Abstract: Active Blind-Thrust Ramps and Growth Folds in the Los Angeles Basin, California

John H. Shaw, John Suppe

Several active, blind-thrust ramps capable of generating large earthquakes are recognized in the Los Angeles basin, California, beneath Pliocene and Quaternary growth folds imaged in seismic reflection profiles. Active folding and coeval thrust and strike-slip faulting are described in balanced kinematic models and a geologic cross section developed using fault-bend fold theory. We conclude from a comparison of lateral fault slip distributions that active fault ramps in the Elysian Park thrust system are linked to blind thrusts beneath the Compton-Los Alamitos and Palos Verdes trends. An analysis of folded syntectonic strata along the Compton-Los Alamitos trend indicates a Pliocene and Quaternary slip rate on the Compton thrust of 1.4 mm/yr. Based on the Compton fault slip rate, our s ructural interpretation yields a 0.7 mm/yr uplift rate of the Palos Verdes anticline that is similar to rates derived from uplifted marine terraces. Using fault-bend fold theory, we predict that the Compton fault slip rate should increase to 1.7 mm/yr on the Elysian Park ramp, which bounds the northern Los Angeles basin. Segmentation of the Elysian Park and Compton thrust ramps, revealed by distinct offsets of overlying fold trends, may limit the area that ruptures in individual earthquakes. Sizes of potential earthquakes based on fault ramp areas are estimated at M >= 6.8 for the Compton ramp, and M >= 6.4 and M >= 6.5 for the Whittier and Los Angeles segments of the Elysian Park thrust system. In addition, recurrence intervals of these blind-thrust earthquakes are predicted fr m long-term slip rates using empirical relationships between moment magnitudes and coseismic displacements.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994