HR: 16:30h
AN: T44B-03    [Abstracts]
TI: Geometrical and Structural Asperities on Fault Surfaces
AU: * Sagy, A
EM: asagy@pmc.ucsc.edu
AF: University of California Santa Cruz, 1156 High St., Santa Cruz, Ca 95064, United States
AU: Brodsky, E E
EM: brodsky@pmc.ucsc.edu
AF: University of California Santa Cruz, 1156 High St., Santa Cruz, Ca 95064, United States
AU: van der Elst, N
EM: nvanderelst@pmc.ucsc.edu
AF: University of California Santa Cruz, 1156 High St., Santa Cruz, Ca 95064, United States
AU: Agosta, F
EM: fabrizio.agosta@unicam.it
AF: Camerino University, Via Gentile 3 Varano, Camerino, 62032, Italy
AU: Di Toro, G
EM: giulio.ditoro@unipd.it
AF: Universita degli Studi di Perugia, Piazza dell'Universita 1, Perugia, 06100,
AU: Collettini, C
EM: colle@unipg.it
AF: Universita' di Padova, Via Giotto 1, Padova, 35137,
AB: Earthquake dynamics are strongly affected by fault zone structure and geometry. Fault surface irregularities and the nearby structure control the rupture nucleation and propagation, the fault strength, the near-field stress orientations and the hydraulic properties. New field observations demonstrate the existence of asperities in faults as displayed by topographical bumps on the fault surface and hardening of the internal structure near them. Ground-based LIDAR measurements on more than 30 normal and strike slip faults in different lithologies demonstrate that faults are not planar surfaces and roughness is strongly dependent on fault displacement. In addition to the well-understood roughness exemplified by abrasive striations and fracture segmentation, we found semi-elliptical topographical bumps with wavelengths of a few meters. In many faults the bumps are not spread equally on the surface and zones can be bumpier than others. The bumps are most easily identified on faults with total displacement of dozens to hundreds of meters. Smaller scale roughness on these faults is smoothed by abrasive processes. A key site in southern Oregon shows that the topographic bumps are closely tied to the internal structure of the fault zone. At this location, we combine LiDAR data with detailed structural analysis of the fault zone embedded in volcanic rocks. Here the bumps correlate with an abrupt change in the width of the cohesive cataclasite layer that is exposed under a thin ultracataclasite zone. In most of the exposures the cohesive layer thickness is 10-20 cm. However, under protruding bumps the layer is always thickened and the width can locally exceed one meter. Field and microscopic analyses show that the layer contains grains with dimensions ranging from less than 10 μ up to a few centimeters. There is clear evidence of internal flow, rotation and fracturing of the grains in the layer. X-Ray diffraction measurements of samples from the layer show that the bulk mineralogy is identical to that of the host rock, although thin section analysis suggests that some alteration and secondary mineralization of the grains also occurs. We infer that the cohesiveness of the layer is a consequence of repacking and cementation similar to deformation bands in granular material. By comparing the thickness of the cohesive layer on several secondary faults in this fault area we found that the average thickness of the layer increases with total slip. The correlation is nonlinear and the thickening rate decreases with increasing slip. We conclude that granular flow decreasing with increasing slip and thus the deformation is continually localized.
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8010 Fractures and faults
DE: 8030 Microstructures
DE: 8034 Rheology and friction of fault zones (8163)
DE: 8094 Instruments and techniques
SC: Tectonophysics [T]
MN: 2007 Fall Meeting