SHEAR-WAVE SPLITTING AND ANISOTROPY FROM THE AFTERSHOCKS OF THE NAHANNI, NORTHWEST TERRITORIES, EARTHQUAKES

Two large earthquakes (Ms = 6.6, 6.9) occurred in 1985 near the North Nahanni River. The earthquakes were followed by long aftershock sequences. Two distinct shear waves were generally observed. In September 1986, the plane of polarization of the first arriving shear wave was well defined and averaged 156° ± 8° measured clockwise from north. The distribution of the anisotropy is not uniform. The pressure axes determined from fault plane solutions for the two main shocks are nearly E-W, normal to the compressive direction found in the September 1986 field survey from shear wave splitting data. Four out of seven of the aftershocks for which focal mechanism orientations could be determined showed pressure axis orientations that agreed with the stress direction inferred from the initial shear wave polarizations from the field survey. -from Author