Andrée Blais-Stevens

A Revised Earthquake Chronology for the last 4,000 Years Inferred from Varve-Bounded Debris-Flow Deposits beneath an Inlet near Victoria, British Columbia

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.

Canadian Technical Guidelines and Best Practices Related to Landslides: a National Initiative for Loss Reduction

In the IPL Project #149 – Canadian Landslide Best PracticeManual, the Earth Science Sector of Natural Resources Canada, through its Public Safety Geoscience Program, has initiated the development of national technical guidelines and best practices related to landslides.With this initiative, Canada, as a world leader in the field, will actively contribute to reducing losses from landslides. The guidelines will provide Canadian engineers, geoscientists and other landslide practitioners with a state-of-the-art document related to the science and applied science of landslides and associated loss reduction. This paper summarizes the approach taken to develop the national technical guidelines on landslides and presents the proposed contents of the document.

Landslide Inventory and Susceptibility Mapping for a Proposed Pipeline Route, Yukon Alaska Highway Corridor, Canada

In Canada’s north, slope stability is a critical issue affecting infrastructure. There are several types of landslides and ground hazard features that are directly related to the presence of permafrost: active layer detachment slides, retrogressive thaw flows, solifluction, thermo-karstic depressions, and rock glaciers.

Current Status of Landslide Guidelines Around the World

The Geological Survey of Canada (GSC) initiated a project in 2009 to develop national landslide guidelines and best practices as part of its natural hazard loss reduction effort. This project is part of the International Program on Landslides sponsored by the International Consortium on Landslides. A literature review was carried out as part of this project. More than 30 landslide guidelines from around the world were collected. This paper presents a brief review of these guidelines. The review will assist in the development of the GSC’s landslide guidelines, as well as aid professionals and other stakeholders who wish to learn more about or develop their own landslide guidelines.

Landslide susceptibility modelling using the quantitative random forest method along the northern portion of the Yukon Alaska Highway Corridor, Canada

The random forest method was used to generate susceptibility maps for debris flows, rock slides, and active layer detachment slides in the Donjek River area within the Yukon Alaska Highway Corridor, based on an inventory of landslides compiled by the Geological Survey of Canada in collaboration with the Yukon Geological Survey. The aim of this study is to develop data-driven landslide susceptibility models which can provide information on risk assessment to existing and planned infrastructure. The factors contributing to slope failure used in the models include slope angle, slope aspect, plan and profile curvatures, bedrock geology, surficial geology, proximity to faults, permafrost distribution, vegetation distribution, wetness index, and proximity to drainage system. A total of 83 debris flow deposits, 181 active layer detachment slides, and 104 rock slides were compiled in the landslide inventory. The samples representing the landslide free zones were randomly selected. The ratio of landslide/landslide free zones was set to 1:1 and 1:2 to examine the results of different sample ratios on the classification. Two-thirds of the samples for each landslide type were used in the classification, and the remaining 1/3 were used to evaluate the results. In addition to the classification maps, probability maps were also created, which served as the susceptibility maps for debris flows, rock slides, and active layer detachment slides. Success and prediction rate curves created to evaluate the performance of the resulting models indicate a high performance of the random forest in landslide susceptibility modelling.

Active Layer Detachment Slides and Retrogressive Thaw Slumps Susceptibility Mapping for Current and Future Permafrost Distribution, Yukon Alaska Highway Corridor

The Yukon portion of the Alaska Highway Corridor traverses the discontinuous permafrost zone. Air-photos and high resolution satellite imagery were used to produce an updated landslide inventory (2013) that identified 1,600 landslides in the corridor. Landslide susceptibility models were developed for the corridor for two types of landslides triggered in permafrost, active layer detachment slides (ALD) and retrogressive thaw slumps (RTS), which comprise about 3 % of the inventory. A qualitative heuristic approach was used to combine data layers for slope, vegetation, surficial geology unit, slope aspect, and permafrost distribution. ALD and RTS susceptibility maps were produced for present day permafrost distribution and also equilibrium permafrost distribution resulting from air temperature increases of 1–5 °C. The resulting susceptibility maps indicate that with warming and reduced permafrost extent, there will be fewer zones of high susceptibility. The maps for warmer conditions give a “snapshot” of a potential decrease in zones of high landslide susceptibility, but they do not show the potential landslide occurrence as permafrost warms and thaws. It is expected that as permafrost warms and thaws, ALD and RTS activity will increase until conditions stabilize as permafrost disappears.

Late Glacial and Holocene Sedimentation and Investigation of Fjord Tsunami Potential in Lower Howe Sound, British Columbia

Preliminary investigation of the history of sedimentation in lower Howe Sound fjord near Vancouver, B.C. indicates that no rock slide capable of generating a destructive displacement wave has occurred during the latter half of the Holocene. Possible rock slide run-out deposits were detected along the western margin of Bowen Island. These could date from any time during the Holocene. Investigation of the adjacent slope suggests it is creeping and is not an imminent large rock slide failure.

(Table 1) AMS radiocarbon ages from ODP Sites 169-1033 and 169-1034, supplement to: Blais-Stevens, A; Clague, John J (2001): Paleoseismic signature in late Holocene sediment cores from Saanich Inlet, British Columbia. Marine Geology, 175(1-4), 131-148

This paper explores the paleoseismic record potentially preserved in the upper 40 m of hydraulic piston cores collected in 1996 at two sites in Saanich Inlet, British Columbia, during ocean drilling program (ODP) Leg 169S. The ODP cores are missing 1-2 m of water-rich sediment directly underlying the seafloor, but this sediment is preserved in shorter piston cores collected in 1989 and 1991. The upper part of the ODP cores consists of rhythmically 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 yr ago. Earthquakes may also be responsible for debris flows in about AD 1600, 1500, 1250, 1150, 850, 450, 350, 180, and BC 200, 220, 500, 900, and 1050. If so, the average recurrence interval for moderate to large earthquakes, which trigger debris flows in Saanich Inlet, is about 150 yr. This recurrence interval is broadly consistent with the frequency of moderate to large earthquakes in the region during the historical period. Debris flows, however, can also be triggered by non-seismic processes, making it difficult to assemble a complete earthquake record from the Saanich Inlet cores. We propose that extensive debris flow deposits, emplaced by single large failures or many smaller coincident failures, probably have a seismic origin.