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Durmid ladder structure and its implications for the nucleation sites of the next M >7.5 earthquake on the San Andreas fault or Brawley seismic zone in southern California

Susanne U. Jänecke, Miles Kenney, Patricia Persaud, Daniel K. Markowski, & James P. Evans

Published June 15, 2018, SCEC Contribution #7224

We integrate new geologic data and published geophysical data along the southernmost the San Andreas Fault zone (SAFZ), southern California to document the presence of a transpressional 1-3 km wide highly damaged San Andreas fault zone. A newly described active fault zone, the East Shoreline fault zone of the San Andreas Fault, which has its own ~1 km wide damage zone is parallel to the main strand of the San Andreas Fault for at least 30 km, and forms the southwest margin of the Durmid damage zone that swells to 4-4.5 km width across Durmid Hill, north of Bombay Beach, California. The East Shoreline fault zone is expressed by faults and folds in Pleistocene sedimentary rocks 1 km southwest of the localized main strand of the San Andreas fault between Bombay Beach and North Shore, California. From its southern tip ~1 km SE of Bombay Beach northward at least to Corvina Beach, California, the main and East Shoreline strands of the San Andreas Fault (ESF) form a coordinated ladder-like fault zone in map view that bounds hundreds of intervening left- and right-lateral cross faults and folds with classic geometries of rotating fault blocks. A dense set of right-lateral and oblique-slip faults with mostly steep dips comprise the East Shoreline strand of the San Andreas Fault zone. Based on significant volumes of uplifted basin fill deposits, reflection data across the Mecca Hills, and other subsurface data, the East Shoreline fault persists into the subsurface as a major structure, likely continues northward for over 100 km past Indio, California along the northeast margin of the Coachella Valley, and might be related to the Garnet Hills strand of the San Andreas fault zone.

Both strands of the SAFZ cut Miocene (?) to modern sedimentary rocks and sediment and produce fault-subparallel folds by fault-bend folding across ramps and flats. Sigmoidal geometries define faults and folds, and left-lateral faults strike east, ~40-60° clockwise of their ideal geometry. Transpression and clockwise rotation and right-lateral shear of fault blocks and folds between the ESF and main strand of the San Andreas Fault in the Durmid ladder structure produced roughly 45-55° of rotation during the last 1-2 my. Slip across the master right-lateral faults of the SAFZ are exhuming, rotating, and shortening the latest Miocene (?) to Pleistocene sediment between them, and uplifting higher-density metasedimentary rocks and basement rocks at depth. The uplift and shear deformation are expressed in gravity data, have a major imprint on the landscape, and explain the overall seaward slope of Durmid Hill.
The San Andreas-Brawley fault zone forms a complex band of southward widening deformation that increased from about 1 km wide belt near North Shore, California, to a 5-6 km wide fault zone near Bombay Beach, to a 6-11 km-wide cloud of earthquakes along the south edge of the Salton Sea. As the San Andreas Fault zone bends and gradually transforms southward into the Brawley Seismic Zone, uplift and transpression give way to subsidence and transtension within the ladder-like fault zones. The southernmost exposures of damaged rocks in the San Andreas fault zone share characteristics with both adjacent faults across a broad, interpenetrating, and gently curving subregion.

If the San Andreas Fault zone and Brawley Seismic Zone share some major earthquakes with the southern tip of the SAFZ nucleation of some of the the M>7.5 earthquakes that ruptures the southern SAFZ might occur at any branch point with an active left-lateral faults west of the Brawley-SAFZ and/or an active right-lateral faults east of the zone.
The many-kilometer wide Durmid ladder structure and Brawley Seismic Zone are not unique and there are active ladder-like fault zones in populated areas. Wide ladder structures may pose a different set of seismic hazards because surface faulting is dispersed across several kilometers perpendicular to the main fault traces. Surface displacements during major earthquakes in such a fault zone should be smaller across any particular fault trace because energy would be dispersed through a much larger volume. Recognition of the ESF in Coachella Valley and modeling of more realistic, kilometer(s) wide damage zones will significantly change estimates of seismic hazards in the region.

Key Words
Southern San Andreas Fault, Structural Characterization

Jänecke, S. U., Kenney, M., Persaud, P., Markowski, D. K., & Evans, J. P. (2018). Durmid ladder structure and its implications for the nucleation sites of the next M >7.5 earthquake on the San Andreas fault or Brawley seismic zone in southern California. Lithosphere, 10(5), 602-631. doi: 10.1130/L629.1.

Related Projects & Working Groups
Testing a step-over model of the southern San Andreas fault at Durmid Hill, Earthquake Geology, UCR, SoSAFE