Deep-sea borehole fluid pressure and temperature observations at subduction zones and their geodynamic implications

Seafloor and subseafloor temperature and pressure monitoring has been carried out with CORK ("Circulation Obviation Retrofit Kit") sealed deep-ocean borehole observatories since their original design and deployments in 1991 in hydrologically and tectonically active settings. In this paper, we review results from 20 installations at six subduction zones that have provided novel insights into the thermal and hydrologic state, time-dependent deformation, and elastic properties of the subseafloor. Monitoring depths reached the levels of the megathrust faults beneath the outer subduction prisms at Nankai, Costa Rica, and Barbados, but only at the last were substantially elevated average pressures observed (excess fluid-pressure ratios of similar to 0.3 and 0.5). Otherwise, all records are characterized by fluid pressures only slightly super-hydrostatic. Transient pressure anomalies are observed to be common within both the subduction prisms and the incoming subducting plates. These reflect deformation (volumetric strain changes) caused by distant seismogenic fault slip at the time of large earthquakes, and by more local slow slip that is seen to propagate to the outermost reaches of the prisms from greater depths at rates of several kilometers per day. No such activity is seen at the Cascadia prism site, however, suggesting that its subduction fault is more completely locked. Where CORK observatories are co-located with or near seismometers and other seafloor monitoring instruments and are connected to cable systems for power, real-time high-rate data acquisition, and precise timing, the observations allow relationships among seismic ground motion, seafloor pressure, and formation pressure to be defined. These, in turn, constrain formation elastic properties, and confirm the efficiency and fidelity with which strain is converted to formation pressure.