Date of Award:

2016

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

Susanne U. Janecke

Abstract

The southern - 100 km long Coachella section of the San Andreas fault is the only section of the fault in southern California that has not experienced a historical earthquake, and it may be the most overdue section of the fault. Numerical models of rupture propagation shows that a large earthquake with a nucleation one in the Durmid Hill field area would produce particularly destructive and deadly ground shaking in southern California. This is used as the model earthquake for the ShakeOut exercises in southern California because it is may represent the worst-case scenario for southern California but does not appear to be a very likely scenario following this research.

Building on existing geologic mapping that shows major Pleistocene to Holocene contraction near the hypothesized nucleation, we use geologic mapping to develop and validate a competing geometric and kinematic model for the southern tip of the San Andreas Fault. A ladder-like-fault model explains the widespread contraction in the Durmid Hill study area as the result of contraction between the main strand of the San Andreas fault and East Shoreline strand. The East Shoreline strand of the San Andreas fault is the newly discovered fault and is dispersed across a zone between 0.5 to 1 km wide, and encompasses an area on the northeast shore of the Salton Sea. There is persistent and strong contraction across the entire - 1.5 to 3.5 km wide San Andreas fault zone because both dextral "side-rail" faults are counterclockwise, and in a contractional bend, relative to current plate motions. This contractional bend was previously documented for the main strand of San Andreas fault.

A new digital geologic map and field studies document the stratigraphy and structures at a range of scales between Bombay Beach and Salt Creek. Numerous folds, narrow strike-slip and oblique-slip faults, and sheared damaged rocks in latest Miocene (?) to Holocene sediment lie within the wide and very complex damage zone of the main strand of the San Andreas fault zone. The East Shoreline strand of the San Andreas fault system buffers the main strand from major stress changes produced by deformation along the sinistral to sinistral-normal Extra fault array under the Salton Sea.

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