PARTITIONING OF SEISMOGENIC STRAIN OVER VARYING SPATIAL SCALES AT THE SIERRAN-NORTH AMERICA PLATE BOUNDARY

Using a numerical adaptation of micropolar continuum theory we find evidence that seismogenic strain partitions spatially into distinct geometries at both kilometer- and sub-kilometer scales. Inversion of seismic focal mechanisms from the Indian Wells Valley of eastern California reveals strain tensors with subhorizontal, approximately E-W maximum lengthening axes (d1) and either subhorizontal or subvertical minimum lengthening axes (d3). The seismic events that define both tensors are spatially intermixed over length scales no greater than the spatial resolution of the events (i.e., hundreds of meters). The nodal planes that display slip directions best fit to the inverted strain tensors occur dominantly at high angles to the large-scale shearing along the north northwest trending Sierran-North America boundary. These results suggest fine-scale partitioning of large-scale deformation and suggest that single faults may accommodate two strain geometries by slipping in different directions at different times. Likewise, inversion of seismic focal mechanisms from the Wild Horse Mesa, located less than 30 km to the north, reveals two strain tensors. The events that define these two tensors, however, occur within distinct depth ranges. Seismic events from 0-5 kms are somewhat uniformly distributed in 3 dimensions, and indicate a subhorizontal, approximately E-W oriented d1 and a subvertical d3. The events from 5-8 kms display a more linear distribution when viewed in a profile normal to the Sierran-North America boundary. These events indicate a d1 that is subparallel to that defined for the shallow events, and a shallowly south southeast plunging d3, nearly normal to that defined for the shallow events. The nodal planes with slip directions best fit to the inverted strain tensors also display variations with depth. The favored nodal planes for the shallow events are dominantly at high angles to the Sierran-North America boundary whereas the favored nodal planes for the deep events display variable orientations. These results highlight very different spatial scales over which seismogenic deformation can be partitioned and have implications for understanding active deformation (e.g., akin to analog models of transtension) and interpreting the rock record (e.g., multiply-lineated faults).