Long‐Term Offshore Borehole Fluid‐Pressure Monitoring at the Northern Cascadia Subduction Zone and Inferences Regarding the State of Megathrust Locking
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...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
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Wiley
2023-06-01
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| Series: | Geochemistry, Geophysics, Geosystems |
| Subjects: | |
| Online Access: | https://doi.org/10.1029/2023GC010910 |
| _version_ | 1797638078325587968 |
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| author | Earl E. Davis Tianhaozhe Sun Martin Heesemann Keir Becker Angela Schlesinger |
| author_facet | Earl E. Davis Tianhaozhe Sun Martin Heesemann Keir Becker Angela Schlesinger |
| author_sort | Earl E. Davis |
| collection | DOAJ |
| description | 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. |
| first_indexed | 2024-03-11T12:57:29Z |
| format | Article |
| id | doaj.art-5aba1d1539924eeaa2fe3b0348d6e710 |
| institution | Directory Open Access Journal |
| issn | 1525-2027 |
| language | English |
| last_indexed | 2024-03-11T12:57:29Z |
| publishDate | 2023-06-01 |
| publisher | Wiley |
| record_format | Article |
| series | Geochemistry, Geophysics, Geosystems |
| spelling | doaj.art-5aba1d1539924eeaa2fe3b0348d6e7102023-11-03T16:56:13ZengWileyGeochemistry, Geophysics, Geosystems1525-20272023-06-01246n/an/a10.1029/2023GC010910Long‐Term Offshore Borehole Fluid‐Pressure Monitoring at the Northern Cascadia Subduction Zone and Inferences Regarding the State of Megathrust LockingEarl E. Davis0Tianhaozhe Sun1Martin Heesemann2Keir Becker3Angela Schlesinger4Pacific Geoscience Centre Geological Survey of Canada Sidney BC CanadaPacific Geoscience Centre Geological Survey of Canada Sidney BC CanadaOcean Networks Canada University of Victoria Victoria BC CanadaRosenstiel School of Marine, Atmospheric and Earth Science University of Miami Miami FL USAOcean Networks Canada University of Victoria Victoria BC CanadaAbstract 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.https://doi.org/10.1029/2023GC010910subduction megathrust lockingoffshore geodesyfluid pressure monitoringborehole observatoryIODPevent detection threshold |
| spellingShingle | Earl E. Davis Tianhaozhe Sun Martin Heesemann Keir Becker Angela Schlesinger Long‐Term Offshore Borehole Fluid‐Pressure Monitoring at the Northern Cascadia Subduction Zone and Inferences Regarding the State of Megathrust Locking Geochemistry, Geophysics, Geosystems subduction megathrust locking offshore geodesy fluid pressure monitoring borehole observatory IODP event detection threshold |
| title | Long‐Term Offshore Borehole Fluid‐Pressure Monitoring at the Northern Cascadia Subduction Zone and Inferences Regarding the State of Megathrust Locking |
| title_full | Long‐Term Offshore Borehole Fluid‐Pressure Monitoring at the Northern Cascadia Subduction Zone and Inferences Regarding the State of Megathrust Locking |
| title_fullStr | Long‐Term Offshore Borehole Fluid‐Pressure Monitoring at the Northern Cascadia Subduction Zone and Inferences Regarding the State of Megathrust Locking |
| title_full_unstemmed | Long‐Term Offshore Borehole Fluid‐Pressure Monitoring at the Northern Cascadia Subduction Zone and Inferences Regarding the State of Megathrust Locking |
| title_short | Long‐Term Offshore Borehole Fluid‐Pressure Monitoring at the Northern Cascadia Subduction Zone and Inferences Regarding the State of Megathrust Locking |
| title_sort | long term offshore borehole fluid pressure monitoring at the northern cascadia subduction zone and inferences regarding the state of megathrust locking |
| topic | subduction megathrust locking offshore geodesy fluid pressure monitoring borehole observatory IODP event detection threshold |
| url | https://doi.org/10.1029/2023GC010910 |
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