Abstract

Mapping of surface rupture associated with the 28 June 1992 Landers earthquake along a portion of the north-central Emerson fault permits us to characterize the fractures and to interpret them in terms of a contractional bend in the fault trace. The main rupture exhibits right-lateral and reverse slip and is localized in a zone about 50-m wide along the present-day trace of the Emerson fault. The surface deformation within its northeastern block, the hanging wall, is accommodated by a dome structure about 2-km long and 0.5-km wide and a complex assemblage of fractures. The complexity of the fracture pattern around the fault bend results in part from (1) interaction between the main rupture and many shorter subparallel faults, (2) the initiation and propagation of fractures at geometric complexities such as fault segment tips and fault bends, and (3) bedding plane faulting. Both geological data and rupture kinematics indicate that uplift and shortening are associated with the fault bend.

The state of stress around the Emerson fault bend is determined for co-seismic conditions (fault slip boundary conditions) and interseismic conditions (the remote applied stress and frictional fault boundary conditions) using both displacement discontinuity boundary element and dislocation methods, respectively. We also investigate a possible fault propagation path at an idealized bend using the maximum distortional strain-energy density criterion. Results of the theoretical analyses provide support for an understanding of the location and the asymmetry of the dome structure and the mechanisms for some of the observed fracture systems in terms of the state of stress and fault propagation paths around the fault bend and other smaller-scale complexities along the fault trace.

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