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The Cascadia subduction megathrust off the Pacific Northwest follows an “end member” seismogenic behavior, producing large (up to moment magnitude 9) but infrequent (every several hundred years) earthquakes and tsunamis. Crustal deformation associated with the ongoing plate convergence has been characterized by land‐based geodetic observations, but the state of locking across the full breadth of the seismogenic fault is poorly constrained. We report results of offshore monitoring of borehole fluid pressure, as a proxy for formation volumetric strain, at a site ∼20 km landward of the Cascadia subduction deformation front since 2010. The multi‐depth pressure records were plagued by hydrologic noise, but noise at the deepest monitoring level (303 m sub‐seafloor) abated in 2015. Subsequently, including at the times of regional large earthquakes that caused significant dynamic stressing, no persistent pressure transients are present above a threshold of 0.08 kPa imposed by unremovable oceanographic signals, corresponding to a strain detection limit of ∼16 nanostrain. Simple dislocation models using local megathrust geometry suggest a resolvable slip of <1 cm along a trench‐normal corridor beneath the borehole for a range of slip‐patch dimensions. A large slip patch can be well resolved even at considerable along‐strike distances from the borehole; for instance, ∼10 cm slip is detectable over a 200‐km strike range for a slip‐patch radius of ∼50 km. This high sensitivity for detecting slip, along with the lack of observed events, stands in stark contrast to observations at other subduction zones, and suggests that the Northern Cascadia megathrust is most likely fully locked.

Six-year records of ocean bottom water temperatures at two locations in an isolated, sedimented deep-water (∼4500 m) basin on the western flank of the mid-Atlantic Ridge reveal long periods (months to >1 year) of slow temperature rises punctuated by more rapid (∼1 month) cooling events. The temperature rises are consistent with a combination of gradual heating by the geothermal flux through the basin and by diapycnal mixing, while the sharper cooling events indicate displacement of heated bottom waters by incursions of cold, dense bottom water over the deepest part of the sill bounding the basin. Profiles of bottom water temperature, salinity, and oxygen content collected just before and after a cooling event show a distinct change in the water mass suggestive of an incursion of diluted Antarctic Bottom Water from the west. Our results reveal details of a mechanism for the transfer of geothermal heat and bottom water renewal that may be common on mid-ocean ridge flanks.

A new instrument developed for monitoring acceleration, tilt, and pressure at the ocean floor also measures sediment temperature 1 m below the seafloor. Four deployments have been completed and connected to the Ocean Networks Canada cabled observatory, one on the inner Cascadia accretionary prism, two on the outer prism, and one on the sedimented eastern flank of the Juan de Fuca Ridge. Relative amplitudes and phases of temperature variations measured at the seafloor and in the sediment at periods greater than roughly 1 week constrain the thermal diffusivity of the upper meter of subseafloor sediment to be 4 × 10⁻⁷ m²/s. Clear ±0.1‐mK amplitude tidal sediment temperature variations are also resolved. These are too large and regular to be the consequence of downward thermal diffusion from the seafloor and too large to be the consequence of fluid migration driven along the sediment geotherm by poroelastic response to tidal loading. The variations are closely correlated with tidal pressure variations, however, and we infer that these temperature signals reflect adiabatic heating and cooling. The lapse rates inferred from the observations at two of the sites are close to the values for seawater but significantly higher than predicted for a mixture that includes sediment grains. The values observed by both instruments at the outer prism site, located near methane‐bearing‐fluid springs, are particularly high, 20% higher than predicted for a sediment‐seawater mixture. This discrepancy could be reconciled if free gas or methane hydrate were present within the pore volume.