Focal mechanisms have been derived for 30 earthquakes in the Adirondack Mountains-western Quebec seismic zone from a modified version of the computer program FOCMEC using first motions of Pn and Pg waves, and SV/P amplitude ratios measured from Sg and Pg phases. Reverse faulting on medium to steeply dipping planes is prevalent. Focal depth and velocity structure can vary within realistic limits without much influence on the focal mechanism solution. Deviatoric compressive-stress axes have a strong horizontal component, but the trend varies. Most align approximately northeast-southwest, in accordance with both the orientation predicted by models based on plate tectonics and that derived from most other studied earthquakes in the region. However, for several earthquakes, mechanisms have been obtained that deviate from this pattern, notably for events near 45.5°N to 46°N, 75°W, and the North Gower main shock-aftershock series of October 1983. Possible explanations for these anomalies in terms of local structural disturbances, or reactivation of preexisting zones of weakness with different structural orientation than those created during the current tectonic regime, are presented.
Different hypotheses for the seismotectonic evolution and processes of the area are presented and discussed. In addition to orientations of nodal planes and deviatoric stress axes, there are other indicators, such as the seismicity distribution and patterns of earthquake sequences, of inhomogeneity in the proposed Adirondack Mountains-western Quebec seismic zone.