Abstract
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.
Plain Language Summary
Subduction thrust faults can produce Earth's largest earthquakes and tsunamis. Estimating the seismic and tsunami potential requires knowing how complete the fault locking is. The Cascadia subduction zone is known to have hosted very large earthquakes, but with the existing measurements of deformation being on land, the state of locking of the shallow sub‐marine portion of the fault remains unclear. Addressing this question requires offshore deformation measurements directly above the shallow fault. In this study, we report results of formation fluid pressure measurement—as a volumetric strain indicator—from a deep‐ocean borehole off Vancouver Island near the deformation front of Cascadia. While data from shallow monitoring depths suffer from noise likely caused by formation gas dynamics, the record from the deepest level (300 m beneath the seafloor) has been sufficiently quiet since mid‐2015 to allow detection of any slip‐triggered pressure variations. Lack of such variations, together with deformation modeling, limits local slip events to be <1 cm. This suggests that the slip deficit of the plate convergence is being accumulated at a substantial rate and it may be released in the next megathrust earthquake. This contrasts with other subduction margins where slip‐related pressure transients have been frequently detected.
Key Points
8‐year formation pressure records from northern Cascadia suggest a lack of tectonic transients despite occasional large dynamic stressing
Noise level of the record defines a detection threshold of 0.08 kPa, equivalent to 16 nanostrain for shallow accretionary prism sediment
Deformation modeling shows that the data would resolve local slip down to <1 cm, and thus suggests a fully locked state of the megathrust
