Georgian Bay (Ontario) earthquake with magnitude m (sub N) 4.3 (October 20, 2005) and its foreshock-aftershock sequence; : tectonic implications

• On October 20 2005 at 21:16 GMT, a magnitude mN 4.3 earthquake occurred in the southern part of Georgian Bay, approximately 12 km north of Thornbury, Ontario, Canada (latitude 44.67° N and longitude 80.46° W). This earthquake is the largest one in southern Ontario recorded by a local seismograph network and is of particular interest due to its location 90 km from a proposed long-term storage facility for high-level nuclear waste. The earthquake was felt along the southern shore of Georgian Bay with maximum intensity of IV MM. During the first 24 hours after the earthquake occurred, four portable ORION seismograph systems were installed to record possible aftershocks. The main shock on October 20 2005 was preceded by a foreshock 30 sec before it, and was followed by 5 aftershocks within 4-day period. All the epicenters of the foreshock and aftershocks were within 2.5 km from the epicenter of the main shock. The large amount of available data from the recently installed broad-band POLARIS seismograph stations, as well as the permanent CNSN stations and the temporary stations, gave us a unique opportunity to study the parameters of this event. The analysis of the foreshock-main shock-aftershock sequence indicated focal depths around 7 to 12 km. The focal mechanism calculated from the polarities of P-arrivals showed predominantly thrust mechanism of the main shock, with nodal planes oriented almost NW-SE. The focal mechanism is very similar to the predominant focal mechanism of the earthquakes in Western Quebec Seismic Zone but different from the predominant strike-slip focal mechanisms south of Lake Erie and the oblique slip mechanisms in western Lake Ontario. Aeromagnetic data reveal a prominent NW-SE structural fabric for the basement rocks beneath Georgian Bay, in good agreement with the orientation of the nodal planes. This structural fabric probably reflects mafic dykes (the Matachewan dyke swarm). The spectra of S-waves, recorded at 13 bedrock stations, were fitted with Brune’s model and used to calculate the seismic moment (3.6e+14 N.m), source radius (~ 400 m), stress drop (~ 20 bars), and moment magnitude (Mw 3.7). This seismic moment and calculated focal mechanism were used as initial approximation for seismic moment tensor inversion. The results of the inversion showed correspondence between the seismic moment and double-couple focal mechanism calculated from the moment tensor

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