HR: 0800h
AN: T21A-0439    [Abstracts]
TI: Spectral Analysis of Localized Stress Variations, the Spatial Distribution of Faults, and the Scaling of Physical Properties near the San Andreas Fault
AU: * Day-Lewis, A
EM: adaylewis@stanford.edu
AF: Stanford University, Mitchell Building, Room 360, 397 Panama Mall, Stanford, CA 94305-2215 United States
AU: Zoback, M D
EM: zoback@pangea.stanford.edu
AF: Stanford University, Mitchell Building, Room 360, 397 Panama Mall, Stanford, CA 94305-2215 United States
AU: Hickman, S H
EM: hickman@usgs.gov
AF: United States Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, CA 94025 United States
AB: Statistical characterization of stress-induced wellbore failures and rock property heterogeneity from well logs offers potential insight into the scaling properties and mechanisms of stress heterogeneity. Wellbore breakouts identified in acoustic wellbore image data obtained adjacent to the San Andreas Fault, from both the San Andreas Fault Observatory at Depth (SAFOD) and the Cajon Pass Scientific Borehole, reveal multi-scale rotations in the direction of maximum horizontal compressive stress (SHmax) as a function of depth. Similar breakout rotations are frequently observed in other deep wellbores and, in most cases, reflect small variations in the directions and/or magnitudes of the in situ principal stresses superimposed on a relatively uniform regional stress state. To determine possible physical causes for these rotations, we employ spectral and statistical methods to investigate the relationships between the breakout rotations observed in our study wells and stress drops associated with slip on faults in highly fractured crust adjacent to a major fault zone. We also address the possible role of rock property variability as a controlling mechanism, taking into account drilling and data acquisition artifacts. We find that physical property heterogeneity in the SAFOD Pilot Hole behaves as self-similar, flicker noise (i.e., 1/f) over wavelengths from one meter to one kilometer, a result that agrees with similar investigations at Cajon Pass and a variety of other locations throughout the world. The stress orientations in both wells, however, exhibit behavior between that of flicker noise and Brownian motion over wavelengths from one decimeter to several kilometers, which is similar to how earthquake frequency has been shown to scale with fault size. The fractal scaling of observed stress heterogeneity appears to be more closely related to the distribution of faults in the crust adjacent to the study wells than to heterogeneity of elastic or other in-situ physical properties. In addition, we are able to model specific breakout rotations at SAFOD as resulting from stress perturbations created by slip on existing fractures in response to the current stress field. This result further supports our hypothesis that stress heterogeneity near the San Andreas Fault is controlled by slip on secondary, active faults at a variety of scales.
DE: 3255 Spectral analysis (3205, 3280)
DE: 4440 Fractals and multifractals
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8150 Plate boundary: general (3040)
DE: 8164 Stresses: crust and lithosphere
SC: Tectonophysics [T]
MN: Fall Meeting 2005