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 Southern California Regional Tectonics - Constraints from Geodetic Data Courtesy of JPL

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

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

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)

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

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

Strike-Slip Rates from Geodesy Courtesy of B. Meade

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

Examples of Differences in Rate  Garlock fault  Geologic rate 7 +/- 2 mm/yr  Geodetic rate 2 +/- 2 mm/yr  Geodesy 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 mm/yr  Geodetic rate of b a + b ~ 6-8 mm/yr

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?

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? Bayarsayhan et al., 1996 Kurushin et al., San Andreas 1957 Gobi-Altay

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

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

Closure Rates from Geodesy Courtesy of B. Meade

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

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 D. Argus, JPL Figure Courtesy of Don Argus and co- authors Complex Problem:

Rotations Figure Courtesy of Chris Sorlien and co- authors

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

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