A TECTONIC IMPLICATION OF THE SEISMICITY IN THE CENTRAL METASEDIMENTARY BELT OF THE GRENVILLE PROVINCE

Synthetic seismograms for a point-source dislocation in a multilayered medium were computed for eastern Canada earthquakes using stations of the Eastern Canada Telemetered Network (ECTN). These synthetic seismograms were then inverted to determine focal mechanisms from the maximum amplitude ratio of body waves at different stations. Different numerical tests were performed. The results show that the focal mechanism of microearthquakes (M(N) < 2.8) in the region covered by ECTN stations can be determined with an accuracy better than 10-degrees for the strike and dip angles and 20-degrees for the slip angle, provided that data are available from at least four stations (four quadrants) with good azimuthal coverage. Seismic activity in western Quebec and vicinity (45-48-degrees-N and 73-79-degrees-W) is summarized for the period between January 1970 and July 1986. Most earthquakes occurred in the basement of the Central Metasedimentary Belt (CMB); this region is characterized by a complex Bouguer gravity anomaly. The orientation of the maximum horizontal compression taken form the earthquake P axes varies considerably within the CMB but it tends to be subparallel to the gradients in the Bouguer gravity anomaly contours. This correlation could indicate that the pattern of the seismicity is affected by small-scale heterogeneities within the crust beneath the CMB, or alternately that local structures (reflected in the gravity field) are responsible for reorientation of the stress field.