An active seismic zone extends along the passive margin of eastern North America from Baffin Island to the Grand Banks of Newfoundland. We have determined the focal mechanisms of several of the earthquakes, including the 1933 Ms 7.3 Baffin Bay earthquake, the largest event ever recorded along the eastern North American margin. The mechanisms show thrust faulting for earthquakes seaward of the 1000 m contour, and primarily normal faulting for earthquakes landward. We propose that these earthquakes are induced by the removal of the Pleistocene glacial loads which extended onto the continental shelf. The deglaciation reactivated basement faults remaining from rifting associated with the opening of the Labrador Sea. Baffin Bay and the Atlantic. A simple flexure calculation yields horizontal extension (normal faulting) in the previously glaciated region, and horizontal compression (thrust faulting) farther seaward, in good general agreement with the observed earthquake mechanisms. The magnitude of the stresses, 100 to 150 bars, is sufficient to reactivate preexisting basement faults. Large passive margin earthquakes may occur as far south along the coast as glaciation extended.

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We made a direct measurement of the distance from a hypocenter in the Benioff‐Wadati zone to the boundary between the downgoing slab and overlying asthenosphere. This was accomplished by identifying a low amplitude P wave reflection off the slab‐asthenosphere contact which arrives at teleseismic stations several seconds after the initial P arrival. The measured delay time (after P) of the reflected phase shows a consistent azimuthal variation, thus eliminating possible source or receiver effects. The relative amplitude, polarity, and delay time of this observed phase are consistent with a model in which the distance between hypocenter and slab‐asthenosphere contact at this depth is about 38±5 km. The event studied was an intermediate depth (181 km) earthquake in the southern Kuriles which occurred in the lower layer of a double seismic zone. Our measured distance, when compared to the distance between the two seismic zones, indicates that the upper layer of seismicity is within 15 km of the slab‐asthenosphere contact. Appendix is available with entire article, on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, U.C. 20009. Document JB1‐010; $1.00. Payment must accompany order.