The 28 October 2012 Haida Gwaii, British Columbia, Canada, earthquake with a moment magnitude (MW) of 7.8 occurred along an east-dipping poorly known thrust fault beneath the Queen Charlotte Terrace. It was the largest thrust event ever recorded in this dominated by strike-slip motion region. We studied the focal mechanism and the source rupture process for the event. The retrieved geometric parameters of the fault plane were a strike of 329°, dip of 24°, and slip of 114°. The isotropic moment was negative, and its value was about one-fifth of the total seismic moment released. The earthquake ruptured an area of about 160 km × 60 km, and major slip occurred in an area of about 100 km × 60 km. The maximum slip was about 5.8 m. The slip distribution on the fault plane was highly heterogeneous, with four slip patches. The main slip lay on a large zone above the hypocentre to the sea floor. The maximum and average stress drops calculated using the Brune model were 16.5 and 4.6 MPa, respectively. The major rupture occurred about 10 s after the rupture initiation, and lasted about 25 s. During a subducting earthquake, the leading edge of the overriding plate is assumed to spring seaward and upward, while the landward portion is assumed to extend and drop down, and the generated rapid motions set off a tsunami. The falling-down process seems to be consistent with a negative isotropic moment.