Ladders, stair-steps, and crossing faults: Insights from southern California’s active strike-slip faults

Susanne U. Janecke, Benjamin E. Belgarde, Ann Bykerk-Kauffman, James P. Evans, Stefan Kirby, Daniel Markowski, Alexander Steely, & Steve J. Thornock

Submitted August 15, 2017, SCEC Contribution #7834, 2017 SCEC Annual Meeting Poster #131

Detailed mapping and structural analyses of the San Andreas, San Jacinto, Elsinore, and cross faults, southern California, reveals geometries of strike-slip (ss) faults that do not match classic models. Many fault zones exhibit ladder-like geometries in map view and/or pitchfork geometries in cross section.  Crossing and mutually interfering faults rotate one another and create map-scale breccias.  Overpasses and/or interpenetrating geometries form when faults cross though one another, and mis-oriented and stair-step fault traces are diagnostic of this process. These geometries involve large volumes of rock and many damage zones are 1-5 kilometers wide. Transformation from a classic strike-slip fault to an unusual one occurs over short lateral distances and is common. Relocated seismic alignments confirm these geometries and show that ladders persist to depth and deform 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 anticlines and synclines 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. Ladders can be the central feature of fault zones and replace fault cores. Block rotation within ladders and combs produces structures out of alignment with regional 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 connection between the Clark and the Superstition Hills faults, despite the presence of continuous marker beds and the Extra fault array between them. Oversimplification of the actual fault geometries has implications for shaking and surface-faulting hazards. Key implications: 1) Strike-slip faults may be many kilometers wide.  The SAFZ in Durmid Hill and the Brawley seismic zone have this geometry. 2) Strike-slip faults may cross through one another and connect in unexpected ways. 

Janecke, S. U., Belgarde, B. E., Bykerk-Kauffman, A., Evans, J. P., Kirby, S., Markowski, D., Steely, A., & Thornock, S. J. (2017, 08). Ladders, stair-steps, and crossing faults: Insights from southern California’s active strike-slip faults. Poster Presentation at 2017 SCEC Annual Meeting.

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