The crust and upper mantle beneath four major seismic zones — Boston-Ottawa, Charleston-Cumberland, Grand Banks, and New Madrid — are compared with that of surrounding areas by examining differences in travel-time residuals for P waves from large nuclear explosions at teleseismic distances, geologic features, particularly the distribution of Mesozoic structures and igneous rocks that postdate the initial opening of the Atlantic Ocean, and other geophysical data. Many, but not all, of the larger historic earthquakes located east of the Rocky Mountains occurred in these zones, which are a few hundred kilometres wide, and do not appear to be related to single through-going faults; hence, their locations may be governed by deep-seated structures. A zone of early P-wave arrivals (negative travel-time residuals) is found in northern New York, in Canada along the St. Lawrence Valley from Ottawa to Seven Falls, Quebec, and in central Vermont. The anomaly, which is between −0.6 and −1.5 s for sources to the northwest, indicates that appreciable differences in seismic velocity (and presumably in petrology) extend into at least the uppermost mantle. This difference, which cannot be attributed to reading errors, is found at nine stations and is nearly as large as that between stations in the eastern and western United States. This negative anomaly may be caused by ultramafic rocks in the crust and upper mantle, such as those in the nearby Monteregian Hills, or by velocity inhomogeneities related to an ancient suture zone. Positive travel-time residuals between 0.4 and 1.2 s are associated with stations from central New York to West Virginia and Virginia in the miogeosynclinal Appalachians. These anomalies may be explained at least in part by a greater crustal thickness.
In eastern North America considerable seismic activity, particularly large earthquakes, appears to occur along the inferred continental extensions of major fracture zones that were active in the early opening of the Atlantic. The Boston-Ottawa seismic zone appears to be nearly spatially coincident with Mesozoic alkalic igneous rocks of the White Mountain Magma Series and the Monteregian Hills. These rocks are similar in age to the New England (Kelvin) Seamounts, a major transform fault across which magnetic lineations of Mesozoic age in the western Atlantic change strike and appear to be offset. The Boston-Ottawa seismic zone, the Mesozoic igneous rocks, and the seamount chain appear to define a major tectonic zone about 2,000 km long. The trend of this zone fits a small circle (line of constant latitude or transform direction) about the center of rotation for the early (Mesozoic) opening of the North Atlantic. Other major offsets of Mesozoic magnetic anomalies (interpreted as major transform faults active in the early opening of the western Atlantic) occur at the Blake and Newfoundland fracture zones. Two small circles drawn about the same pole of rotation as that used to fit the Boston-Ottawa seismic zone and the new England Seamounts fit (1) the Blake Fracture Zone and Charleston-Cumberland seismic zone; and (2) the Newfoundland Fracture Zone and the epicenter of the large Grand Banks earthquake of 1929. A zone of major seismic activity in southeast Missouri and alkalic intrusive rocks of Cretaceous age are found at the northern end of the Mississippi embayment, which has been sinking since Cretaceous time. Any mechanism for the localization of earthquakes in eastern and central North America must explain the apparent association of offshore fracture zones, seismic activity, and Mesozoic alkalic rocks as well as their fit about a small circle constructed for the early opening of the Atlantic and the lack of a spatial progression in ages of the alkalic rocks.
- Geological Society of America