Shear wave velocity, especially when used with Compressional wave velocity, is an important constraint on estimates of the composition and physical state of the continental lithosphere. We present shear velocity (νs) models derived from high‐quality S wave data along three profiles in Southwest Germany, which combine with existing P wave velocity (νp) models to give Poisson's ratio (σ) in the crust and uppermost mantle. We then construct a laterally varying petrological model of the crust by comparing our νp and νs models with published laboratory‐measured rock velocities. In general, Poisson's ratio is 0.25 in the uppermost crust, 0.22–0.23 in the mid‐crust (but reaching a low of 0.15 in the Black Forest low‐νp zone), and varies laterally from 0.24 to 0.30 in the lower crust; the average Poisson's ratio of the whole crust is everywhere about 0.25. The laminated signature of the lower crust prominent on reflection and P wave refraction data is absent on the S wave refraction data, implying a strong high/low alternation of Poisson's ratio in the lower crust. This suggests that the lower‐crustal laminae represent mafic/silicic compositional layering (for example, basic sills in an originally more quartz‐rich lower crust) rather than alternating zones of high and low pore pressure. There is no evidence for a shear wave low‐velocity zone (LVZ) in the Black Forest mid‐crust, despite the presence of a strong P wave LVZ; this suggests high quartz content and the presence of pore fluids at low (not high) pore pressure. A clear P‐to‐S converted Moho reflection (Pm S) is visible on several shots, indicating that the Moho is, at least locally, a first‐order discontinuity. No shear wave refractions from the upper mantle are present, even where strong P wave refractions are observed, possibly indicating an increase of Poisson's ratio with depth in the upper mantle. The petrological modeling indicates that the upper and middle crust are probably of granitic composition, that the mid‐crustal low‐νp zones have high quartz content, and that the lower crust probably consists of granulites of laterally varying bulk composition, from granodioritic to gabbroic. The lower crust is more mafic in areas of high reflection density on seismic reflection profiles, suggesting that the laminations are caused by mafic intrusions into the lower crust, either prior to the Variscan orogeny or during (Tertiary?) volcanic activity.

Analysis of borehole televiewer and Formation MicroScanner data from Ocean Drilling Program (ODP) Holes 829A and 83 IB, located in the Vanuatu subduction zone, reveals that both boreholes are elongated. However, borehole televiewer data from neither borehole show the classical form of borehole breakouts (enlargements on opposite sides in an otherwise circular wellbore); rather, the data indicate smooth, varying borehole surfaces. The orientation of the borehole elongation in Hole 829A (123° ± 16°) parallels topography and structural lineaments. In Hole 83 IB the orientation of elongation (148° ± 19°) is roughly perpendicular to the direction of plate convergence (76°).Focal mechanism solutions show that in the area of investigation the overriding plate is characterized by an east-northeastwest-southwest directed compressional stress regime. This indicates, that the orientation of the minimum horizontal stress direction, where compressive shear failure occurs, is parallel to the strike of the Australia/Pacific Plate boundary. Calculation of finite element modelling also show that high compressive stresses are assumed in accretionary wedges (Bott et al., 1989).The correlation of the orientation of the elongations in both wellbores with the local and regional stress field implies that the orientation of borehole elongation results from the present day stress field. Orientation of borehole elongation of Hole 83 IB strikes parallel to the Australia/Pacific Plate boundary indicating that it is consistent with the stress field deduced from earthquake focal mechanisms. However, orientation of borehole elongation of Hole 829A is affected by the local rather than the regional stress field. Observed local tectonics and topography superimpose the regional stress field at this drill site and lead to a 43° northward rotation of the maximum horizontal stress field.Additionally, the existence of a smooth borehole surface rather than borehole breakouts implies that despite the assumed high compressive stresses, additional processes, for example decoupling along thrust faults, must exist to reduce these stresses.

Hole 504B, about 200 km south of the Costa Rica Rift in the eastern equatorial Pacific, has been drilled over a succession of Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) legs to a total depth of 2000.4 meters below seafloor (mbsf). Borehole televiewer (BHTV) measurements were recorded over the intervals 275-497 mbsf on Leg 137, and 1497-1715 mbsf and 1868-1990 mbsf on Leg 140. Detailed investigation of the BHTV data revealed stress-induced wellbore breakouts in all depth intervals with variable occurrence. The statistical analysis of the wellbore breakouts shows that the orientation of the maximum horizontal stress (S Hmax ) varies from N53°E ± 28° in the interval 410-490 mbsf, to N21 °E ± 35° in the interval 1497-1715 mbsf, and N82°E ± 38 in the interval 1868-1990 mbsf. Whereas the intermediate depth interval shows a mean azimuth that is consistent with previous observations from borehole breakout analysis between 700 and 1500 mbsf, the upperand lowermost breakout intervals are rotated clockwise by 22° and 61°, respectively. As none of the observed intersecting structures correlates with the observed rotation ofS Hmwc , it is assumed that the rotation is affected by non-intersecting structures or structures beyond the wellbore head. The onset of the breakout interval from 410 to 490 mbsf correlates well with the Layer 2A/2B boundary and corresponds to a seismic anisotropy observed in the upper 500 m of the crust. For breakouts to occur in shallow depths, high S Hma jS hmin ratios are required or rock strengths must locally vary (to values

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