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Seismicity Results (1991-2011)

Seismicity map from 1991 to 2009

The accompanying seismicity map shows the location of all local earthquakes recorded in the time period 1991-2011. The map shows that most of the earthquakes occur in clusters with the largest being in the western Quebec seismic zone shown in the north eastern corner of the map. There is a large cluster along the south shore of Lake Erie and sets of smaller clusters scattered in western Lake Ontario and Georgian Bay.

Over the past decade, an average of 6-10 local earthquakes per year in the western Lake Ontario area was detected by the SOSN. Most of the events were in the 1.5 - 3 magnitude (mN) range. The estimated location uncertainty (+/- 2 km) is an order of magnitude better than that which was possible before 1991. The focal depths for all the events lie down to 15 km, well within the Grenville rocks of the Precambrian Shield.

The largest events which took place during the 1992-2011 time period in the surrounding regions are as follows:

The largest two earthquakes in Lake Ontario area were of magnitude (mN=3.8). They are:

Historical seismicity maps (Stevens, 1994) of the western Lake Ontario region show that all the earthquakes were located on land areas surrounding the Lake with no events in the Lake. These events were probably located on the basis of felt reports. An examination of the revised seismicity map for the past 18 years shows that a definite pattern has emerged in the SOSN data set in Lake Ontario which is significantly different from the past historical earthquake patterns obtained when the instrumental coverage was poor. Most events occur in scattered clusters in the western part of Lake Ontario and the northwestern corner of New York State. The area of seismicity does not extend significantly to the north of western Lake Ontario and appears to end rather abruptly along a line running from south of Hamilton in a north-northeasterly direction towards Toronto, Ontario. The cause of the seismicity is speculated to be related to water flows along various fissures below the lake ( Mereu et al, 2002). It is known from induced seismicity studies of reservoirs that the presence of fluids can cause earthquakes by changing the pore pressure and reducing the friction along any faults which may be present. From seismic reflection studies, dipping structures and shear zones have been imaged to extend southeastward under Lake Ontario. This may explain why most of the earthquakes are occurring under the lake or southeast of the lake.

Although the area of seismicity coincides with a region of linear magnetic anomaly trends (suggesting a strong structural fabric in the basement rocks), the correlation of seismicity of the new SOSN data set with magnetic lineaments is still unclear. In a recent study, Dineva et al. (2004) showed that the seismicity and magnetic anomalies of the area exhibit statistically significant preferred orientations at N40E-N45E. Three preliminary focal mechanisms of earthquakes with magnitudes mN 3.1-3.8 show unusual normal faulting with nodal planes in almost the same direction as the magnetic trends. The proximity of the earthquake clusters to large bodies of water coupled with the colinearity of the magnetic trends, suggests that both surface water and preexisting basement structures may play significant roles in controlling inter-plate seismicity in the southern Great Lakes region.


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