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Recent geological and geophysical exploration of several accretionary complexes reveals that pore pressures inside them are much higher than the hydrostatic values. Indirect inferences further suggest that such high pore pressures may be present inside most accretionary complexes. In this study the mechanisms for generating excess pore pressures is investigated by focusing on the Barbados Ridge Complex which is an end-member of sediment-rich accretionary prisms. Modeling is carried out using a two-dimensional finite element procedure, with constraints provided by the following data: Deep Sea Drilling Project results on stratigraphy and lithology, seismic results on sediment structure and thickness, regional geology on tectonic history, and laboratory measurements on mechanical properties of sediments. Coupling among the equations for fluid flow, heat transfer, and sediment deformation is made throughout the computation. The results of the modeling predict that the principal decollement beneath the Barbados Ridge Complex is a zone of high pore pressure, high porosity, and reversal in seismic velocity. These characteristics may be responsible for the mechanical weakness of the decollement, which allows continuous subduction of sediments below it, and for its seismic signatures. Results of the modeling further suggest that tectonic overburden may be the most important mechanism for the generation of high pore pressures inside accretionary prisms. The direct effect of tectonic compression is of secondary importance, and the effect due to temperature increase with subduction is of little significance. Thrust faulting near the toe of accretionary prisms can disturb pore pressure patterns and produce localized highs. Rich sediment supply, low permeability, high subduction angle, and high subduction velocity are all favorable factors in creating high pore pressures inside accretionary prisms. "subduction channels" are essentially lithostatic. Only a few direct measurements of high pore pressures within active subduction zones have been reported (Barbados, Middle American Trench off Guatemala), probably due to the difficulty of drilling into such complexes. Indirect estimates of the magnitude of pore pressure in subduction prisms have been made [Davis et al., 1983] Paper number 7B5021. 0148-0227/88/007B-5021 $05.00 for several accretionary prisms; pore pressures in these complexes are found to be close to the lithostatic values. The Barbados Ridge Complex provides a good opportunity for the study of pore pressure in an accretionary prism, since abundant geological and geophysical data have been accumulated in this region. The distribution of mud volcanoes [ Westbrook and Smith, 1983] indicates the existence of high pore pressure [Gretener, 1976]. Studies of the overall shape of the accretionary prisms also add support to the hypothesis that the average pore pressure may be close to the lithostatic load [Davis et al., 1983]. Most importantly, Deep Sea Drilling Project (DSDP) leg 78A near the Barbados Ridge Complex [Moore e...

Estimated b values in log N = a − bM are widely used in seismicity comparisons and risk analysis, but uncertainties have been little explored. In this paper, the usual F probability density distribution for b is given and compared with an asymptotic form for temporally varying b. Convenient tables for the standard error of b are given that allow statistical tests to accompany investigations of both temporal and spatial variations of b. With large samples and slow temporal changes in b, the standard error of b is σ ( b ^ ) = 2.30 b 2 σ ( M _ ) , where σ 2 ( M _ ) = ∑ i − 1 n ( M i − M _ ) 2 / n ( n − 1 ) . In an example from central California, stable estimates of b require a space-time window containing about 100 earthquakes. From 1952 to 1978, the average b and 90 per cent confidence limits are 0.95 (+0.94, −0.30). Some fluctuations of b are statistically significant but some are not. Within 90 per cent confidence limits, b changes from a low of 0.60 (+0.11, −0.09) in 1955 to a high of 1.39 (+0.25, −0.21) in 1967 and drops to 0.72 (+0.13, −0.10) in 1975. In this example, no correlation between large earthquakes (M > 5) and b variations occurred.

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The principal mechanism for abnormally high pore pressure generation has been an important matter of debate for more than a decade. Among the various mechanisms proposed, mechanical overloading and aquathermal pressuring have probably received the greatest attention. Most previous discussions, however, have been based upon qualitative conceptual models. Here we propose a quantitative analysis by first examining the basic principles of porous flow and the fundamental properties of sediments and, second, by comparing the relative importance of the overloading versus the aquathermal mechanisms. The coupling between pore pressure and compaction as well as the nonlinear and path‐dependent material properties are found to be critical in the present analysis. We find that although changes in stress and temperature both contribute to pressure generation, mechanical overloading is the main mechanism under normal geological conditions and is capable of producing the observed pore pressure profiles.

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