THE STRANGE GEOMETRIES OF STRIKE-SLIP FAULTS: INSIGHTS FROM SOUTHERN CALIFORNIA’S ACTIVE FAULTS

Detailed mapping and structural analyses of basins and faults, southern California reveals geometries of strike slip (ss) faults that do not match classic models. The San Jacinto, Elsinore, Earthquake Valley, Brawley seismic zone, Fontana trend, Extra, Elmore Ranch, Superstition Hills, Mecca Hills, Hidden Springs, and southern San Andreas fault zones exhibit ladder and comb-like map patterns, stair-stepping traces, ramps, pitchforks, zippers, overpasses and/or interpenetrating geometries. Relocated microseismic alignments confirm these geometries in both basins and basement. Strike-slip faults commonly ramp through strong rocks and flatten in weaker units, producing bends, flats, ramps, and cryptic bed-parallel faults. Ramp-related folds form adjacent to and parallel to many ss faults. The Brawley seismic zone, the southern San Andreas fault in Durmid Hill, the Fontana trend, and the Earthquake-Valley-Elsinore fault zones have ladder-like geometries in map view and widths of 2-25 km perpendicular to strike. Ladders can be the central feature of fault zones and replace fault cores. Combs have one master fault on their margin. Block rotation within ladders and combs produces structures out of alignment with stresses. Abrupt laterally changes in geometry along strike of active ss faults create damage zones with variable widths. The Elmore Ranch earthquake ruptured a 10-km wide fault array at the surface yet deformation coalesced into a single vertical plane at depth. Its geometry is like a pitchfork in cross section-a flower structure with many flat detachment horizons. Some ss faults pass through and over one another and are mutually active at geologic and hazards time scales. This explains the cryptic connection between the Clark and the Superstition Hills faults through continuous marker beds and the Extra fault zone at the surface. The dextral Earthquake Valley fault zone similarly persists through the left-normal Split Mountain fault zone in the Fish Creek basin. Misidentification of fault zone geometries has implications for hazards analyses. Wide dispersed fault zones or localized knots of complexity may be hazardous and underlie large areas, but rarely make it into unified fault models.