non-uniqueness – Observed– Predicted

  31. Non-uniqueness – Observed– Predicted Fialko, 2006

  32. Non-uniqueness – Observed– Predicted

  33. Conceptual issues Friedrich et al., JGR, 2003; Weldon et al., GSA Today, 2004

  34. Conceptual issues Friedrich et al., JGR, 2003; Weldon et al., GSA Today, 2004 Variable fault loading rate?

  35. Conceptual issues Friedrich et al., JGR, 2003; Weldon et al., GSA Today, 2004 Constant loading rate?

  36. Conceptual issues Weldon et al., 2004

  37. Reconciliation of different slip rates Bennett et al., 2004

  38. Recent hypothesis (weak version): co-varying clusters? – Observed– Predicted Dolan et al., 2007 See also Press and Allen, 1995

  39. Recent hypothesis for present-day behavior: new fault forming – Observed– Predicted Du and Aydin, 1996 Nur et al., 1993 Li and Liu, 2006

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