Date of Award
Master of Science in Geology
Reinhard K. Frohlich
The causes of the seismicity of the St. Lawrence River Valley are not well understood. As is the case for the entire east coast of North America, epicentral zones often occur in regions where no correlation exists between seismicity and mapped geologic structures. There are several explanations for such a phenomenon: a) earthquakes occur along unmapped surface faults, b) earthquakes occur along subsurface faults showing no surface expression, or c) the structures are not fault related.
Conventional filtering techniques, such as the upward continuation, downward continuation and second derivative methods were applied to gravity data from the St. Lawrence River Valley in an attempt to delineate possible seismic-related structures. The gravity survey indicates that the anomalies trend in a north-northeast direction similar to the structural trends of the Precambrian rocks. The major feature of the Simple Bouguer map is an extensive gravity high centered at Massena, New York.
Analyses by the filtering methods and subsequent modeling by the two-dimensional Talwani technique reveal the existence of two anomaly-producing bodies responsible for the Massena High: 1) a wedge (8x35 km) located 6 km below sea-level with a density contrast of +.11g/cc, and 2) a smaller body (2x6 km) located 3.3km below sea level with a density contrast of +.2g/cc. The large wedge may represent a sequence of interlayered metasesiments and meavolcanics related to the Grenville sequence. The smaller body may represent a mafic intrusive. In addition, the Simple Bouruer map reveals a circular gravity of low intensity (diameter=20km) centered at Cornwall, Ontario.
The Cornwall Low suggests an association of high gradients of gravity (toward positive) produced by mafic intrusives and earthquakes in the southeastern United States. The possible existence of a mafic intrusive near Massena, New York and its proximity to epicentral zones suggests a similar origin for some earthquakes in the study area.
Albert, Robert Louis, "A Gravity Study of Earthquake-Related Structures in the St. Lawrence River Valley" (1977). Open Access Master's Theses. Paper 990.