Comparison of Microscale Fault Fabrics Formed in Creeping and Stick-Slip Faults

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This research applies an innovative technique, microstructural analysis of faultedsediments, to evaluate more accurately the hazard associated with creeping faults. This project extends ongoing microstructural studies of faulted sediment texture from stick-slip faults (Cashman and Cashman, 2000; Baldwin et al., 2001, in review) to a selected study site on the creeping portion of the San Andreas Fault. The research goal is to develop microstructural criteria that will allow paleoselsmologists to differentiate shear zones produced by aseismic creep from those formed by coselsmic fault rupture.Digital image acquisition and analysis enable researchers to quantify characteristics ofrock fabrics such as grain dimensions and grain orientations in order to test hypotheses about processes governing the development of rock texture. For example, computer-based image analysis has been used to measure crystal size distribution (e.g. Cashman and Marsh, 1988) and vesicle size distribution (e.g. Cashman et al. 1994) in volcanic rocks with the goal of constraining dynamic rates of crystallization and vesiculation. This approach has also proved useful for measuring particle size distribution, grain shape, and grain orientation in shear zones in unconsolidated sand adjacent to the active McKinleyville fault, California, (Cashman and Cashman, 2000) and New Madrid North fault, Missouri, (Baldwin et al., 2001, in review) with the goal of assessing the relative importance of compaction, grain breakage, and grain rotation in shear zone development. Fault zone sediments from these two stick-slip faults have several textural features in common, including compaction, pronounced grain size reduction, preferred grain orientation, and development of anastomosing micro-scale shear zones. Of particular interest is the high degree of grain breakage in the unconsolidated sand.This study is a microstructural analysis of fault zone sediments from a site on the creeping portion of the San Andreas Fault. Preliminary data indicate significant quantifiable differences in the degree of grain breakage between unconsolidated sand subjected to stick-slip faulting and that defon-ned by ascismic creep. Image analysis of fault zone sediment from a well-documented creeping fault site will provide data on grain breakage and other textural changes produced by creep. These data will be used to evaluate the relative importance of grain breakage, grain rotation, and compaction in sediment defon-ned by stable fault creep, and to determine if specific textural criteria are unique to stick-slip or to creeping faults.