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Poster Abstracts
and the Christoffel equation. This allows prediction of the expected lower crustal contribution to seismic anisotropy and
improves our ability to interpret crustal anisotropy in terms of composition as well as deformation mechanisms. In the Mojave
Desert region of southern California the crust is under-plated by the Pelona Schist, which due to its high mica content is
expected to have significant seismic anisotropy. We measured mineral CPOs in 10 samples of the Pelona Schist using electron
backscatter diffraction (EBSD) and then 148c 148 a 148 l 148c 148 u 148l148 a 148 t 148e 148d 148 148t 148h148e 148i 148r 148
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ACCURACY OF NON-DESTRUCTIVE TESTING OF PBRS TO ESTIMATE FRAGILITIES (B-080)
J.N. Brune, R. Brune, G.P. Biasi, R. Anooshehpoor, and M.D. Purvance
Prior studies of Precariously Balanced Rocks (PBRs) have involved various methods of estimating fragilities. These have
included non-destructive testing (NDT) methods such as photomodeling, and potentially destructive testing (PDT) such as
forced tilt tests. PDT methods have the potential of damaging or disturbing the rock or its pedestal so that the future
usefulness of the PBR is compromised. To date we have force-tilt tested approximately 28 PBRs, and of these we believe 7
have been compromised. We suggest here that NDT methods are now sufficiently advanced as to be adequate for comparison
with Ground Motion Prediction Equations (GMPEs) and seismic hazard maps (SHMs). We compare tilt-test static toppling
estimates to three non-destructive methods: (1) 3-D photographic modeling (2) profile analysis assuming the rock is 2-D, and
(3) expert judgments from photographs.
3-D modeling uses the commercial Photomodeler program and photographs in the field taken from numerous directions
around the rock. The output polyhedral shape is analyzed in Matlab determine the center of mass and in Autocad to estimate
the static overturning angle alpha.
For the 2-D method we chose the photograph in profile looking perpendicular to the estimated direction of toppling. The rock
is outlined as a 2-D object in Matlab. Rock dimensions, rocking points, and a vertical reference are supplied by the photo
analyst to estimate the center of gravity and static force overturning angles. For the expert opinion method we used additional
photographs taken from different directions to improve the estimates of the center of mass and the rocking points.
We used 7 rocks for comparisons. The error in estimating tan alpha from 3-D modeling, 2-D estimates, and expert opinion are
about 0.05, 0.1, and 0.06, respectively. Tilt-testing differs materially in one case from 3-D (*) because an irregular base on the
rock allowed the rock to begin to tilt at a lower angle onto a second rocking point with higher alpha. 2-D methods generally
perform well enough to use as a screening method for the larger archive. The following is a list of the tan alpha data:
Method: Tilt, 3-D, 2-D, Expert
Grass Valley 2: .31*, .50, .50, .47
Lovejoy Buttes 5(NE): .33, .34, .20, .31
Mead Valley West: .53, .53, .55, .53
Pacifico: .22, .18, .29, .27
148 | Southern California Earthquake Center