Does Subsurface Fault Geometry Affect Aleatory Variability in Modeled Strike-Slip Fault Behavior?

ABSTRACT The nonplanar geometry of faults influences their seismotectonic behavior, affecting the initiation, propagation, and termination of individual earthquakes as well as the stress–slip relationship and probability of multisegment rupture. Consequently, computer simulations that aim to resolve the earthquake rupture process and make predictions about a fault’s future behavior should incorporate nonplanar fault geometries. Although surface traces of faults can be mapped with high accuracy, a key challenge is to define a fault’s detailed subsurface geometry due to a general lack of data. This raises the question of which geometry to use. Does it matter which subsurface geometry is utilized in earthquake rupture simulations, as long as at least the fault trace is considered? How different is the simulated fault behavior for faults that share the same surface trace but different subsurface geometries? Using the physics-based earthquake-cycle simulator MCQsim, we generate seismic catalogs for 100 × 20 km strike-slip faults, assuming variations in fault surface trace, subsurface geometry, and strength distribution. We investigate how the long-term fault behavior—in the form of magnitude–frequency distribution, earthquake interevent time, and maximum earthquake size—is affected by fault geometry and strength distribution. We find that the simulated behavior of strike-slip faults with identical fault traces is interchangeable—even if their subsurface fault geometries differ. Implementing the fault trace constrains possible fault geometries to a level that makes the long-term behavior indistinguishable—at least for strike-slip faults with “known” strength distribution and length-to-width aspect ratios that are equal or larger than what we used here. The fault trace can provide a satisfactory representation of subsurface geometry for assessing long-term fault behavior.