Paleoseismic signature in late holocene sediment cores from Saanich Inlet,British Columbia

This paper explores the paleoseismic record potentially preserved in the up per 40 m of hydraulic piston cores collected in 1996 at two sites in Saanic h Inlet, British Columbia, during ocean drilling program (ODP) Leg 169S. Th e ODP cores are missing 1-2 m of water-rich sediment directly underlying th e seafloor, but this sediment is preserved in shorter piston cores collecte d in 1989 and 1991. The upper part of the ODP cores consists of rhythmicall y laminated (varved) marine mud with intercalated massive beds, interpreted to be debris flow deposits. Some of the debris flow deposits are linked to past earthquakes, including the 1946 Vancouver Island earthquake (M7.2), a great (M8-9) plate-boundary earthquake at the Cascadia subduction zone in January 1700, and a large crustal or plate-boundary earthquake about 1000 y r ago. Earthquakes may also be responsible for debris flows in about AD 160 0, 1500, 1250, 1150, 850, 450, 350, 180, and BC 200, 220, 500, 900, and 105 0. If so, the average recurrence interval for moderate to large earthquakes , which trigger debris flows in Saanich Inlet, is about 150 yr. This recurr ence interval is broadly consistent with the frequency of moderate to large earthquakes in the region during the historical period. Debris flows, howe ver, can also be triggered by non-seismic processes, making it difficult to assemble a complete earthquake record from the Saanich Inlet cores. We pro pose that extensive debris flow deposits, emplaced by single large failures or many smaller coincident failures, probably have a seismic origin. (C) 2 001 Elsevier Science B.V. All rights reserved.