Laboratory Study of Fault Healing and Frictional Properties: Role of Fluids

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).This project will investigate slip on tectonic faults and important processes that occur during earthquakes. We will focus on the frictional strength of faults and the role of underground fluids in modifying fault strength. Damaging earthquakes occur on major tectonic faults in a sequence that is often separated by 50 to 100 years or more. One important question for seismic hazard analysis is that of how fault strength is regained between earthquakes ?so called fault healing. This project will investigate fault healing via a combination of detailed laboratory experiments coupled with computational modeling and microscope-based studies of the fault zone textures. Faults in Earth's crust undergo a range of slip behaviors including earthquakes and non-damaging 'creep' events in which slip occurs without releasing damaging seismic waves. One of the goals of this project is to determine the microscopic and larger-scale factors that cause each type of slip behavior. This information will help us build more realistic models for seismic hazard around the country. Results of the project are expected to have significant impact on understanding faults and earthquakes including triggering of seismic and aseismic fault slip, fault interaction, and seismic hazard assessment. Fault healing plays a central role in earthquake rupture processes at time scales ranging from tectonic to elastodynamic. Frictional healing (as evidenced by increasing static friction during quasi-stationary contact) is considered the most likely mechanism of interseismic and dynamic fault strengthening, and there is good agreement between laboratory-based friction laws and field observations of fault healing in some cases. However, laboratory data are limited in quantity and scope. Existing lab data do not provide a consistent explanation of fault healing as observed via repeating earthquakes, which indicate both increases and decreases in seismic moment as a function of time between successive events. Moreover, the physical processes of fault healing and, more generally, the micro-mechanisms of frictional rate/state effects are poorly understood. This project will support a multidisciplinary investigation of fault healing. The work includes two general tasks. 1) Laboratory study of frictional healing and fault zone transport properties for a range of conditions (shearing rate, gouge material, normal stress, fluid properties, temperature). Experiments will be conducted under true-triaxial stress conditions using the double-direct shear configuration with controlled pore fluid pressure and flow through. We will measure healing via frictional strength, elastic properties of the fault zone, and hydraulic transmissivity during shear. Detailed microstructural studies of the deformed samples will be used to identify processes responsible for healing. 2) Coupled numerical, laboratory, and microstructural studies aimed at identifying the physico-chemical processes that determine fault healing, creep consolidation, and time-dependent fault weakening. Preliminary data are available in each area of proposed study