The San Andreas Fault System
Presiding: N Fay, University of Oregon; G Schmalzle, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami
G21A-01 INVITED 08:30h
Interseismic strain accumulation and anthropogenic motion in metropolitan Los Angeles
We use global positioning system (GPS) geodesy and synthetic aperture radar (SAR) interferometry to distinguish between interseismic strain accumulation and anthropogenic motion in metropolitan Los Angeles. We establish a relationship between horizontal and vertical seasonal oscillations of the Santa Ana aquifer, use this relationship to infer cumulative horizontal anthropogenic motions from cumulative vertical motions caused by water and oil resource management, and estimate horizontal interseismic velocities corrected for anthropogenic effects. Vertical anthropogenic rates from 1992 to 1999 are slower than 3 mm/yr in the Santa Ana and San Gabriel aquifers and faster than 5 mm/yr in the Chino aquifer and in many oil fields. Inferred horizontal anthropogenic velocities are faster than 1 mm/yr at 18 of 46 GPS sites. Northern metropolitan Los Angeles is contracting, with the 25 km south of the San Gabriel mountains shortening at 4.5 ±1 mm/yr (95% confidence limits). The thrust fault in an elastic edge dislocation model of the observed strain is creeping at 9 ±2 mm/yr beneath and north of a position 6 ±2 km deep and 8 ±8 km north of downtown Los Angeles. The model fault is near the Los Angeles segment of the Puente Hills thrust but south of the Sante Fe Springs segment of the thrust. Disagreement between the 6 km locking depth in the model and the 15 km seismogenic depth inferred from earthquakes suggests that the elastic continuum model may be unsatisfactory; models with different stiffnesses of sedimentary basin and crystalline basement must be investigated.
G21A-02 08:45h
Slicing up the San Francisco Bay Area: Insights from regional block modeling of GPS data
Utilizing a new compilation of GPS velocities for the San Francisco Bay Area, we present a 3-D block model to solve for the slip rates of individual fault segments within the San Andreas fault system. Block models employ similar assumptions to traditional dislocation modeling (deep dislocations representing plate-boundary deformation, elastic medium), but add the additional physical constraint that individual faults must form connected boundaries around coherent blocks. Our slip rate estimates have important implications for regional seismic hazard, but here we use our model as a case study to focus on the advantages and limitations of this simple block modeling. We describe the importance of global constraints on estimating regional slip rates, the use of heat flow and seismicity to provide a first order constraint on the the seismic/aseismic transition depth, and the ability to test different scenarios of fault connectivity when geologic data alone is ambiguous. We also discuss the fundamental challenge of discriminating between deep deformation on closely spaced faults and the difficulty of defining 3-D fault geometries.
http://www.seismo.berkeley.edu/~burgmann/RESEARCH/BAVU/
G21A-03 09:00h
Kinematics and Dynamics of the Southern San Andreas Fault System
We use geodetic data in the Salton Trough region of southern California to constrain elastic and viscoelastic finite element modeling of San Jacinto and San Andreas fault slip rates and the effects of heterogeneous upper crustal elastic and lower crustal viscous rheology. Results suggest that the San Andreas is the dominant plate-boundary fault, the effects of the sediment-filled Salton Trough on regional crustal deformation is minimal and the lower crust must be fairly strong (~> 5e19 Pa-s) to maintain the high strain gradient near the San Andreas fault. This high strength lower crust couples upper crustal deformation to motion of the lithospheric mantle, and together with shear stress transmitted laterally across faults drives mountain building, basin formation and the generation of earthquakes. Analysis of the force budget acting on the Salton block suggests tectonic shear stress on major faults in the San Andreas system are ~30-60 MPa at 15 km, larger than earthquake stress drops but lower than that predicted by laboratory experiments.
G21A-04 INVITED 09:15h
Holocene deceleration of the San Andreas fault zone in San Bernardino and implications for the eastern California shear zone rate debate
New geodetic inferences for the rate of strain accumulation on the San Andreas fault associated with tectonic loading are ~20 mm/yr slower than observed Holocene surface displacement rates in the San Bernardino area, south of the fault's intersection with the San Jacinto fault zone, and north of its intersection with the eastern California shear zone (ECSZ). This displacement rate "anomaly" is significantly larger than can be easily explained by locking depth errors or earthquake cycle effects not accounted for in geodesy-constrained models for elastic loading rate. Using available time-averaged fault displacement-rates for the San Andreas and San Jacinto fault zones, we estimate instantaneous time-variable displacement rates on the San Andreas-San Jacinto-ECSZ fault zones, assuming that these fault zones form a closed system in the latitude band along which the fault zones overlap with one another and share in the accommodation of steady Pacific-North America relative plate motion. We find that the Holocene decrease in San Andreas loading rate can be compensated by a rapid increase in loading/displacement rate within the ECSZ over the past ~5 kyrs, independent of, but consistent with geodetic and geologic constraints derived from the ECSZ itself. Based on this model, we suggest that reported differences between fast contemporary strain rates observed on faults of the ECSZ using geodesy and slow rates inferred from Quaternary geology and Holocene paleoseismology (i.e., the ECSZ rate debate) may be explained by rapid changes in the pattern and rates of strain accumulation associated with fault loading largely unrelated to postseismic stress relaxation. If so, displacement rate data sets from Holocene geology and present-day geodesy could potentially provide important new constraints on the rheology of the lower crust and upper mantle representing lithospheric behavior on time-scales of thousands of years. Moreover, the results underscore that disagreement between geodetic and geologic fault displacement rates may reflect changes in strain accumulation rates associated with far-field elastic loading and thus earthquake potential, and not just transients.
G21A-05 INVITED 09:30h
Historical Deformation Models of the San Andreas Fault System: Integrating 1000 Years of Earthquake Activity With Modern Deformation Measurements
Geologic, geodetic, and seismic data are integrated into a semi-analytic earthquake cycle model to simulate deformation and stress along the entire San Andreas Fault System over the past 1000 years. The 3-D time-dependent model consists of multiple interacting fault strands embedded in an elastic layer overlying a viscoelastic half-space. The model efficiently simulates interseismic stress accumulation on the upper locked portion of faults, major earthquakes on prescribed fault segments, and the viscoelastic response of the underlying half-space following major ruptures. We simulate 1000 years of the earthquake cycle along the San Andreas Fault System using estimates of both long-term (geologically determined) interseismic slip and coseismic slip estimated from paleoseismic and historical earthquake data. The model is further refined by present-day horizontal and vertical geodetic observations in order to provide bounds on elastic plate thickness (> 60 km) and half-space viscosity (> 2x10 18 Pa s). Using these model parameters, we calculate a model time-series for both 3-D deformation and Coulomb stress spanning the past 200 years. Coulomb stress models show large amounts of accumulated stress prior to major events (ie., 1857, 1906), while the present-day model snapshot identifies significant regions of stress along the majority of the southern San Andreas, as expected from the absence of major earthquakes in the region for over the past 150-300 years. Finally, a time-series of the vertical deformation component of the model is compared with relative sea level changes recorded by coastal tide gauges since 1917. Using these data, we explore both our best geodetically-determined model and a suite of alternative models in order to place further constraints on acceptable model parameters.
http://topex.ucsd.edu/body_force
G21A-06 09:45h
Strain Accumulation Across the Central San Andreas Fault: Impact of Laterally Varying Crustal Properties
Major strike slip fault systems, such as the San Andreas Fault, have one common characteristic: lateral juxtaposition of geologically dissimilar terrains. Terrains on opposite sides of the fault may vary in both geometry of the elastic upper crustal layer and in their material properties. The Carrizo segment of the San Andreas Fault is a prime area to study the effects of asymmetry imposed by strike slip faulting because it is a straight segment and exhibits relatively simple seismic behavior. We present new GPS data on the Carrizo segment to quantify the asymmetry, as well as a series of numerical models designed to investigate various classes of asymmetry. Our models are implemented with the finite element technique, and investigate differences in elastic layer thickness and variable material properties of the upper crust. We find that available data are well fit by a simple model whereby a relatively weak zone (approximating the upper and middle crust) 10-20 km wide exists on the northeast side of the fault.