Late Holocene subaqueous landslide activity within Horne Lake, Vancouver Island, British Columbia, Canada

Sub-bottom acoustic profiling and multibeam bathymetry surveys were undertaken at Horne Lake, Vancouver Island, British Columbia, to investigate subaqueous landslide activity. These revealed 57 late Holocene landslides within the three lake sub-basins. Event horizon mapping revealed that stratigraphically older landslides are also present. Three of these maps depict multiple mass transport deposits (MTDs), and a fourth map shows a single laterally extensive turbidite bed. Three mechanisms were assessed to account for the late Holocene landslide activity. A rock slide-generated tsunami was rejected because there is no evidence of a recent, major rock slide entering the lake. Two MTDs within Paradise and Deadhead bays are covered with numerous submerged tree trunks and are interpreted to be drawdown failures most likely triggered in 1964, when the greatest net drawdown happened during initial operations of the Horne Lake reservoir. To further evaluate the drawdown mechanism, an effective stress analysis yielded factors of safety (FoS) of 1.1 - > 1.8 at 31 of 43 studied failure sites during the 1964 drawdown. These 31 sites were probably stable, but up to nine sites with FoS < 1.0 could be drawdown failures. Published estimates of shaking intensity (MMI ~ VII) in the Horne Lake area indicate that the M9 1700 Cascadia and/or M7.3 1946 Vancouver Island earthquakes probably triggered most of late Holocene landslides, but additional research is needed to determine the relative importance of each earthquake. This study demonstrates the importance of assessing both seismic and aseismic mechanisms when investigating subaqueous landslides.