Abstract

Prominent gravity anomalies, consisting of paired positive-negative belts, occur in Canada at structural boundaries between geological provinces. The associated anomalous masses produce what are termed gravitationally induced stresses. These stresses may contribute to the failure of rocks along preexisting faults, or other zones of weakness. In the case of a typical structural boundary, failure at shallow depths in the crust is likely to occur in the region outlined by the negative gravity anomaly, whereas failure deeper within the crust and upper mantle may occur beneath the positive anomaly. Along the lower St. Lawrence valley, good spatial correlation is found between regions of high seismicity and those negative free-air anomaly areas which are adjacent to prominent free-air gravity highs. It is suggested that in a heavily faulted region, such as the lower St. Lawrence valley, gravitationally induced stresses may be a contributing factor to the production of earthquakes in regions which are otherwise already close to failure.

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