Sheila Peacock

Abnormal fluid pressures and fault-zone dilation in the Barbados accretionary prism: Evidence from logging while drilling

Logs collected while drilling measured density in situ, through the accretionary prism and decollement zone of the northern Barbados Ridge. Consolidation tests relate void ratio (derived from density) to effective stress and predict a fluid pressure profile, assuming that the upper 100 m of the prism is at a hydrostatic pressure gradient. The calculated fluid pressure curve rises to >90% of lithostatic below thrusts in the prism, presumably due to the increase in overburden and lateral tectonic loading. Thin (0.5–2.0 m) intervals of anomalously low density and resistivity in the logs through the basal decollement zone suggest dilation and perhaps hydrofracturing. A peak in hydraulic head in the upper half of the decollement zone requires lateral influx of fluid, a conclusion consistent with previous geochemical studies. Although the calculated fluid-pressure profile is model dependent, its inherent character ties to major structural features.

Consolidation patterns during initiation and evolution of a plate-boundary decollement zone: Northern Barbados accretionary prism

Borehole logs from the northern Barbados accretionary prism show that the plate-boundary decollement initiates in a low-density radiolarian claystone. With continued thrusting, the decollement zone consolidates, but in a patchy manner. The logs calibrate a three-dimensional seismic reflection image of the decollement zone and indicate which portions are of low density and enriched in fluid, and which portions have consolidated. The seismic image demonstrates that an underconsolidated patch of the decollement zone connects to a fluid-rich conduit extending down the decollement surface. Fluid migration up this conduit probably supports the open pore structure in the underconsolidated patch.

Stress-forecasting (not predicting) earthquakes: A paradigm shift?

After 120 years of unsuccessful endeavor, a paradigm shift is required before earthquakes can be predicted. The most sensitive diagnostic of low-level changes of stress in in situ rock, variations in microcrack geometry, can be monitored by analyzing shear-wave splitting. The suggested paradigm shift is that, instead of investigating the source zone, we monitor stress accumulation before earthquakes at, possibly, substantial distances from the source. Characteristic temporal variations of shear-wave time delays have been observed in retrospect before 14 earthquakes worldwide. On one occasion, when changes were recognized early enough, the time, magnitude, and fault break of an M = 5 earthquake in southwest Iceland were successfully stress-forecast in a narrow time-magnitude window. Such stress accumulation can be theoretically modeled and is believed to be at least partially understood. When sufficient shear-wave source earthquakes are available, increasing time delays also show an abrupt decrease shortly before the impending earthquake occurs. This is not fully understood but is thought to be caused by stress relaxation as microcracks coalesce onto the eventual fault break. The new result confirming these ideas, and justifying the paradigm shift, is that logarithms of the durations of both increases and decreases in time delays are found to be proportional (self-similar) to the magnitudes of impending earthquakes.

Shear-wave vibrator signals in transversely isotropic shale

The experiments of Robertson and Corrigan (1983) on shale are among the first three‐component field observations of shear waves in transversely isotropic media to be published. Their data are reprocessed to highlight the effects of the shale’s anisotropy on shear waves. Two results emerge. First, shear‐wave splitting in a transversely isotropic substrate is most easily observed when the vibrator baseplate is oriented so that both SH‐ and SV‐waves reach the geophone. Second, the SV‐wave polarization deviates significantly from perpendicular to the raypath. Both results may significantly affect the interpretation. Both are found to agree with theoretical results and are modeled successfully by synthetic seismograms.

Shear wave velocities and anisotropy in the Barbados accretionary complex

S wave velocities at the base of the Barbados accretionary wedge were determined from mode-converted S waves in two vertical seismic profiles in Ocean Drilling Program boreholes. At Hole 949C the profile extends from 92 to 397 m below seafloor (mbsf), ending ∼17 m into the decollement zone separating the wedge from the underthrust sediment on the subducting plate beneath. At Hole 948D the profile extends from 105 to 471 mbsf, ending ∼19 m above the decollement zone. S wave velocities are 303–549 m/s at Hole 949C and 453–679 m/s at Hole 948D. The profiles show that the lowest 70 m of the accreted sediment, just above the decollement, has lower S wave velocity (379±26 m/s at Hole 949C and 453±11 m/s at Hole 948D) and higher ratio of P wave to S wave velocity (Vp/Vs) (4.59±0.31 at Hole 949C and 3.99±0.10 at Hole 948D) than in the overlying layer. This coincides with high clay content that has a high proportion of smectite. Shear wave splitting observed in records from the lowest 100 m of the accretionary wedge at both sites indicates little or no anisotropy in the low-velocity zone. A single measurement at Hole 949C of the horizontal component of polarization of the leading split S wave from an ocean bottom shot is N68°E±20°. At Hole 949C, split shear wave delays in the downward propagating S wave originating at a fault at 275 mbsf indicate strong anisotropy in the range 275–306 mbsf and little or no anisotropy in the range 306–380 mbsf. The rate of increase of delay between 275 and 306 mbsf, 0.52±0.16 ms/m, can be modeled by either a Hudson cracked medium with crack density 0.186±0.054 or (preferably) a Hornby model of shale with 44% of clay particles aligned within 5° of the symmetry plane. This strong anisotropy is attributed to “scaly fabric,” alignment of clay particles by pervasive shear in the fault zone, which is observed in core. The low anisotropy in the remainder of the section shows that there is very little overall alignment of pore space in the lower part of the accreted sediments, supporting the hypothesis that the sediments are undercompacted rather than hydrofractured. The low S wave velocities and high Vp/Vs ratios indicate undercompaction and low effective stress despite the tectonic burial of the lowermost accreted sediments.

Seismic evidence for fluid-driven deformation

Abstract The behaviour of seismic shear waves shows that fluid–rock interactions control both low-level deformation of the intact rock mass before fracturing takes place, and the fracturing or faulting process itself in deep in situ rock. The splitting (birefringence) of shear-waves directly indicates that the pre-fracturing deformation of intact unfractured rock is the result of fluid movement along pressure gradients between adjacent microcracks at different orientations to the stress field. This is the mechanism for low-level pre-fracturing deformation of almost all in situ rocks. Further seismic evidence shows that fracturing and faulting at depth only occur when pore-fluid pressures on seismically active fault planes are sufficiently high to relieve frictional forces and allow asperities to be overcome. This is comparatively direct evidence that fluids control low-level small-scale (pre-fracturing) deformation of the intact rockmass, and that most if not all fracturing only occurs when fluid pressures on the fault plane are critically high.

Satellites that see the Earth move

Sheila Peacock summarizes the British Geophysical Associations annual Advances in Geophysics meeting, which took place on 10 and 11 February 2005, on the theme of “The Impact of Satellite Measurements on the Observation and Modelling of Continental Deformation”.