CGRG Bibliography of Canadian Geomorphology
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Author : Eaton, D.; O'Dowd, C.; Dineva, S.; Mereu, R.; and Asmis, H.
Date : 2002.
Title : Crustal structure and seismicity in the Greater Toronto Area.
Publication : American Geophysical Union 2002 Fall Meeting, Moscone Center, San Francisco, California, 6-10 December 2002.
Issue :
Page(s) :
Abstract
Earthquake activity in southern Ontario is mainly concentrated in western Lake Ontario and environs. Although this region is not as seismically active as other areas of eastern Canada, the seismic risk remains a significant social and economic issue due to its high population density and concentration of critical facilities. Beneath major population centres such as the greater Toronto area (GTA), the deep structural framework of the crust has not been previously studied in detail. We have obtained initial results from a new 75-km seismic-reflection profile, acquired northeast of the GTA as part of a 340-km crustal seismic program. We have interpreted the seismic profile in conjunction with recent aeromagnetic data and revised earthquake hypocentre information. The seismic section provides a clear image of the Central Metasedimentary Belt boundary zone (CMBbz), a major tectonic boundary that formed 1.08 - 1.03 billion years ago. This zone appears to dip toward the southeast to a depth of at least 21 km (7 seconds two-way time), at a moderate angle ($<40°$). We have recalculated hypocentres of earthquakes recorded since 1991 by the Southern Ontario Seismograph Network (SOSN), to reduce positional uncertainty to smaller than a few km. Most of the seismicity occurs at less than 20 km depth. There is some evidence that at least one cluster of microearthquakes in western Lake Ontario may occur along a southeast-dipping plane, albeit with a steeper dip ($~70°$) than the CMBbz. On the basis of correlation of our seismic image and aeromagnetic maps, we propose a revised location for the subsurface trace of the western limit of the CMBbz. In our interpretation, this boundary follows a SW-NE trending magnetic low near the western shore of Lake Ontario. Our hypothesis reconciles the location of the CMBbz with a geochronologic drillcore measurement near Hamilton, and helps to resolve discrepancies from previous workers at the Canada-U.S. border. According to our model, the CMBbz also coincides approximately with the western limit of Lake Ontario seismicity. Since most of the seismicity is confined to areas under the lake, the circulation of water below Lake Ontario may be playing a major role in reactivating the CMBbz faults. The apparent contrast in level of seismicity on either side of the CMBbz, in our interpretation, may have important implications for understanding whether Precambrian structures influence the location of present-day earthquake activity.
Bibliography of Canadian Geomorphology