Integration of Surface Slip and Aftershocks to Constrain the 3D Structure of Faults Involved in the M 7.3 Landers Earthquake, Southern California

Abstract

Investigation of the mechanics of complex earthquakes requires insight into fault geometry. We use linear‐elastic, quasi‐static mechanical models of faults involved in the M 7.3 Landers earthquake to test the influence of fault continuity, dip, and strike on fault slip and the perturbed stress field in regions of complex surface deformation and dense aftershock activity. Subsurface fault structure is constrained with both geological and geophysical data by comparing observed right‐lateral surface slip with slip along model faults and by comparing focal mechanisms with model Coulomb planes at the locations of large aftershocks. The refined structure of major faults at Landers includes a southward extension of the Johnson Valley fault (JVF) at depth, beyond the mapped fault trace, a 75° W dip along the central Johnson Valley fault, and a near‐surface discontinuity along the southern Homestead Valley fault (HVF). The final model better captures observed slip deficits along the Johnson Valley and Homestead Valley faults, improves quantitative fit to the observed offset data, and captures the local stress state in 9 of 12 locations studied. Relating aftershock orientations to local stresses resulting from mainshock fault slip provides a mechanical basis for deciphering between the two nodal planes provided by each focal mechanism. Slip along nonplanar faults with friction of 0.6 produces model failure planes in a variety of orientations, indicating that heterogeneous aftershocks do not necessarily result from low fault friction. Mechanical selection of nodal planes suggests that aftershocks do not consistently reflect the orientation of their cluster or the orientation and slip direction of mainshock faults.