Present-day CGPS-derived Crustal Strain Rate Field of the Saint Lawrence River Valley

Abstract:

The Saint Lawrence River valley (SLRV) is one of the most seismically active areas in eastern Canada. Along the SLRV and the Ottawa valley, earthquakes are concentrated on three distinct zones of western Quebec along the Ottawa River, Charlevoix, and Lower Saint Lawrence. The entire area is also subject to the glacial isostatic adjustment (GIA). We studied the earth’s surface deformation of the area using the velocity field of 51 continuous GPS (CGPS) stations and the least-squares collocation method. While the intraplate horizontal velocities showed a coherent horizontal motion towards southeast with the typical magnitude of ~1.3 mm/yr for stations along the SLRV, the interpolated vertical velocities demonstrated a coherent uplift with the average rate of 3.1 mm/yr. We estimated strain rate tensors including the effect of vertical velocity. A NNW-SSE shortening with a typical rate of ~3.6–8.1 nstrain/yr was observed over Lower Saint Lawrence. In Charlevoix, an extension with a typical rate of ~3.0–7.1 nstrain/yr was oriented in ENE-WSW parallel to the SLRV. In western Quebec, the deformation has a shear straining mechanism with a typical shortening rate of ~1.0–5.1 nstrain/yr and extension rate of ~1.6–4.1 nstrain/yr. The extension over the northern model is consistent with the prediction of the GIA models. The range of the estimated strain rates of the area (~1.0–8.1 nstrain/yr) is between typical values of rigid blocks (< 0.1 nstrain/yr) and active tectonic regions (> 100 μstrain/yr). A strong correlation was observed between epicenters of earthquakes and areas with the highest rate of shear strain. We found a good agreement between the orientations of the principal axes of strain rate tensors and the maximum horizontal compressional stress σ_H from World Stress Map 2008 for both strike-slip and thrust faulting regimes especially those derived from focal mechanisms. This shows our CGPS intraplate velocities are representative of the current crustal deformation associated with the GIA and that they constrain the crustal strain rates in magnitude, direction and origin.