LATE HOLOCENE SEDIMENTATION IN SAANICH INLET, BRITISH-COLUMBIA, AND ITS PALEOSEISMIC IMPLICATIONS

Eight piston cores of sediment spanning the last 1500 years were colle cted from Saanich Inlet, an anoxic fiord on southern Vancouver Island, to obtain information on sedimentation and prehistoric earthquake act ivity. The cores consist mainly of fine-grained varved sediments, but include massive layers deposited by subaqueous debris flows. The debri s flows may have been triggered by earthquakes or by the buildup of fi ne sediment on the walls of the inlet. Cesium-137 and Pb-210 data, C-1 4 ages, and varve counts were used to date and correlate massive layer s in the eight cores. The uppermost massive layer in two cores may rec ord a magnitude 7.2 earthquake that occurred in 1946 near Comex, Briti sh Columbia, 200 km north-northwest of Saanich Inlet. Seven older laye rs 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 la yers may correlate with previously documented earthquakes in the regio n. 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 I ntensity VII or VIII per century. Saanich Inlet may contain a proxy re cord of all moderate and large earthquakes that have affected southwes tern British Columbia during Holocene time, but some of the massive la yers do not appear to correlate from core to core and undoubtedly are nonseismically generated deposits.