Seismic experiments have been undertaken along the Atlantic coast of Nova Scotia, and an attempt has been made to determine by what mechanism the compressional wave which is recorded as the first event in the range of distance 10 km to 180 km propagates, and to describe its attenuation. In this part of the paper the change in amplitude of the first half-cycle of the first event is investigated, and compared with the change in amplitude which might be expected if propagation occurred as a "head wave", and if propagation occurred because of the presence of a velocity gradient in the upper few kilometers of the earth's crust. We find that we cannot distinguish between the two mechanisms of propagation with any certainty, and although the suggestion is made that the value of Q is in the range 90 to 300, depending upon the propagation mechanism, the assumptions which underlie the approach are criticized, and a more reasonable one put forward.

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