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Unraveling the San Gorgonio Knot, California: A Three-Dimensional Numerical Investigation of San Andreas Fault Configuration through the San Gorgonio Pass

Laura Dair and Michele Cooke
University of Massachusetts – Geosciences Department Amherst, MA [email protected]

The San Andreas fault poses significant earthquake hazards for the 24 million residents of southern California. While geologic evidence shows that the San Andreas fault in the San Gorgonio knot has complex, dipping and discontinuous fault strands, predictive earthquake models of this region have disregarded these complexities. We investigate alternative three-dimensional configurations of the San Andreas fault through the San Gorgonio Pass to show the influence of fault geometry on uplift patterns and slip rates. The different models include both a simplified vertical, continuous fault and a north-dipping discontinuous fault. The north-dipping model corresponds better to observations of recent uplift within the San Bernardino Mountains. The vertical fault configuration has greater net slip and more efficiently transmits deformation through the San Gorgonio Pass, but the north-dipping fault configuration may better match uplift patterns. There has been an ongoing debate over the slip partitioning between the Coachella Valley segment of the San Andreas and the San Jacinto fault. Our study indicates that the slip portioning between the two faults has less of an effect on the slip rates and deformation than fault geometry.

Fieldwork in this area has been invaluable to the creation of a realistic present day fault configuration; the more geologically accurate the initial configuration the more accurate the prediction of earthquake hazards. Further study of this area will incorporate secondary faults in an attempt to understand the fault evolution and provide greater insight into future hazards along the San Andreas fault.


DSCN8986 - Laura Dair, fault at top dipping shallowly toward right side of picture (north)

DSCN9004 – Laura Dair extinct strand of the San Andreas fault (straight line)

DCSN9006 – Laura Dair and Jon Matti (USGS) same stand of San Andreas

DSCN9015 – Laura Dair and Jon Matti looking at the fault deformation zone


AAPG Search and Discovery Article #90070 © 2007 AAPG Foundation Grants in Aid