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Quantifying Natural Fault Geometry: Statistics of Splay Fault Angles

Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964-8000

^* Also at Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan 305-8567.

Online Material: Event information and color figures.

We propose a new approach to quantifying fault system geometry, using an objective fit of the fault geometry to a test function, specifically here a fault branch. Fitting a Y-shaped object using a cost function to dextral faults in California, we find a number of significant results arising from use of a a systematic, objective, quantitative approach. (1) The largest angle of the branch structure is generally very close to 180°, implying that the branch is a splay fault off the primary through-going fault. (2) The distribution of the smallest angle, the splay angle, has a peak near ±17°, symmetric about the primary fault. (3) These features appear independent of scale. These results are not yet explained by any theory, and they pose new questions and constraints for the physics of fault system formation and behavior.