b t e Holocene sedimentation in Saanich Inlet, British Columbia, and its paleoseismic implications1 A. Blais-Stevens, J.J. Clague, P.T. Bobrowsky, and R.T. Patterson Abstract: 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 2BoPb data, I4C 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 5946 near Comox, 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. WQum4 : Huit Cchantillons de sediment prClevCs par carottier a piston, s'tchelonnant sur les derniers 1500 ans, ont CtC collectks dans le bras de mer Saanich, un fjord anoxique de l'ile de Vancouver, pour obtenir des donndes sur la ~Cdimentation et I'activitC de tremblements de terre prkhistoriques. Les carottes sont composCes principalement de skdiments varcrCs a grain fin, mais elles incluent des couches massives dCposCes par des coulCes de debris sous-aquatiques. Ces coulCes de dCbris ont pu Ctre provoqukes par des tremblements de terre ou par une accumulation excessive de stkliment fin sur les versants du bras de mer. Les donnkes de '33Ce et 2ioPb,les dges au I4C et le comptage de varves ont CtC utilisCs pour dater et la mise en corrClation des couches massives dans les huit carottes. La couche massive sommitale dans deux carottes correspond probablement au tremblement de terre de magnitude 7,2 survenu en 1946 prks de Comox, en Colombie-Britannique, h 200 km au nord-nord-ouest du bras de rner Saanich. Sept couches plus anciennes apparaissnt dans deux carottes ou plus, et leurs fges sont estimCs a 200, 440, 550, 808-850, 1050- 1100, 1100- 1150 et 1450- 1500 ans. Beux de ces anciennes couches peuvent Ctre corrClCes avec d'anciens tremblements de terre documentks pour cette rCgion. Dans la carotte possCdant le plus grand nombre de telles couches, on observe en moyenne une couche massive par 116 varves, ce qui concorde grossi2rement avec la pCriodicitC estimCe des tremblements de teme de magnitude forte ou modCrCe dans la rCgion-il y a en moyenne un tremblement de terre d'intensitC VII ou VIII a 1'Cchelle locale modifike de Mercalli par sikcle. I1 est possible que les skdiments du bras de mer Saanich aient pu enregistrer tous l a tremblements de terre, forts et modCrCs, qui ont affect6 le sud-ouest de la Colombie-Britannique durant l'Holockne, mais pour certaines couches la correlation de carotte carotte n'est pas Cvidente, et incontestablement ces couches reprbentent des dCpiits qui ont kt6 engendrCs par un processus autre que sismique. [Traduit par la rddaction] Received October 16, 1996. Accepted May 7, 1997. A. Blais-Stevens.' Geological Survey of Canada, 601 Booth Street, Ottawa, ON KIA 0E8, Canada. J.J. Clague. Geological Survey of Canada, 101 - 605 Robson 1 Street, Vancouver, BC V6B 5J3, Canada, and Department of Earth Sciences, Simon Fraser University, Burnaby, BC V5A LS6, Canada. P.T. Bobrowsky. Geological Survey BrancR, P.O. Box 9320, Victoria, BC V8W 9N3, Canada. R.T. Patterson. Ottawa - Carleton Geoscience Centre and Department of Earth Sciences, Carleton University, Ottawa, ON K1S 5B6, Canada. <eological Survey sf Canada Contribution 1996249; British Columbia Geological Survey Contribution 0059. Corresponding author (e-mail: ablaisagsc. nrcan. gc. ca) . Can. J. Earth Sci. 34: 1345- 1357 (1997) introduction Southwestern British Columbia is one of the most tectoni- cally active areas in Canada, and there is concern that a moderate to large earthquake may damage the cities and eeo- nomic infrastructure of the region. Seismic activity is related to subduction of tRe oceanic Juan de Fuca plate beneath North America along the Cascadia subduction zone (Riddi- hough and Hyndman 1976) (Fig. 1). Rare, great (magnitude (M) r 8) earthquakes occur at the boundary between the Juan de Fuca and North America plates (Rogers 1988; Atwater et al. 1995; Hyndman 1995), and smaller, more frequent earthquakes are centred within the two plates (Shedlock and Weaver 1991 ; Rogers 1994). Four large earthquakes have struck the region in historical time, in 1918 (M = 7) and 1946 (M = 7.2) on Vancouver Island, in 1872 (M = 7.4) O 1997 NWC Canada