Crustal structure from an ocean bottom seismometer survey of the Nootka fault zone
Bibliographic record
Abstract
The Nootka fault zone is the boundary between the small Explorer and Juan de Fuca plates which lie off western Canada between the America and Pacific plates. To investigate the crustal structure in the region, three refraction lines were shot with explosives and a large airgun into three ocean bottom seismometers (OBS's) each equipped with three-component geophone assemblies. The P and S wave profile data are analysed primarily by synthetic seismogram modelling using the WKBJ algorithm. The interpretation gives relatively consistent results for the upper crust. Below the 1 km thick sediment layer, the P wave velocity ranges from 3-7 to 4.7 km/s and increases with depth at a moderate velocity gradient of 0.5 km/s/km to a depth of 1.9 km. A zone of high velocity gradient marks the transition from layer 2A to 2B, below which the velocity again increases moderately with depth to a range of 6.0 to 6.4 km/s at the base of layer 2B. The S wave velocity also increases with depth in the upper crust but no detail is available for the v[sub s] structure in the shallow region of layer 2A. Within layer 2B, the P and S wave velocity values give a Poisson's ratio with an unusually low value of 0.24. This may be related to the presence of quartz-rich trondhjemites. Both v[sub p] and v[sub s] are relatively uniform at the top of layer 3A. Good agreement is found between the seismic velocities of layer 3A and the seismic velocities of rock samples from the corresponding depths of ophiolite complexes measured in the laboatory, resulting in consistent values for Poisson's ratio. Sub-bottom crustal thickness varies from 6.4 to 11.2 km among the various profiles. Some aspects of this variation can be explained by consideration of a recent tectonic model for the development of the fault zone. This requires, within the past one million years, variation in the process of crustal formation at the ridge, crustal 'maturing', or both. The abnormally thick crust may result in part from the complex interaction of the Juan de Fuca and Explorer plates with the larger and older America and Pacific plates. Apparent velocities of compressional waves in the upper mantle, calculated for areally distributed ray paths, show significant anisotropy. A maximum velocity of 8.3 km/s is found in the inferred direction of plate motion and a minimum velocity of 7.5 km/s is found parallel to the spreading ridge. This type of velocity variation can be approximated by a mixture of 42% transversely isotropic olivine and an isotropic material with a constant velocity of 7.0 km/s. Velocity of shear waves in the upper mantle, varying from 4.5 to 4.6 km/s, is isotropic within the resolution of the interpretation. This causes a prominent anisotropy in the values of Poisson's ratio. The variation in velocity with depth in the crust, values of Poisson's ratio, the P wave velocity anisotropy, and the approximately isotropic S wave velocity are consistent with laboratory measurements on rock samples from ophiolite complexes.
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".