Kenneth W. Hudnut U. S. Geological Survey Pasadena, California

1. Southern California Regional Tectonics - Constraints from Geodetic Data Courtesy of JPL Kenneth W. Hudnut U. S. Geological Survey Pasadena, California Southern California Earthquake Center --- Workshop on Tectonophysics of Southern California Caltech; Pasadena, California --- November 11, 2004

2. San Andreas system Basin and Range ECSZ & Walker Lane Transverse Ranges Borderlands Colorado Plateau, Sierra Nevada, Peninsular Ranges (and Baja California) The Plate Boundary ? From Dickinson & Wernicke (1997, Geology)

3. SoCal Regional Tectonics • San Andreas fault and major subparallel faults; San Jacinto, Elsinore, etc. & The Big Bend • Eastern California Shear Zone • Thrust fault systems; San Bernardino and San Gabriel ranges, Los Angeles, etc. • Cross-faults; Garlock, Big Bear, Salton Trough, Yorba Linda trend, etc. • Block rotations; Transverse Ranges, e.g., Santa Monica Mtns., Salton Trough, transition zones • A little bit of everything - complicated

4. Statement of Problems • Must understand complex fault interactions to attain a system-level understanding • Some questions: • How does the San Andreas fault interact with abutting and nearby structures? • How do these secondary and tertiary structures interact with the San Andreas? • How is the Big Bend influencing the region - has approach to frictional lock-up caused bypasses such as the Eastern California Shear Zone and San Jacinto fault? • How may ruptures propagate along these fault systems? • Fundamental goals: • Unique natural laboratory opportunity to capture large events and fault interaction • Tectonics; Stress interaction - static and also dynamic triggering • Source physics; Fault and rock mechanics • Hazards high due to proximity of faults to Los Angeles greater metro area • Devise large experiments & additional new instrumentation (and obtain funds)

5. San Andreas Fault • 35 mm/yr slip rate; • >70% of plate motion • 1685, 1812, 1857 eq’s • Big Bend compression • 1971 Sylmar (M 6.7) • 1994 Northridge (M 6.7) • SoCal is now heavily ‘wired’ - need more? What’s missing? • Catalog; SCEC CMM3 • ‘Natural laboratory’ • Likely source of most future ‘Big Ones’ • Southern SAF Interest Group

6. SCEC Tectonic Geodesy • CMM3 & future work: • Integrate InSAR • with GPS for • vertical defor- • mation rates • Resolve rate dis- • crepancies • between geology • and geodesy

7. Strike-Slip Rates from Geodesy Courtesy of B. Meade

8. Recent Results Bennett et al., Geology 2004 San Andreas and San Jacinto variable & alternating slip rates Anderson et al., BSSA 2003 San Andreas and San Jacinto rates are the same

9. Examples of Differences in Rate • Garlock fault • Geologic rate 7 +/- 2 mm/yr • Geodetic rate 2 +/- 2 mm/yr • Geodesy < Geology => weak lower crust • Eastern California Shear Zone • Geologic rate summed over all faults is ~6 mm/yr • Geodetic rate across ECSZ is ~10–12 mm/yr • Geodesy > Geology => clustering or new higher tectonic rate? • Imperial Valley • Geologic rate of 20 mm/yr • Geodetic rate across valley of ~50 mm/yr => missing a major fault? • Sierra Madre – Cucamonga fault zone • Geologic rate of 0.5 mm/yr • Geodetic rate of a • Raymond fault • Geologic rate of 1.5-4 mm/yr • Geodetic rate of b • a + b ~ 6-8 mm/yr

10. Alternating Slip • Peltzer et al., Geology 2001 • Garlock fault and ECSZ slip rate discrepancies can be explained by alternating activity between the two fault zones (over ~1000-yr. time scales) • May correspond to ECSZ clustering?

11. Fault Interaction • Emerging view of large events as a composite of sub-events or asperities • Dynamic triggering • Static triggering • Important to study analogous events • Cascading rupture - order in chaos? 1857 San Andreas Bayarsayhan et al., 1996 Kurushin et al., 1998 1957 Gobi-Altay

12. Understanding Temporal Changes • Temporal variations do occur: • Clustering (e.g., Basin & Range, ECSZ, Asia) • Discrepant geological and geodetic rates • Sequences involving fault interaction (e.g., Joshua Tree - Landers - Big Bear - Hector Mine; Anatolian system, etc.) Courtesy Anke Friedrich

13. ECSZ Temporal Variations • Savage et al. (2004) data re-analysis confirmed Hudnut et al. (2002) model for block breakaway in ECSZ • How does ~1000-yr. temporal clustering in ECSZ relate (if at all) to ~100-yr. clustering along the San Andreas? Hudnut et al., 2002

14. Closure Rates from Geodesy Courtesy of B. Meade

15. LA Deformation Obfuscation • Bawden et al., 2002 Nature paper • Seasonal variations in SCIGN data correlated with water table changes • Removal of this noise enabled a refined velocity map for the urban area

16. LA Contraction • Must integrate many types of information • Combine GPS with the deep fault geometry (from imaging and seismicity, etc.) and 3D structure • Employ novel modeling methods Complex Problem: new geodetic results for LA Figure Courtesy of Don Argus and co- authors D. Argus, JPL

17. Rotations Figure Courtesy of Chris Sorlien and co- authors

18. Uplifting Thoughts for the Future? • How fast are the mountains going up? • Nikolaidis et al. vertical rates from SCIGN -suggest rate changes Courtesy of R. Nikolaidis, UCSD dissertation

19. Summary • We can understand the SoCal fault system in all of its complexity, it’s just not going to be easy • Pursue similar course longer, and more will continue to be learned about deep geometry, activity, and overall geodynamics of the system • We must understand the fault interactions if we are to predict aspects of future behavior within the SoCal fault system • Much remains to be discovered about past evolution, and increasingly sophisticated models will help with interpretation of system dynamics