Abstract:
Hydrologically active areas in the deep ocean are also seismically active, and ever since early "CORK" (Circulation Obviation Retrofit Kit) hydrologic observatories were established in Ocean Drilling Program boreholes, transient signals related to earthquakes have been observed. In holes that are properly sealed, formation pressure anomalies appear to be the consequence of co-seismic and post-seismic strain, and of strain related to slow fault slip. The sign of the anomalies is always consistent with that predicted from the earthquake moment tensors. The magnitude of volumetric strain can be determined from pressure change using the calibration provided by the formation response to oceanographic loading at the seafloor. Examples at both subduction zone (Mariana forearc, Nankai Trough) and ridge and ridge flank settings (Juan de Fuca Ridge) show that co-seismic strain inferred from pressure is typically one to two orders of magnitude greater than that predicted for the corresponding earthquakes. Transients observed in the Middle America Trench off Costa Rica follow slow slip and tremor events landward of the locked, or partially locked portion of the subduction plate interface. This relationship suggests that slow slip observed by way of GPS-constrained deformation and seismic tremor on land can reach all the way to the trench with no seismic energy generation. In holes that are not perfectly sealed, formation temperature transients concurrent with pressure transients have been observed, with temperature being sensitive to changes in the rate of flow within the boreholes. Examples from the Juan de Fuca Ridge flank suggest that formation physical properties may be affected near epicentres where ground motion intensity is large. In one recent example, pressure fell in response to dilatational strain, while the temperature, and hence rate of flow in an overpressured and leaking hole increased. This can only be explained by an increase in permeability. All examples demonstrate the utility of using long-term monitoring of formation pressure in ocean boreholes as a sensitive proxy for strain in the study of seismic and aseismic fault slip.