Abstract

Levelled height differences obtained from first-order vertical control surveys between 1930 and 1984 indicate large apparent elevation changes along the east coast of central Vancouver Island, British Columbia. The levelling survey following the 1946 earthquake (M = 7.3) indicated relative subsidence, likely coseismic, of up to 80 mm for the northern third of the 130 km traverse between Comox and Campbell River. Subsequent surveys indicated relative uplifts within this region at rates averaging 1 mm/year (1946–1977) and 5 mm/year (1977–1984). Tilting along a 90 km traverse across central Vancouver Island between Gold River and Campbell River is not well defined because of uncalibrated magnetic errors present in the 1976–1978 baseline data. Minimal estimates indicate a possible average uplift rate of 1–2 mm/year for the east coast region with respect to the outer coast between 1976 and 1985.Taken at face value, these data are consistent with the crustal deformation sequence expected for an earthquake cycle. The coseismic subsidence has been followed by gradual postseismic recovery totalling 30 mm over 30 years. The apparent recent acceleration of this uplift rate to 5 mm/year could mark a stage of more rapid strain accumulation prior to a seismic event. The sense of tilting, principally upwards to the northwest, is difficult to reconcile with two-dimensional, northeast subduction. If this tilting is being generated at depths directly by the subduction process, it implies that the northern part of the survey area overlies a locked or currently more resistive subduction zone. Alternatively, elastic strain accumulation within the crust across the Beaufort Range (or a subparallel) fault preceding right-lateral strike-slip rupture could also lead to the observed tilt pattern.

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