Title: Micro-mechanisms of dynamic faulting at seismic slip rates
Speaker: Dr. Xiaofeng Chen
Research Specialist I, Texas A&M University
Abstract: How fault strength varies during earthquakes is a fundamental process governing rupture propagation. Instrumentation advancement over the last two decades makes it possible to study dynamic faulting at seismic slip rates. Unlike quasi-static rock friction that can be well-described by the Byerlee’s friction law, dynamic rock friction is complicated and involves multiple processes operating at different temporal and spatial scales. Here I present experimental studies of dynamic rock friction using the high-speed rotary apparatus at University of Oklahoma, focusing on the mechanisms at the micro-meter scale that control macro-scale dynamic friction behavior.
At sub-seismic slip rates (1-10 cm/s) granitic faults frequently show low friction coefficients (~0.3) at slip distances of a few meters. We found tiny cylindrical rolls composed of nanoparticulate gouge spontaneously develop along fault surfaces and lead to drastic dynamic weakening. These rolls are perfect examples of powder lubrication as they serve as natural roller bearing systems by transferring sliding friction to rolling friction. When we continue to shear granitic faults over longer slip distances, the powder lubrication mechanism fails, and complex friction evolution patterns emerge. The initial powder lubrication stage transitions to a temporary strengthening stage, and eventually to another weakening stage with prolonged slip. Microstructural evidence shows frictional melt initiates when fault strengthens, and when the molten layer covers the fault surface a weak fault appears again. This complex weakening-strengthening-weakening evolution is controlled by frictional heat and can explain frictional behavior of igneous rocks at seismic slip rates.
The main feature of carbonate faults sheared at seismic slip rates is the development of polished smooth fault surfaces, and the three-body shear zone structure with gouge in between the fault blocks evolves to a two-body structure with shiny fault surfaces sliding against each other. The polished fault surfaces are composed of pavements of nanoparticles of calcite and calcium oxide. Roughness characterization shows extensive smoothing along the slip direction, and micro-scale friction measurements demonstrate that such smoothing process can reduce fault friction, along with other mechanisms such as nano-metric flow, super-plasticity, and grain-boundary flow.