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1 Department of Earth Sciences
University of Western Ontario
London, Ontario, Canada N6A 5B7
On 20 October 2005 at 21:16 UTC, a moderate earthquake
(mN 4.3) occurred in an area of low seismicity within
Georgian Bay, approximately 12 km north of Thornbury, Ontario
(44.67° N,
80.46° W). Despite
its moderate magnitude, it was exceptionally well recorded and is of particular
interest because of its location 90 km from a proposed long-term storage
facility for low- and medium-level nuclear waste. No damage was reported, but
ground shaking was felt to a distance of 100 km. Within 24 hours after the
mainshock, four portable seismograph systems were installed in the epicentral
region. In total, eight events were recorded over a 4-day period, including a
foreshock and six aftershocks. The unusually rich dataset from this moderate
earthquake sequence enabled robust determination of hypocentral parameters,
including well-constrained focal depths for most events. For the mainshock, we
estimated a seismic moment of M0 4.5
x 1014 N
m and corner frequency of
3.7 Hz, based on a
spectral fit using
Brune’s source
model. Least-squares waveform inversion of P and S phases
yielded a double-couple focal mechanism with a reverse- sense of slip and
northwest-striking nodal planes. The reverse mechanism and midcrustal focal
depths (10–12 km)
are characteristic, in general, of more abundant seismicity located
200 km northeast
of this event in the western Quebec seismic zone. These parameters differ,
however, from shallow
(2–6 km)
earthquakes, with predominantly strike-slip mechanisms, observed near Lake Erie
200 km to the
south. We attribute this
north–south change
in rupture mechanism to variations in crustal stress induced by postglacial
isostatic rebound. Aeromagnetic data in and around the epicentral region reveal
prominent northwest-striking lineations caused by Precambrian mafic dykes. Under
midcrustal conditions, the dyke material is mechanically stronger than generally
more felsic upper-crustal host rocks. We propose that where large dykes are
favorably oriented with respect to the stress field, they may strongly influence
the locations of intraplate earthquake rupture in Shield regions.
Related articles in Bulletin of the Seismological Society of America:
This article has been cited by other articles:
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  G. Ranalli and M. Lamontagne Comment on "The October 2005 Georgian Bay, Canada, Earthquake Sequence: Mafic Dykes and Their Role in the Mechanical Heterogeneity of Precambrian Crust" by S. Dineva, D. Eaton, S. Ma, and R. Mereu Bulletin of the Seismological Society of America, February 1, 2010; 100(1): 402 - 405. [Abstract] [Full Text] [PDF]
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S. Dineva and D. W. Eaton Reply to "Comment on 'The October 2005 Georgian Bay, Canada, Earthquake Sequence: Mafic Dykes and Their Role in the Mechanical Heterogeneity of Precambrian Crust' by S. Dineva, D. Eaton, S. Ma, and R. Mereu" by G. Ranalli and M. Lamontagne Bulletin of the Seismological Society of America, February 1, 2010; 100(1): 406 - 407. [Full Text] [PDF]
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 S. Ma and D. W. Eaton Anatomy of a Small Earthquake Swarm in Southern Ontario, Canada Seismological Research Letters, March 1, 2009; 80(2): 214 - 223. [Full Text] [PDF]
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