western quebec and southern ontario - Department of Geology

5.3 State of StressStress data for intraplate North America up to 2008 are summarized by Heidbachet al., (2008). Through examining the focal mechanisms of 32 earthquakes in easternNorth America, she estimated a horizontal stress orientation throughout the region,varying between northeast and east and averaging east-northeast. This orientation isconsistent with the North American plate motion and mainly attributed to plate boundaryforces such as Mid-Atlantic Ridge push (Zoback, 1992a; Richardson, 1992).P and T axes from earthquake focal mechanisms are one of the most commonlyused tectonic stress indicators, although borehole breakouts yield more consistent results(Mazzotti and Townend, 2010). Principal stress axis orientations obtained fromindividual fault-plane solutions, however, are not reliable measures because P and T axescannot be converted with certainty to the greatest and least principal stress directions(Zoback and Zoback, 1980; Scholz, 1990). These uncertainties result from the fact thatmost earthquakes occur on pre-existing faults that can be mis-oriented with respect to thecurrent stress tensor. Therefore, individual focal mechanisms can only constrain the stressdirection within 35°–45° (Zoback and Zoback, 1980). McKenzie (1969) proposed arestriction on the greatest principal stress imposed by the fault plane solution in whichthis stress direction should lie in the same quadrant as the P axis but could, in fact, benearly normal to the P direction. It appears that the best method of analysing focalmechanism data is to consider a number of events that occur on different faults in aparticular area and then to rely on their average P and T directions as measures of theorientations of the maximum and minimum principal stresses (Zoback and Zoback,1980). A large number of mechanisms from a small volume can be used for a stress63