Late Holocene sedimentation in Saanich Inlet, British Columbia, and its paleoseismic implications

Eight piston cores of sediment spanning the last 1500 years were collected from Saanich Inlet, an anoxic fiord on southern Vancouver Island, to obtain information on sedimentation and prehistoric earthquake activity. The cores consist mainly of fine-grained varved sediments, but include massive layers deposited by subaqueous debris flows. The debris flows may have been triggered by earthquakes or by the buildup of fine sediment on the walls of the inlet. Cesium-137 and Pb-210 data, C-14 ages, and varve counts were used to date and correlate massive layers in the eight cores. The uppermost massive layer in two cores may record a magnitude 7.2 earthquake that occurred in 1946 near Comex, British Columbia, 200 km north-northwest of Saanich Inlet. Seven older layers are found in two or more cores and are about 200, 440, 550, 800-850, 1050-1100, 1100-1150, and 1450-1500 years old. Two of these older layers may correlate with previously documented earthquakes in the region. There is an average of one massive layer per 116 varves in the core with the greatest number of such layers, which is broadly consistent with the expected periodicity of moderate to large earthquakes in the region, on average, one earthquake producing local Modified Mercalli Intensity VII or VIII per century. Saanich Inlet may contain a proxy record of all moderate and large earthquakes that have affected southwestern British Columbia during Holocene time, but some of the massive layers do not appear to correlate from core to core and undoubtedly are nonseismically generated deposits.