The Lower St. Lawrence Seismic Zone Ocean Bottom Seismometer Deployment

The Lower St. Lawrence Seismic Zone, a paleorift zone in Quebec, is one of the most seismically active areas in eastern Canada. It underlies the St. Lawrence Estuary, which is itself an important habitat for endangered baleen whales. Between 2023 and 2025, we carried out two deployments of an amphibious seismic network with eight broadband ocean-bottom seismometers (OBS) and four coastal stations to monitor earthquakes and baleen whales. The OBS were deployed via free-fall; we present the required preliminary analysis including 1) clock-drift calculation using ambient noise cross-correlation, 2) tilt estimation using transfer functions, and 3) horizontal-component orientation using passing ships as a noise source. 

We apply machine learning pickers in combination with probabilistic earthquake phase association to construct an earthquake catalogue. Initial results indicate a detection rate of roughly twice that of the Canadian National Earthquake Database, although some detections may be rock blasts. Fin and blue whale calls are monitored using their characteristic internote intervals. An increase in vocal activity of both species is observed between the first (2023-24) and second (2024-25) deployments, despite the second being shorter in duration and missing the typically active month of October. In terms of per-OBS averages, fin whale activity increased from 63 active days to 97, and the number of detected calls per active day increased from 270 to 960, whereas blue whales remained more consistent in terms of active days (83 to 76) but the number of calls per active day increased from 106 to 173.