western quebec and southern ontario - Department of Geology

5.4 Spatial variation of focal mechanism parametersThe spatial variations of the focal mechanism parameters were studied using the focalmechanism data obtained in the current study combined with all available focalmechanism solution data in the literature (Appendix D). The solutions were selected andmanually analysed based on the accuracy and quality of the results provided for eachevent. In some cases the strike and dip of the nodal planes were calculated manually fromthe trend and plunge of the T and P axes. Only one focal mechanism solution for eachearthquake was considered, based on the quality, for the subsequent analysis. Allearthquake epicentres of the events with focal mechanisms and with magnitude m N > 3.5are shown in Figures 5.4a. The events were combined into clusters using the densitydistribution of the earthquakes in the study area (Figure 5.4b) and manually checked forconsistency of the focal mechanisms of closely spaced events. The clusters outlined byprevious researchers within the study area were also taken into account. This clusteringcan be improved in future by including more earthquakes with well-defined hypocentresfor longer period of time and wider magnitude range. The data for the focal mechanisms(nodal planes, P- and T-axes) were analysed separately for each cluster.The earthquake activity in western Quebec is suggestive of a strong NNW-NWtrend, where most of events are focused along the Ottawa River, from Montreal toBaskatong (Figure 2.4). Earthquake epicentres along the Ottawa River appear to correlatewith a zone of Paleozoic or younger normal faults along the Ottawa River (Forsyth,1981). Another series of epicentres are located from Montreal to west of the BaskatongReservoir trend northwest. Forsyth (1981) suggested that earthquakes of the second band(from Montreal to the west of the Baskatong) are occurring on or between structural67