Three-Dimensional Characterization of a Pseudotachylyte-Bearing Fault

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Fault slip is the dominant mode of deformation in the earth's upper crust. Faults within the crust, however, exhibit a range of behavior, from earthquake-generating stick-slip to aseismic creep. Laboratory-derived friction laws can explain this spectrum of fault behavior; however, the scaling of these laws to the conditions of natural earthquakes and the interpretation of the laws' physical meanings have proven to be formidable challenges. This research will bridge laboratory and earthquake scales by imaging the three dimensional geometry of pseudotachylyte-bearing faults. Pseudotachylytes, fault rocks formed from frictional melts, are generally considered the only unequivocal evidence ofearthquake slip velocities preserved in fault zones. The solidification of frictional melt "freezes in" earthquake source geometries; however, it also precludes the development of extensive fault surface exposures that have enabled previous studies of fault zone geometry. We have overcome this difficulty by imaging the intact 3D geometry of the fault using high-resolution X-ray computed tomography (CT). In this study we will use CT imagery of natural and experimental fault surfaces to quantify surface roughness, frictional contact area, and pseudotachylyte thickness and will thus determine the evolution of fault roughness with slip and estimate key frictional parameters at earthquake nucleation and propagation scales. The proposed project will improve the understanding of earthquakes and their effects, one of the primary goals of the National Earthquake Hazard Reduction Program. Furthermore, the project will establish a new collaboration between Wesleyan University, a primarily undergraduate institution in the US, and scientists from the Istituto Nazionale di Geofisica e Vulcanologia (INGV), a world-renowned research institute in Italy. Undergraduate students will play an integral role in the project, planting the seeds for future international and interdisciplinary research into the processes of brittle deformation in the earth's crust. These students will experience the full scope of the scientific process, from hypothesis generation, to study design, to presentation of results at professional meetings and in a written thesis. The project is being supported by the EAR Tectonics Program and the Office of International Science and Engineering's Global Venture Fund.