It is well established that some segments of the San Andreas Fault reveal frequent earthquakes of small magnitude, occasional earthquakes of moderate magnitude, and aseismic creep. Other segments feature no aseismic creep, but rare fatal ruptures, such as the 1857 and 1906 earthquakes. The high rates of aseismic creep along the San Andreas Fault north of Parkfield are best explained by the low shear strength of abundant hydrothermally altered Coast Range mantle rocks. While serpentine, the dominant mineral of hydrated mantle rock, is mechanically too strong to allow the observed high displacement rates, there is a growing recognition that talc plays a key role in weakening the San Andreas Fault. Yet the mechanisms, conditions and consequences of talc formation in the San Andreas Fault remain controversial. The hypothesis to be tested in this project is that the high creeping rates of up to 28 mm/yr between Cholame Valley and San Juan Bautista are controlled by the reaction of carbon dioxide-rich fluids with serpentine to form mechanically weak talc and magnesite (soapstone). In addition, it is hypothesized that prolonged interaction of soapstone with carbon dioxide-rich fluids causes the formation of mechanically much stronger and velocity-weakening quartz (or opal) and magnesite, promoting micro-earthquakes in a nominally weak serpentinite. To test both hypotheses this project will merge petrographic results with fluid-inclusion analysis and friction experiments to determine: 1) mineral replacement reactions, 2) formation conditions, and 3) geomechanical characteristics of progressively carbon dioxide-altered Coast Range rocks cropping out between Cholame and San Juan Bautista. Finally, three-dimensional time-dependent seismicity data along the SAF north of Parkfield will be analyzed to examine its correlation with mineralogical zoning.
California has endured fatal, high-impact earthquakes, which claimed hundreds of lives and caused billions of dollars of damage; hence there is a critical need to understand how fluid-rock interactions influence the mechanical properties of the San Andreas Fault system. This project explores a new idea that might explain why certain portions of the fault exhibit aseismic creep whereas other portions a characterized by earthquakes. Ultimately, this study of carbonation of serpentinite will promote a deeper understanding of the seismicity and earthquake deformation cycles in one of the most densely populated regions of the United States.
