Micorstructural Analysis of Late Holocene Sediments Deformed by the San Andreas Fault at Point Arena, California

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Differentiating between earthquake rupture-related (coseismic) structures and creep-related (aseismic) structures in active faults such as the San Andreas, Hayward, and Calaveras faults in northern California is commonly not possible, yet assessing the presence or absence of past earthquake-related surface ruptures is critical for evaluating seismic hazard. Recognition of individual surface-rupture earthquakes in trenches excavated across faults is commonly the basis for interpreting the paleoearthquake history of a site and thus for evaluating earthquake recurrence intervals along a given fault segment. However, aseismic creep also deforms surficial deposits and can produce features in trench exposures that resemble those from coseismic rupture. Uncertainty in distinguishing these two types of fault features may result in an erroneous interpretation of earthquake timing and recurrence. This research is extending traditional paleoseismologic investigations to include analysis of microtextural features in sediment samples deformed by active faults. The Alder Cr. study area, on the segment of the San Andreas Fault that last ruptured in 1906, is providing microstructural information from a site that records unambiguous coseismic rupture and allowing direct comparison with an earlier study of the creeping segment of the San Andreas Fault.Computer-based image analysis is a new tool that can be used to measure microscopic-scale characteristics of faulted sand samples. Scanning electron microscope images of oriented sample of faulted sand collected from the San Andreas Fault are being studied using image analysis software. Features such as degree of grain size variation and preferred grain orientation of fault zone samples are measured, and then compared to those from creeping fault zones, from liquifaction vents, and from undeformed sand near the fault zone.This study is: (1) characterizing the degree to which coseismic strain is localized on individual fault strands and describing microstructures formed at high strain rate (coseismic slip); (2) providing a better understanding of fault zone architecture in porous, poorly lithified materials. (3) adding to a small but crucial data set describing deformation processes and strain rates in the time period of hundreds to tens of thousands of years, and (4) yielding information that is directly applicable to assessing the presence or absence of past surface ruptures on faults throughout the San Francisco Bay region.