ABSTRACT

: Recent field studies conducted in a valley setting have provided direct evidence of an exchange with depth in the orientations of the two horizontal principal stresses, SHMAX and SHMIN. This observed 90-degree rotation with depth of SHMAX and SHMIN beneath a valley is predicted by a previously developed analytical model for the effect of topography on near-surface tectonic and gravitational stress fields in idealized, isolated, symmetric valleys. These new modeling results demonstrate that valley topography may cause significant local perturbation of regional stress fields; effects that should be considered whenever regional stress orientations are inferred from near-surface hydraulic fracturing experiments and/or from well-bore breakout data.

1 INTRODUCTION

Recent field studies conducted in a valley setting provide direct evidence of an exchange with depth in the orientations of the two horizontal principal stresses, SHMAX and SHMIN (Morin, et al., 1999, Savage and Morin, in preparation, Morin and Savage, in preparation). Analytical models for the effects of idealized, isolated, symmetric, long ridges and valleys on near-surface tectonic and gravitational stress fields developed by Savage, et al. (1985) and Savage and Swolfs (1986) have been applied to a variety of investigations on the effects of topography on regional stress fields (Swolfs, et al. 1988, Liu and Zoback, 1992, and Miller and Dunne, 1996). In this paper we show that the analytical models predict 90-degree rotations of principal stresses with depth below valleys subject to gravity and far-field tectonic stresses.

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