We are working to understand the physical properties of large-scale tectonic faults in Earth's crust, with particular focus on the San Andreas Fault in California. As part of the San Andreas Fault Observatory at Depth (SAFOD) drilling recently penetrated the fault at a depth of ~ 2.5 km. We are studying core recovered during this drilling (SAFOD phase III) along with synthetic fault gouge, samples collected at the surface in the vicinity of the SAFOD borehole, and cuttings from drilling in and around the fault. Only a small amount of core was recovered from the active strand of the fault; therefore, we are using the other materials to expand our data set and investigate physical processes within the fault. Our work is focused on: 1) detailed measurements of frictional properties and permeability of the samples, 2) development of process-based models for controls on fault strength and stability, 3) development of innovative methods to study the role of fault zone fabric on frictional stability and physical properties, and 4) integration of laboratory results with models for fluid flow and heat transport, which will allow critically evaluation of hypotheses for apparent fault weakness. Outstanding questions that we are addressing include: 1) What causes spatial variability in fault slip behavior and seismicity? 2) Are elevated fluid pressures within the SAFOD 3-D volume plausible? 3) How are geophysical observations such as low velocity or resistivity linked to in situ conditions of stress and fluid pressure? 4) Does the fault zone act as a barrier for regional and local fluid flow? 4) Does significant fault healing occur in materials comprising the active SAF, and if so, what is the acoustic (seismic) signature, if any? 5) How are fault zone frictional, elastic, and transport properties linked with those of the surrounding protolith (in other words, how does protolith composition influence fault zone properties)?
Our work complements studies by other groups working on SAFOD samples as well as a substantial body of work at Penn State focused on other fault zone drilling projects, such as NanTroSEIZE. Results of the proposed experiments will have societal impact through improved understanding of fault mechanics and earthquake physics, applied to the best-studied and best-instrumented plate boundary fault on Earth.
We investigated large-scale tectonic faults in Earth's crust with particular focus on the San Andreas Fault in California. Our work involved making careful measurements of physical properties of fault materials, including frictional strength and fluid flow properties. We used rock samples from the San Andreas Fault recovered in drilling, as part of the San Andreas Fault Observatory at Depth (SAFOD) project, to depths of ~ 2.5 km. Our work focused on: 1) detailed measurements of frictional properties and fluid permeability of the samples, 2) development of process-based models for controls on fault strength and stability, 3) development of innovative methods to study the role of fault zone fabric on frictional stability and physical properties, and 4) integration of laboratory results with models for fluid flow and heat transport, which allow critically evaluation of hypotheses for the apparent fault weakness of the San Andreas Fault. We find that material from the actively creeping faults penetrated in the SAFOD borehole are weak, with frictional strengths µ of ~ 0.1. They also exhibit frictional properties that are inherently stable, which suggests that the fault in this area should slip slowly and without large earthquakes. In contrast, samples of wall rock near the active faults have much higher frictional strength and exhibit the frictional behavior that can cause stick-slip friction such as in earthquakes. Results of the proposed experiments will improve understanding of fault mechanics and seismic hazard.
