The October 2005 Georgian Bay, Canada, Earthquake Sequence: Mafic Dykes and Their Role in the Mechanical Heterogeneity of Precambrian Crust
Bibliographic record
Abstract
On 20 October 2005 at 21:16 UTC, a moderate earthquake (mN 4.3) occurred in an area of low seismicity within Georgian Bay, approximately 12 km north of Thornbury, Ontario (44.67 N, 80.46 W). Despite its moderate magnitude, it was exceptionally well recorded and is of particular interest because of its location 90 km from a proposed long-term storage facility for low- and medium-level nuclear waste. No damage was reported, but ground shaking was felt to a distance of 100 km. Within 24 hours after the mainshock, four portable seismograph systems were in- stalled in the epicentral region. In total, eight events were recorded over a 4-day period, including a foreshock and six aftershocks. The unusually rich dataset from this moderate earthquake sequence enabled robust determination of hypocentral pa- rameters, including well-constrained focal depths for most events. For the mainshock, we estimated a seismic moment of M0 4.5 10 14 N m and corner frequency of 3.7 Hz, based on a spectral fit using Brune's source model. Least-squares waveform inversion of P and S phases yielded a double-couple focal mechanism with a reverse- sense of slip and northwest-striking nodal planes. The reverse mechanism and mid- crustal focal depths (10-12 km) are characteristic, in general, of more abundant seismicity located 200 km northeast of this event in the western Quebec seismic zone. These parameters differ, however, from shallow (2-6 km) earthquakes, with predominantly strike-slip mechanisms, observed near Lake Erie 200 km to the south. We attribute this north-south change in rupture mechanism to variations in crustal stress induced by postglacial isostatic rebound. Aeromagnetic data in and around the epicentral region reveal prominent northwest-striking lineations caused by Precam- brian mafic dykes. Under midcrustal conditions, the dyke material is mechanically stronger than generally more felsic upper-crustal host rocks. We propose that where large dykes are favorably oriented with respect to the stress field, they may strongly influence the locations of intraplate earthquake rupture in Shield regions.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 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 teacher head, 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".