Researchers at Texas A&M University are using structural observations of active and exhumed faults of the San Andreas system to provide new constraints on the earthquake energy budget. In particular, work focuses on characterizing the physical processes of earthquake slip, and quantifying the specific fracture energy of rupture, which is defined as the energy consumed by the formation of the rupture surface and loss of frictional strength. Constraining the specific fracture energy is important to modeling the dynamics of earthquake rupture propagation. The research involves detailed field mapping, sampling, and optical and electron microscopy and microprobe study of several key surface exposures of the active San Andreas fault and exhumed faults of the San Andreas system to quantify particle size distributions and fracture surface area within the faults, particularly focusing on the nanoscale particles within the fault zones. The mechanisms of particle size reduction, healing and particle growth in seismic faults are being determined. The results contribute to developing synoptic models that describe the 3-D structure of seismogenic fault zones and attempt to relate structural/petrologic characteristics to the mechanical parameters important for earthquake rupture nucleation, propagation, and arrest. The project is contributing to broad research programs and initiatives of national interest, such as National Earthquake Hazard Reduction Program, the Southern California Earthquake Center, and the San Andreas Fault Observatory at Depth. The scientific benefits of this work are maximized through coordination with other researchers in the broader scientific community. The research provides education-through-research opportunities for both undergraduate and graduate students, and addresses the fundamental physics of earthquakes, which ultimately may contribute to reduction of earthquake hazards and improved mitigation.