Active folding within LA Basin

Readings: Shaw, J., and P. Shearer, An elusive blind-thrust fault beneath Metropolitan Los Angeles, Science, 283, 1516-1518, 1999Davis, T. L., and J. S. Namson, A balanced cross-section of the 1994 Northridge earthquake, southern California

Ge 277Xiangyan Tian 6 December 2008

Argus et al. (2005)

From Aron’s presentation

Argus et al. (2005)

Observations of interseismic strain accumulation along the two profiles are compared with predictions of elastic edge dislocation models of reverse slip along thrust faults. In all models the horizontal component of slip is 8 mm/yr.

Profile A-A' is (light blue) a model in which the Puente Hills thrust and upper Elysian Park thrust are creeping beneath a locking position 6 km deep with slip split evenly between the two faults, (orange) a model in which the Puente Hills thrust is creeping beneath a locking position 8 km deep, (red) a model in which the LARSE decollement is creeping beneath a locking position 15 km deep, and (dark blue) the model best fitting the geodetic observations.

Profile B-B' is (light blue) a model in which the Whittier fault is creeping beneath a locking position 6 km deep, (orange) a model in which a hypothetical fault is creeping beneath a locking position 8 km deep, (red) a model in which the LARSE decollement is creeping beneath a locking position 15 km deep, and (dark blue) the model best fitting the geodetic observations.

The edge dislocation model suggests that the LARSE decollement cannot be the only fault that is creeping. A model (red dashed lines above) in which the horizontal component of slip along the decollement is 8 mmyr and the locking position is 15 km deep predicts the gradient in the velocity component along the profile in northern metropolitan Los Angeles to be much gentler than it is observed to be. The horizontal component of slip would have to be increased to 16 mm/yr to fit the high strain rate there, but then convergence between the Palos Verdes peninsula and the San Gabriel mountains would be predicted to be much too high.

The edge dislocation model suggests that along profile A-A' the Puente Hills-Upper Elysian Park thrust system is creeping at about 9 mm/yr beneath a locking position about 6 km deep. Creep may be occurring along only the upper Elysian Park thrust, along only the Puente Hills thrust, or along both thrusts. The locking position can be no shallower than 8 km deep. A model (orange dashed line above) in which the Puente Hills thrust is creeping beneath a locking position 8 km deep with a horizontal component of slip of 8 mm/yr fits the observations fairly well. Models with locking positions greater than 8 km deep predict the gradient in the velocity component along the profile between USC1 and the southern front of the San Gabriel mountains to be gentler than it is observed to be.

The edge dislocation model suggests that along profile B-B' a hypothetical fault halfway between the Puente Hills thrust and the upper Elysian Park thrust is creeping at about 9 mm/yr beneath a locking position about 6 km deep. A model (dark blue dashed line above) in which a hypothetical fault 14 km northnortheast of the Puente Hills thrust is creeping beneath a locking position 6 km deep with a horizontal component of slip of 8 mm/yr fits the observations well. In the model the hypothetical fault is creeping at 9.0 mm/yr. The hypothetical fault lies near the western part of the reverse-slipping San Jose fault [Yeats, 2004], but the San Jose fault trends at a 45¡ angle to the model fault.

The locking position can be no shallower than 8 km deep. A model (orange dashed line above) in which the hypothetical thrust is creeping beneath a locking position 8 km deep with a horizontal component of slip of 8 mm/yr fits the observations fairly well. However, models with locking positions greater than 8 km deep predict the gradient in the velocity component along the profile between WHC1 and the southern front of the San Gabriel mountains to be gentler than it is observed to be.

The Whittier fault-Puente Hills thrust system cannot be taking up most of the slip. A model (light blue dashed line above) in which the Whittier fault is creeping beneath a locking position 6 km deep with a horizontal component of slip of 8 mm/yr fits the observations poorly, predicting the place at which the velocity component along the profile changes quickly from that of southern Los Angeles to that of the San Gabriel mountains to be 12 km south of where it is observed to be. In the model slip along the Whittier fault goes into the Puente Hills thrust at 9 km depth.

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.

From Aron’s presentation

Fuis et al. (2003) Shaw and Shearer (1999)

Maps showing the locations of the following profiles

Contour map of the Puente Hills thrust system overlain

on a LandsatTM imageShaded relief map of Los

Angeles region

Fuis et al. (2003)

Velocity models and reflectivity of LARSE lines 1 and 2, aligned along surface trace of San Andreas fault

Fault slip rates 0.5 – 2 mm/yr

The geometry of folded growth strata in the Santa Fe Spring structures indicates that at least 800 m of slip occurred on the underlying blind thrust in the Quaternary. Use of the maximum age of Quaternary strata (1.6. million years ago) yields a minimum slip rate of 0.5 mm/year. Maximum slip rate (2.0 mm/year) is taken as the portion of the shortening (7.5 to 9.5 mm/year) measured by geodesy across the Los Angeles basin that remains unaccounted for on previously recognized fault systems.

Shaw and Shearer (1999)

The best-fitting trishear models produce a good match to the modeled part of the structure. The initial tip line position of the Puente Hills thrust was located in the same part of the crust as the Whittier Narrows event (Allmendinger and Shaw, 2000) .

Davis and Namson (1994)

Balanced cross-section across the Northridge portion of the Transverse Range fold-and-thrust belt

Main event of the Northridge 1994 earthquake and aftershocks below 5 km occurred along the Pico thrust, a backthrust off the north-dipping Elysian Park thrust ramp. Shallower aftershocks are the result of fold growth (flexural-slip faults) and propagation of the fold hinge into the Santa Clara synclinorium.

b. Balanced cross-section showing the Santa Susana Mountains and Santa Monica Mountains anticlinorium.c. Restoration of the cross-section shows that it balances and is therefore a viable solution.

Fault slip rates: The Pico thrust 1.4-1.7 mm/yrThe Elysian Park thrust 3.9 – 5.9 mm/yr

Davis and Namson (1994)