Scott T. Marshall

Effects of Nonplanar Fault Topology and Mechanical Interaction on Fault-Slip Distributions in the Ventura Basin, California

To assess the control of fault geometry and mechanical interactions on fault-slip distributions in a complex natural system, we present results from three- dimensional mechanical models incorporating both nonplanar and rectangular planar representations of active faults within the Ventura basin region of southern California. We find that the incorporation of geologically constrained nonplanar fault surfaces into numerical models of active deformation results in a better match to available geologic slip-rate data than models utilizing rectangular planar fault surfaces. The model results demonstrate that nonplanar fault geometry and mechanical interactions exert a strong control on resultant slip distributions. Additionally, we find that slip rates at most locations along the surface trace of Ventura faults are not likely to rep- resent average values for the entire fault surface. We propose that results from three- dimensional mechanical models using realistic (i.e., nonplanar) fault geometry can be used to both predict slip rates at specific locations and determine whether existing site- specific slip-rate estimates are representative of average fault-slip rates. Although geo- metric irregularities along-fault surfaces should resist slip, planar faults can have lesser slip than nonplanar faults due to the differing mechanical interactions among nearby faults in the two representations. This suggests that models using simplified or planar fault geometry are likely to inaccurately simulate regional deformation. We assert that detailed knowledge of three-dimensional fault shape as well as the geometry and con- figuration of deep fault intersections is essential for accurate seismic hazard char- acterization of regions of complex faulting such as the Ventura basin of southern California.

Using Vertical Rock Uplift Patterns to Constrain the Three-Dimensional Fault Configuration in the Los Angeles Basin

Comparison of geologic uplift patterns with results of three-dimensional mechanical models provides constraints on the fault geometry compiled by the South- ern California Earthquake Center community fault model in the northern Los Angeles basin, California. The modeled uplift matches well the geologic pattern of uplift as- sociated with the Santa Fe Springs and Coyote Hills segments of the Puente Hills thrust fault but does not match structures to the west of the San Gabriel River. To better match the geologic patterns in this area, alternative fault configurations were tested. The best match to geologic uplift is attained with a model incorporating (1) a steep blind thrust fault at the location of the Los Angeles segment of the Puente Hills thrust system (following interpretations of the Las Cienegas fault geometry at this location), (2) removal of an inferred linking fault between the Raymond and Holly- wood faults, and (3) lateral continuation of the Lower Elysian Park fault, a blind low- angle detachment at >10 km depth, along strike to the northwest. These geometric revisions alter the connectivity of northern Los Angeles basin faults and significantly improve the match of model uplift pattern to geologic data. Model results suggest that fault connectivity may be more important in governing fault slip rate than are fault dip and fault area. The preferred model alters slip rates by >0:2 mm=yr for the Upper Elysian Park, Hollywood, Lower Elysian Park, Raymond, Sierra Madre West, and Verdugo faults. Additionally, the preferred model alters the surface area of several faults in the northern Los Angeles basin, such as the Puente Hills thrust and the Lower Elysian Park fault, which may have important implications for seismic hazard assess- ment in the northern Los Angeles basin.

Influence of fault connectivity on slip rates in southern California: Potential impact on discrepancies between geodetic derived and geologic slip rates

Along the San Bernardino strand of the San Andreas fault (SAF) and across the eastern California shear zone (ECSZ), geologic slip rates differ from those inverted from geodetic measurements, which may partly be due to inaccurate fault connectivity within geodetic models. We employ three-dimensional models that are mechanically compatible with long-term plate motion to simulate both fault slip rates and interseismic surface deformation. We compare results from fault networks that follow mapped geologic traces and resemble those used in block model inversions, which connect the San Jacinto fault to the SAF near Cajon Pass and connect distinct faults within the ECSZ. The connection of the SAF with the San Jacinto fault decreases strike-slip rates along the SAF by up to 10% and increases strike-slip rates along the San Jacinto fault by up to 16%; however, slip rate changes are still within the large geologic ranges along the SAF. The insensitivity of interseismic surface velocities near Cajon Pass to fault connection suggests that inverse models may utilize both an incorrect fault geometry and slip rate and still provide an excellent fit to interseismic geodetic data. Similarly, connection of faults within the ECSZ produces 36% greater cumulative strike-slip rates but less than 17% increase in interseismic velocity. When using overconnected models to invert GPS for slip rates, the reduced off-fault deformation within the models can lead to overprediction of slip rates. While the nature of fault intersections at depth remains enigmatic, fault geometries should be chosen with caution in crustal deformation models.

Influence of Fault Geometry on the Spatial Distribution of Long‐Term Slip with Implications for Determining Representative Fault‐Slip RatesInfluence of Fault Geometry on the Spatial Distribution of Long‐Term Slip

Determining representative slip rates of active faults is essential to seismic hazard assessment and tectonic analysis. Here, we take a two-pronged approach to determine how spatially variable slip over many earthquake cycles impacts the sliprate record at potential sites of geologic investigation near releasing stepovers. First, we use 2D parametric models to estimate the probability that a point measurement is representative of the average slip-rate for continuous strike-slip faults and those with a range of releasing stepover geometries and friction values. All models result in skewed distributions for which a randomly selected site has a higher probability of sampling a slip rate that exceeds the mean. For most configurations, individual point measurements are unlikely (p < 0.5) to yield a slip rate within +/- 1 mm/yr of the mean. The probability can be notably improved (15%-300%) by summing slip rates of overlapping segments. Second, we use 3D mechanical models of a well-studied releasing stepover along the San Jacinto fault to investigate the impact of specific fault geometries on slip-rate distribution and the implications for existing slip-rate estimates. The model-calculated dextral-slip rates are consistent with the abundant geologic slip-rate data available for this system. Although summing the slip rates at geologic sites across the stepover may produce a nonrepresentative slip rate, the summed slip rates from the model are compatible with the representative slip rate. Thus, the complete along-trace slip-rate distribution produced from the geologically consistent models provides a way to select sites for determining slip rates or interpreting isolated geologic slip-rate data. The site-specific models also illustrate how to assess the uncertainty of slip-rate data due to the spatial variability of slip rates in geometrically complex fault systems.