Abstract

A reanalysis of the varve chronology from hydraulic piston sediment cores was carried out to establish better uncertainty estimates on ages of prehistoric debris-flow deposits (DFDs) in the last 4000 yr. Saanich Inlet is an anoxic fiord located in southeast Vancouver Island near the city of Victoria, British Columbia. It contains annually laminated (varved) marine mud deposited in anoxic conditions. Interlayered with these Holocene varves are massive layers of coarser sediments deposited by submarine debris flows. It has been previously interpreted that these flows were induced by earthquake shaking. Two of the DFDs correspond to known earthquakes: A.D. 1946 Vancouver Island (M 7.3) and the A.D. 1700 Cascadia plate-boundary subduction earthquake (M 9). Based on varve counts, 18 DFDs (310, 410–435, 493–582, 767–887, 874–950, 1001–1133, 1163–1292, 1238–1348, 1546–1741, 1694–1811, 1859–2104, 2197–2509, 2296–2483, 2525–2844, 2987–3298, 3164–3392, 3654–4569, 3989–4284 yr ago from A.D. 2010 datum) were correlated among two or more cores during this time period, suggesting an average return period of strong shaking from earthquakes of about 220 yr. Nine of the DFDs overlap with the age ranges for great plate-boundary earthquakes that have been determined by other paleoseismic studies: coastal subsidence and offshore turbidity deposits. The remaining nine events give an average return period of about 470 yr for strong shaking from local earthquakes. The peak ground acceleration calculated from a recurrence relation based on statistics from local earthquakes for a 470-yr period is 0.30g, which corresponds to the upper range of Modified Mercalli Intensity (MMI) VII (seven). Historical data from Vancouver Island and other areas show that this level of shaking (MMI VII) is sufficient to trigger submarine landslides.

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