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western quebec and southern ontario - Department of Geology

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3.3.6. Focal MechanismsThe focal mechanism can be obtained manually or by using s<strong>of</strong>tware. Data used for thefocal mechanism can be the polarities <strong>of</strong> the P-waves or the amplitude information for P-<strong>and</strong> S-waves. An important feature <strong>of</strong> this kind <strong>of</strong> s<strong>of</strong>tware is an output not only as asingle solution but as a set <strong>of</strong> all possible solutions for a specific confidence level. TheFOCMEC program uses both the polarities <strong>and</strong> the amplitude ratios, <strong>and</strong> gives a set <strong>of</strong>possible solutions.FOCMEC programFOCMEC (Snoke et al., 1984) was used to evaluate the focal mechanism for eachearthquake. The program uses the distances, azimuths, <strong>and</strong> the take-<strong>of</strong>f angles from theHYPOCENTRE output. FOCMEC produces four output files: 1) ‘focmec.dat’ containsthe input parameters for FOCMEC, 2) ‘focmec.lst’ shows the input data <strong>and</strong> all details <strong>of</strong>all possible solutions (dip, strike, <strong>and</strong> rake <strong>of</strong> the nodal planes, trend <strong>and</strong> plunge <strong>of</strong> P, T,<strong>and</strong> B axes, total number <strong>of</strong> polarities <strong>and</strong> the stations with wrong polarities); 3)‘focmec.out’ contains only the strike, dip <strong>and</strong> rake for one <strong>of</strong> the nodal planes for allsolutions with the number <strong>of</strong> wrong polarities <strong>and</strong> grid step in degrees; 4) ‘focmec.eps’ isa Postscript plot file <strong>of</strong> the latest plot. An example <strong>of</strong> the output files for one seismicevent is given in Appendix B. The program uses grid-search algorithm over the entirefocal sphere to define all possible solutions with option <strong>of</strong> misfit polarity. The minimumnumber <strong>of</strong> polarity errors <strong>and</strong> degree increment in the grid search can be enteredmanually. After several trials, the best solution <strong>of</strong> each event has been selected based onthe minimum number <strong>of</strong> polarity misfit.43

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