Strike Standard Deviation for Shallow-dipping Rock Fracture Sets

Summary The well-known problem that the observed strike of a shallow-dipping rock fracture can vary widely, even if one re-measures the strike in the same location, is dealt with in this paper. The overall purpose of the paper is to place the statistical estimation of the strike confidence interval for shallow-dipping rock fracture sets on a sound foundation. We have made series of strike measurements at each of various dips on a wooden laboratory “fracture” to allow estimation of strike standard deviation at all shallow dips. This is accurate for smooth rock and serves as a lower bound estimate for rock with a rougher surface. These estimates are used, along with two independent iterative statistical procedures, to re-estimate the dip direction confidence interval of an actual set of nineteen fractures. The dip direction confidence interval is more than three times as wide as the corresponding dip confidence interval. This will be made visible in our transformed confidence region plot for the mean pole.