QUANTIFICATION OF PORE FLUID PRESSURE IN ACTIVE SUBDUCTION ZONES: IMPLICATIONS FOR FAULT STRENGTH AND SLIP BEHAVIOR

At subduction zones, mechanical loading due to burial and tectonic compression, in combination with the release of bound fluids by dehydration, can drive fluid pressures significantly in excess of hydrostatic. Recent advances in documenting pore fluid pressure at convergent margins, and in particular within the subducting sediment layer that lies immediately below the plate boundary interface, have come from drilling, seismic reflection surveys, and numerical modeling. Drilling through the plate boundary at several subduction complexes, including Nankai, Costa Rica, and Barbados, has allowed quantification of effective stress and pore fluid pressure from observed compaction state and from laboratory reconsolidation tests on core samples. Pore pressure has also been quantified using well-constrained P-wave interval velocities from seismic reflection surveys, by application of transforms to compute porosity from acoustic velocity, and effective stress from porosity. Finally, in situ measurements of pore pressure have been obtained in instrumented boreholes, and provide important “ground truth” for values computed from indirect methods.

These analyses show that excess pore pressure ratios of λ = ~0.70–0.95 are common within the underthrusting section, and extend >20 km landward of the trench, consistent with the results of independent numerical modeling studies. In some cases, increased seismic reflectivity along the plate boundary is also correlated with regions of expected dehydration reactions or to locations of high pore pressure estimated from numerical models, although this link remains qualitative and the underlying cause of the reflectivity is not fully understood. Taken together, these findings provide a plausible and quantifiable explanation for the absolute weakness of subduction megathrusts beneath the outer forearc. The high excess pore pressures and concomitant low effective stresses should also suppress the nucleation of unstable slip, consistent with observations of fault failure by slow slip, afterslip, and very low-frequency earthquakes (VLFE) in the outer forearc. Finally, the low absolute strength of the plate boundary near its trenchward edge may offer an explanation for the propagation of rupture from below, potentially allowing coseismic slip all the way to the trench.