Seismotectonics of northeastern United States and adjacent Canada

Abstract

Data for local earthquakes recorded by a network of stations in northeastern United States and adjacent Canada were analyzed to study the seismicity, the relationship between earthquakes and known faults, the state of stress, and crustal and upper mantle velocity structure. In addition, portable seismographs were deployed in the field to study aftershocks. As a result, accurate locations for about 364 local earthquakes (2 ≤ mb ≤ 5) and 22 focal mechanism solutions were determined. A comparison of the spatial distribution of these events (1970–1979) with historical earthquakes (1534–1959) reveals that seismic activity in the northeast is relatively stationary in space: those areas that have had little or no seismicity historically are relatively aseismic today, whereas the historically active areas are also active today. The instrumental locations, historical seismicity, and focal mechanism solutions show an internal consistency that help us distinguish two distinct seismogenic provinces. (1) The Adirondack‐western Quebec province is a northwesterly trending zone of seismic activity, about 200 km wide and at least 500 km long, extending from the SE Adirondacks into western Quebec, Canada. Thrust faulting on planes striking NNW to NW appears to predominate, and the inferred axis of maximum horizontal compression is largely uniform and trends WSW, nearly parallel to the calculated absolute plate motion of North America. Little or no seismicity is found where anorthosite outcrops at the surface. Correlations between gravity anomalies and earthquake locations suggest that seismic activity in this zone is localized to regions of steep NE or SW gradient in Bouguer anomalies. This zone does not appear to extend southeastward to Boston, as proposed by some workers. (2) The Appalachian province is a northeasterly trending zone of seismic activity extending from northern Virginia to New Brunswick, Canada. Highangle reverse or thrust faulting on N to NE trending planes appears to predominate. The western margin of this province, however, appears to be relatively aseismic. We attribute this relative lack of activity to one or more of the following: the presence of igneous activity postdating rifting of Africa from North America, the occurrence of intense metamorphism during the Acadian orogeny which may have annealed preexisting faults, and the predominance of ductile as opposed to brittle deformation in the geologic past. The inferred axis of maximum horizontal compression along the eastern margin of the Appalachians is rather uniform and trends W‐WNW, almost perpendicular to the magnetic lineations offshore. We suggest that this W‐WNW compression reflects the gravitational force arising from horizontal density variations in the oceanic lithosphere as it cools and moves away from spreading centers.

Relationship between Near-Field and Teleseismic Observations of Seismic Source Parameters

“…Gaspe Synclinorium, the Bronson-Hill Anticlinorium, and the Merrimac Synclinorium, New England counterparts with somewhat different features (King, 1977;Taylor and Toks6z, 1979;Yang and Aggarwal, 1981). While their tectonic evolution is similar, these zones do not share the metamorphic and igneous character of the New England province nor the distinctive fold-fault features and longitudinal two-fold division so apparent in the trends of the U.S. Appalachians (Spencer, 1969).…”

Recent Relative Sea-Level Change in Eastern North America

“…Leveling results indicate the same sense of movement. Historical and instrumentally-recorded earthquakes have been reported in the vicinity of this graben, particularly along the eastern border fault (Yang and Aggarwal, 1981).…”

“…Crustal movement due to strain release along these fault systems may be affecting RSL at New London. Portsmouth through Ottawa, Ontario (Sbar and Sykes, 1973), although seismic activity may not be continuous along this zone (e.g., Yang and Aggarwal, 1981). Relative crustal movement between Portsmouth and…”

Structural Elements and Neotectonics of Prince Edward County, Southern Ontario

“…The normal faults formed in response to a stress field with the greatest principal stress, o-,, either vertical, or horizontal and oriented approximately northwest. The other two fault types, however, would have developed as a consequence of a compressional stress field in which <r, was oriented generally northeast, as it is at the present time in eastern North America (Zoback and Zoback, 1980;Yang and Aggarwal, 1981;Adams and Basham, 1987). This implies at least two periods of Paleozoic, or younger, faulting, although the exact ages of neither can be established due to the absence of post upper …”