Mechanical fault interaction within the Los Angeles Basin: A two‐dimensional analysis using mechanical efficiency

Abstract

Mechanical models examine deformation within eight different structural cross sections proposed by Davis et al. [1989] and Shaw and Suppe [1996] along a northeast‐southwest transect across the Los Angeles Basin, California. Horizontal contraction of the models, constrained by geodetic measurements, yields varying dip‐slip rates along frictionally sliding faults within the different cross sections. Mechanical efficiency analysis using effective stiffness and strain energy density assesses the overall fault system deformation as well as the partitioning of work between fault slip and host rock strain. The cross section interpreted by Shaw and Suppe [1996] has the best fit to paleoseismically determined slip rates and the greatest mechanical efficiency (greatest proportion of work toward fault slip); however, this model produces excessive reverse slip along the Newport‐Inglewood fault. A modified fault configuration with a wedge or blind Puente Hills thrust fault rather than a ramp‐detachment configuration better matches the paleoseismic data with slightly lower mechanical efficiency. Slip rates in the mechanical models based on interpretations of Shaw and Suppe [1996] have much closer match to the geologically determined rates than those estimated from kinematic models. This difference is due to (1) differing time spans of slip rate estimates and (2) deformable rather than rigid host rock in the mechanical models. The mechanical efficiency analysis provides quantitative indicators of overall fault system deformation, including the cumulative effect of interaction between individual faults. Assessment of effective stiffness and strain energy density furthers our understanding of two‐dimensional fault interactions in the Los Angeles Basin and offers great potential for future applications.