State of lithospheric stress and borehole stability at Deep Sea Drilling Project site 504B, eastern equatorial Pacific

Abstract

Hole 504B in the eastern equatorial Pacific is the deepest hole to penetrate oceanic basement, extending >1500m beneath the seafloor. Two BHTV records have been combined and processed in terms of both acoustic amplitude and travel time in order to evaluate the extent and distribution of rock failure along the borehole wall. A hsitogram of borehole enlargements versus azimuth depicts a dominant breakout azimuth of N122.5°E which corresponds to the direction of minimum principal stress Sh and correlates well with stress orientations inferred from solutions to regional earthquake focal mechanisms. Furthermore, the bimodality of this histogram, with a secondary mode orthogonal to Sh, indicates that a significant number of enlargements are coalesced tensile fractures occurring along the orientation of SH, the maximum principal stress. The appearance of this orthogonal, bimodal distribution suggests that the regional horizonal stress field is highly anisotropic, a condition supported by seismic data which show all of the earthquakes in the area to be strike-slip. -from Authors