Focal mechanisms were determined for 42 small earthquakes (1.4 < M < 3.4) that occurred in the Adirondacks-western Quebec seismic region. The analysis demonstrates the variability of the focal mechanisms and suggests that the region can be divided into two subprovinces with distinctive seismotectonic regimes: 1. (1) in the Adirondacks, the focal mechanisms are mostly strike-slip with a comparatively smaller thrust component and indicate NNE compression: 2. (2) in the western Quebec seismic region, the stress field is more heterogeneous and is dominated by two directions (NE and NW) of horizontal compression. This variable pattern suggests that the seismicity and stress field are controlled by regional plate stresses but are affected by local structures and/or by the predominant orientation of preexisting faults. © 1989.
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