A pair of Mw = 5.5 earthquakes occurred beneath the Brooks Peninsula on the northwest coast of Vancouver Island on 2 June and 25 July 1978. These are the largest and best-recorded earthquakes to date in the vicinity of northern Vancouver Island and the adjacent coast margin. A detailed study of these earthquakes was undertaken to examine the contemporary tectonics of this region, specifically the poorly understood interaction between the Explorer plate and the North American plate at the northern end of the Cascadia subduction zone. Both earthquakes were followed by well-defined aftershock sequences. A four-station temporary seismograph array deployed on the Brooks Peninsula following the 2 June mainshock allowed for accurate aftershock locations. This earthquake was located at 50.15° N, 127.84° W, based on the center of a 9-km-diameter circular region of aftershocks. The 25 July earthquake was located 4 to 7 km to the northeast of the June epicenter based on waveform comparison of the two events. Both earthquakes occurred at 15 to 16 km depth. The focal mechanisms as determined from body-wave modeling are nearly identical and show left-lateral strike-slip motion along a shallow north-dipping, east-west-striking fault. The focal mechanism and depth of these two earthquakes indicates that they were not megathrust events on the Explorer/North America plate boundary, but rather that they occurred within the North American plate, 5 to 10 km above the megathrust. The northeast-directed pressure axes for these earthquakes suggests coupling across the Explorer-North America segment of the Cascadia subduction zone, consistent with contemporary convergence of the Explorer Plate with the North American plate in a northeast-southwest direction.
