A microstructural study of deformation in currently-inactive fault rocks from the San Andreas Fault Observatory at Depth pertinent to aseismic creep in central California

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Based on historic records, surficial geological evidence, and numerous published studies, approximately 150km (90 mile) of the San Andreas Fault in central California, from Hollister to Parkfield, has been recognized as a creeping segment (segment along an active fault that is unlikely to produce damaging tremors). However, cumulative seismological evidence and impressive advances in monitoring technology have lead some scientists to question if the segment will remain creeping for the foreseeable future. Potential hazards from damaging earthquakes in this section of the fault would directly affect 3.5 million people. Since 2006, the San Andreas Fault Observatory at Depth (SAFOD) has provided material evidence including a large suite of rocks drilled from inside the creeping fault zone down to a vertical depth of about 2.7km (1.6 miles). Direct access to rocks that are being actively sheared off has revealed much about how the fault moves by tectonic forces without causing major tremors. But it is uncertain that the same mechanics apply at depths and higher temperatures below the observatory. This project will use drill core samples from the currently inactive rocks in the observatory for conducting basic-science research into the processes that ultimately determine the fault?s tendency to produce damaging earthquakes along the creeping segment. This expectation is realistic because the inactive rocks of the observatory are believed to hold information about the fault movement at greater depths and higher temperatures. The proposal employs traditional tools of information gathering such as x-ray probes and electron microscopes while applying new and innovative techniques of information analysis and processing (the Geographic Information Science) to accomplish the stated objectives of the project. The use of GIS software also makes it possible to provide the results more readily to the public online through web-based mapping applications and visualizations. The primary objective of the proposed research is to probe the information content of the less studied SAFOD damage zone rocks in order to construct a microstructural-compositional model of the development of aseismic creep in central San Andreas Fault Zone (CSAF). The research will be carried out by combining the traditional methods of data collection including a scanning electron microscope, scanning/transmission electron microscope, electron backscatter diffraction, and x-ray diffraction with non-traditional use of techniques of Geographic Information Science (GIS) to produce results that are easier to visualize and analyze, store, and disseminate. A majority of studies based on the SAFOD samples indicate that shear localization on weak clay mineral phases (coefficient of friction 1-0.01) is responsible for the aseismic creep in the upper sections of CSAF. The current models also point to the presence of serpentinite-derived clay minerals at depths 3km below the SAFOD. However, in view of the presence of deep low velocity zones near SAFOD site, the repeating microearthquakes in CSAF, and evidence from the damage zone gouge bordering the active creep zones, we believe the following questions are worth answering: 1. What cumulative and reactivated deformation processes and patterns define the SAF damage zone and what could we learn about the deformation processes at depths below the SAFOD? 2. What is the spatial extent of possible interactions between damage zone and the actively creeping core of the SAF, and what processes (e.g. deformation mechanisms) drive such interactions? 3. What is the potential for such interactions to affect the aseismic behavior of CSAF? The proposed research provides a material basis for current discussion of whether the creeping section of the San Andreas Fault in central California will remain in a stable aseismic mode as the historical records indicate, or strong seismic events are probable in the future. A directly evidence-based model of aseismic creep in CSAF also contributes to better understanding of similar creeping fault segments in the West Coast plate boundary system. The results of this research are of interest not only to structural geologists, but also serve to provide physical constraints for research in tectonophysics, seismology, and geotechnical engineering. The project contributes to interdisciplinary science by introducing methods of GIS into solid earth and microstructural geology. Graduate and undergraduate students will be trained to use methods and concepts of the two disciplines. Working with analytical equipment and datasets help undergraduate students, in particular, to experience the practice of scientific research and applications of the scientific method.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.