Active Folding within the L.A. Basin with a focus on:

1. Aron Meltzner Ge 277 21 November 2008 Active Folding within the L.A. Basin with a focus on: Argus et al. (2005), Interseismic strain accumulation and anthropogenic motion in metropolitan Los Angeles and Myers et al. (2003), Dislocation modeling of blind thrusts in the eastern Los Angeles basin, California

2. Argus et al. (2005) Methodology • use GPS & InSAR to estimate and model motions in Southern California • make predictions & assumptions about coseismic & postseismic motion, and correct to obtain interseismic velocities • identify seasonal & cumulative anthropogenic motions, and remove from velocity field to obtain tectonic interseismic velocity field

3. Argus et al. (2005) Methodology • use elastic screw dislocation model to estimate strain along SAF & SJF • this elastic strain is removed to get strain within metropolitan L.A. • estimate deep slip rates along SAF & SJF assuming locking depth = max seismicity depth • slip rate estimates along SAF are generally slower than geological rates

4. The Palos Verdes peninsula is moving north relative to the San Gabriel Mtns at ~6 mm/yr. A belt of high contractional strain cutting across northern metropolitan Los Angeles 12–25 km south of the San Gabriel mountains is shortening from north to south at 4–5.5 mm/yr. Site velocities decrease going from the Palos Verdes peninsula to USC1, with the mean velocity of 8 sites in southern L.A. being 5.1 ± 1.2 mm/yr toward N6°E. Because the elastic strain that will be released along the San Andreas and San Jacinto faults has been removed from the velocity field, the San Gabriel mountains appear to be hardly deforming and the west Mojave desert appears to be moving southeast relative to the San Gabriel mountains at 20 mm/yr. Argus et al. (2005)

5. Argus et al. (2005)

6. Argus et al. (2005)

7. Main Problem: Faults creeping to within 6 km of the surface seems inconsistent with large earthquakes breaking a brittle lithosphere down to 15 km depth. In metropolitan L.A. the seismogenic depth is 15–20 km, the approx. maximum depth of seismicity and the depth to which 3 large modern EQs ruptured. This disagreement suggests that the model, in which an edge dislocation occurs along a planar reverse fault in an elastic continuum, may be unsatisfactory. Additional Inconsistency: The total horizontal rate estimated by the elastic dislocation model across the Puente Hills and Elysian Park thrusts is more than double the Holocene rates estimated from geology. The difference between the two estimates could be accounted for by slip along faults outside the zones studied by Dolan et al. (2003) and Oskin et al. (2000), or the convergence rate over the past several years could have been faster than the mean Holocene convergence rate. Argus et al. (2005)

8. Myers et al. (2003) Methodology • use elastic dislocation modeling to determine location & slip rate on blind thrust beneath Coyote Hills anticline (S of Puente Hills) • apply method to Santa Fe Springs anticline (to the W, where 1987 Whittier Narrows blind thrust is well located) • consider implications of the convergence rate across this structure on the distribution of shortening across L.A. Basin, measured by GPS

9. Myers et al. (2003)

10. Myers et al. (2003) Methodology • constrain geometry and displacement of source fault with 3D dislocation modeling • dislocation placed in homogeneous half-space • structure formed over >1 Myr, during which time most shear stressed has been relaxed • crustal deformation is visco-elastic, but effect of viscous relaxation on fold shape occurs at wavelengths longer than that of Coyote Hills, so it has a small impact on fold shape

11. Myers et al. (2003) Methodology 7 fault parameters were fit or assumed: • depth of upper fault tip • N-S location of upper fault tip • E-W location of upper fault tip • depth of lower fault tip • fault length • fault dip • fault displacement

12. Myers et al. (2003) Methodology But there’s not much control on the fault location....

13. Myers et al. (2003) Results

14. Myers et al. (2003) Results

15. Myers et al. (2003) Results • use model results and stratigraphic age control to estimate fault slip rates on the blind thrust: • 1.3 ± 0.5 mm/yr at Coyote Hills, 1.5 ± 0.4 mm/yr at Santa Fe Springs over the past 1.2 Ma • 3 ± 1 mm/yr for last 2.9 Ma at Santa Fe Springs Oilfield • results indicate a decrease in slip rate over time • results are generally consistent with geological observations, including those of Dolan et al. (2003) and Oskin et al. (2000) • main problem: the model’s predicted rates of contraction (like the results of geological studies) are much less than those indicated by geodesy

16. Myers et al. (2003) Results

17. Fuis et al. (2003) Argus et al. (2005)

18. Argus et al. (2005) vs. Myers et al. (2003) • Unlike in the Himalaya, where the highest relief is in the middle of the range, L.A. sees more activity along the range front and in the middle of the basin • Argus et al. need to put some creep in the basin • When using an elastic dislocation model, L.A. Basin geodesy cannot be fit well without unrealistic ‘virtual’ faults

19. Argus et al. (2005) vs. Myers et al. (2003) • Both papers use elastic dislocations for modeling but obtain very dissimilar results: • The geology and geodesy cannot be fit simultaneously! • Perhaps a difference between the Himalaya and the L.A. Basin is that, because L.A. deformation rates are an order of magnitude smaller, secondary processes such as pressure solution are more influential and cannot be ignored