A temporary seismic network located in the La Malbaie region of eastern Quebec during the summer of 1974 for studying the local seismicity of the area was used as a seismic array to measure directly the slowness and azimuth of the P arrivals from 18 teleseismic events. A consistent bias in the arrival vectors was observed for rays approaching the array from the south (i.e. azimuths 181°–227°). This bias can be explained by a thickening of sedimentary strata beneath the south shore of the St. Lawrence River as opposed to the outcropping Precambrian basement rock on the north shore. The sedimentary accumulation can be interpreted in terms of a sloping interface located under the network between sediments and Precambrian rock or a thick horizontal sedimentary layer underlying the south shore of the St. Lawrence River.

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