Slope Failures and Paleoseismicity, Effingham Inlet, Southern Vancouver Island, British Columbia, Canada

Abstract

Sediments from Effingham Inlet, an anoxic fjord on the west coast of Vancouver Island, were studied for their possible relation to paleoseismic activity. Diatomaceous sediments carry a complex history of episodic turbidites and deformation structures set against a background of varved sedimentation. The turbidity currents were initiated as fjord sidewall slope failures many of which were earthquake-induced. Accurate dating of 24 turbidites over approximately 3,300 years of the inner basin record was obtained. Known earthquakes recorded by turbidites include the AD 1946 Vancouver Island earthquake and AD 1700 plate-boundary earthquake. Comparison of this event history and a similar record from Saanich Inlet (130 km away) yielded 8 contemporaneous slope failures over the past 1,500 years.