HR: 11:50h
AN: S41G-06    [PDF]
TI: Slip Surfaces in Fault Breccia From the Sierra Madre Fault Zone: Geometry and Mechanical Implications
AU: Mordohai, P
EM: mordohai@aludra.usc.edu
AF: University of Southern California, Department of Electrical Engineering, PHE 204 MC-0273, Los Angeles, CA 90089-0273 United States
AU: * Dor, O
EM: dor@usc.edu
AF: University of Southern California, Department of Earth Sciences, Science Hall 117, Los Angeles, CA 90089-0740 United States
AU: Zechar, J
EM: zechar@aludra.usc.edu
AF: University of Southern California, Department of Earth Sciences, Science Hall 117, Los Angeles, CA 90089-0740 United States
AU: Sammis, C G
EM: sammis@usc.edu
AF: University of Southern California, Department of Earth Sciences, Science Hall 117, Los Angeles, CA 90089-0740 United States
AU: Ben-Zion, Y
EM: ybz@usc.edu
AF: University of Southern California, Department of Earth Sciences, Science Hall 117, Los Angeles, CA 90089-0740 United States
AB: Fault breccia from the Sierra Madre Fault Zone is imaged using a new technique, resolving a highly detailed structure of internal slip surfaces with characteristic distribution of orientations and senses of slip. We excavated a 25x28x35 cm3 sample of brecciated granite from the hanging wall of the Sierra Madre fault near JPL, La Canada, California. The base of the sample was attached to the principal slip surface, dipping 40$\deg$ to the NW, showing up-dip slip striations. The sample was carefully removed with known orientation while maintaining its integrity. We used a two camera digital imaging system developed in the Department of Electrical Engineering at USC. With the positions of the cameras and sample fixed, we progressively removed the outer layers of the specimen exposing the internal slip surfaces. Each slip plane was marked with a spot; a line was marked if slicknsides were observed, and a stereo photo taken. These 3D images were analyzed to give the normal to each slip plane and the tangent to each slip line. The slip surfaces are few millimeters to several centimeters wide. They branch and coalesce to create a dense micro- and meso-scale network. The polygons bounded by these surfaces are composed of powdered granite that maintains its original fabric. The surfaces are shiny, present clear slicknsides and have silky touch. Larger slip surfaces show macroscopic curvature producing roughness at various wavelengths. This curvature is comprised of the smaller slip planes that were marked for the imaging. We hypothesize that initial fracturing process created the small-scale planes which then coalescenced by rotation and abrasive wear to create the large surfaces. These large slip surfaces are generally sub-parallel to the fault strike and sub-perpendicular to its slip vector. Analysis of 196 planes indicates that the dip direction of 123 planes (63%) is between the east and the south, nearly opposite to the fault. Another 59 planes are dipping in the NE and SW quadrants, 43 of them trend to the ENE and to the SSW. Only 14 planes (7%) trend NW, sub-parallel to the fault. The distribution of inclinations covers the entire range, except for the planes that trend with the fault. Of these, 9 are sub-vertical and 3 are sub-horizontal. The slicknsides were imaged as lines in space but the senses of motion have not yet been determined. They are low-angle 3D lines with no consistent trend indicating mostly horizontal motion. The mechanical origin of this set of surfaces needs a further verification. While they are not consistent with Riedel shear geometry, the characteristics of the surfaces show that they accommodate a small amount of rotation that may reflect simple shear. This requires a relatively homogeneous stress field. As this is not necessarily the case, they might be of dynamic origin, reflecting the heterogeneity of a process-zone stress field ahead of the earthquake rupture. Finally, the dynamic reduction of normal stress during the passage of a rupture can control the formation of this geometry, but requires that the slicknsides (to be verified) will demonstrate a tensile regime.
DE: 7209 Earthquake dynamics and mechanics
DE: 8010 Fractures and faults
DE: 8020 Mechanics
DE: 8025 Mesoscopic fabrics
DE: 8030 Microstructures
SC: Seismology [S]
MN: 2003 Fall Meeting