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 × 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.