Evidence of Strain Partitioning Between the Sierra Madre Fault and the Los Angeles Basin, Southern California from Numerical Models

Abstract

Geodetic data indicate that the northern Metropolitan Los Angeles region is shortening at a rate of 4.5-6.0 mm/yr between downtown Los Angeles and the San Gabriel Mountains. If we assume that all of the uplift of the San Gabriel Mountains is due to the major frontal fault system (the Sierra Madre fault) and use reported values for bedrock uplift, slip per event and recurrence intervals to determine the slip rate on the Sierra Madre fault, we obtain slip rates between 0.6-1.27 mm/yr. Using these slip rates, the horizontal shortening attributable to the Sierra Madre fault accounts for only $33% of the observed shortening across the basin, leaving $67% of the shortening to be accounted for elsewhere. Herein we present a suite of models that test possible shortening mechanisms to account for this strain deficit. The models incorporate a range of fault geometries and have a layered structure with variable vertical and horizontal rheologies. The models demonstrate how lower-crust rheology and the presence of a low rigidity, anelastically deforming sedimentary basin affects the dissipation of stress imposed on the viscous layers by elastic failure of the faults. We found that viscoelastic models with a single fault, vertically strong crust and a compliant sedimentary basin yield a horizontal velocity profile that best matches the geodetically observed velocity profile across the Los Angeles Basin. Our models also indicate that we are still not accounting for all of the observed deformation. Therefore, more complex models that include both laterally varying rheologies and frictional properties on faults must be considered.

Interseismic Strain Accumulation on Faults Beneath Los Angeles, California

“…used here, and Glasscoe et al (2004) showed that postseismic deformation is unlikely to affect surface velocities in Los Angeles for more than a few years under realistic parametrizations, meaning that earthquakes preceding Northridge are likely to have much effect on present-day geodetic velocities either. The 1857 Mw~7.9 Fort Tejon earthquake could have triggered widespread and long-lasting postseismic deformation, but as shown for that case by Hearn et al (2013) and for a thrust setting by Hager et al (1999), such deformation should cause present-day geodetic deformation rates to trail average rates not substantially exceed geologic and paleoseismologic rates.…”

“…These rates are higher than most estimates from geologic and paleoseismologic studies, with the exception of the slip rate of 3.6-5.7 mm/year inferred on the western Sierra Madre Fault by Meigs et al (2003). On the Sierra Madre Fault, Walls et al (1998) estimated a slip rate of 1-2 mm/year on the western section, Tucker and Dolan (2001) estimated a slip rate of 0.6-0.9 mm/year on the central section, and Rubin et al (1998) inferred~11 m of total displacement in two paleoearthquakes since~15 ka, translating into an average slip rate of~0.6 mm/year (Glasscoe et al, 2004). On the Puente Hills Fault, Bergen et al (2017) inferred a recent slip rate of 1.33 + 0.4/À0.2 mm/year on the Los Angeles segment, Leon et al (2009) and Myers et al (2003), respectively, inferred slip rates of 1.1-1.6 mm/ year (1.4-2.4 mm/year in an alternate estimate) and 1.5 mm/year on the Santa Fe Springs segment, and Shaw et al (2002) and Myers et al (2003), respectively, inferred slip rates of 1.28 and 1.5 mm/year on the Coyote Hills segment.…”