Late Pleistocene to Holocene transtension in the northern Cascadia forearc: Evidence from surface ruptures along the Beaufort Range fault
Bibliographic record
Abstract
The active deformation field in subduction forearcs provides critical information about the stress and strain state of the upper plate and its potential for seismogenesis. However, these properties are challenging to quantify in most subduction systems, and in the northern Cascadia forearc, few faults have been identified that can be used to reconstruct the upper plate deformation field. Here we investigate the slip history of the Beaufort Range fault (BRF) on Vancouver Island. This fault was proposed to host the 1946 M7.3 Vancouver Island earthquake, but no surface rupture or evidence of Quaternary activity has been documented, and the stress and strain conditions that promoted this event are poorly understood. We provide the first evidence that the BRF is active, using newly-collected lidar to map topographic scarps along the fault system and to reconstruct slip vectors from offset geomorphic markers. Quaternary deposits and landforms that show increasing magnitude of displacement with age provide evidence for at least three M ~6.5-7.5 earthquakes since ~15 ka, with the most recent event occurring <3-4 ka. Kinematic inversions of offset geomorphic markers show that the BRF accommodates right-lateral transtension along a steeply NE-dipping fault. This fault geometry and kinematics are similar to those modeled for the 1946 earthquake, suggesting that the BRF is a candidate source fault for this event. We find that the kinematics of the BRF are consistent over decadal to millennial timescales, suggesting that this portion of the northern Cascadia forearc has accommodated transtension over multiple earthquake cycles.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.002 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.001 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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".