NWT RESEARCH DATABASE

Tags: geology, permafrost, sediment chemistry, Beaufort Sea, coastal environment

Objective(s): To evaluate possible ground motion amplification due to permafrost variability in the Mackenzie-Beaufort area and to examine coastal sediment exposures and drained lake basins for evidence for paleo-tsunamis and glacial deposits.

Project Description: The objectives of this research project are to evaluate possible ground motion amplification due to permafrost variability in the Mackenzie-Beaufort area and to examine coastal sediment exposures and drained lake basins for evidence for paleo-tsunamis and glacial deposits. Activity 1: Evaluation of possible enhanced ground motion amplification due to permafrost variability in the Mackenzie-Beaufort area. In addition to the understanding of the regional tectonic setting, understanding the local site effects is an important part of any seismic hazard assessment. Site effect or “ground motion amplification” is an estimate of the response of soil layers when an earthquake occurs and how this variation is felt at the ground surface where people and structures are located. A particular concern in the Mackenzie-Beaufort area is the effect of variable permafrost thickness on ground motion amplification. Given the substantial changes in sediment strength associated with different permafrost regimes, there are many locations where significant ground motion amplification could have occurred. Activity 2: Examination of coastal sediment exposures and drained lake basins for evidence for paleo-tsunamis and glacial deposits. There is potential for a tsunami to occur in the outer Mackenzie Delta, caused by under ocean slides and or large magnitude earthquakes. In an effort to understand this potential, we can look for evidence of past tsunami events (paleo-tsunamis) in low-lying thermokarst lake sediments along the outer Mackenzie Delta, which have the potential to preserve the record of past tsunami events going back thousands of years. Activity 1: A portable, low-cost engineering geology instrument (Tromino) will be used to passively record ambient vibrations of the soil. Sites will primarily be at industry well locations with varying permafrost conditions where existing geological, geophysical, and geotechnical data exist that will aid in the site response analysis. A transect is also proposed where the Geological Survey of Canada conducted a detailed EM survey in the early 1990s. Additional sites may be chosen on an opportunity basis in collaboration with other Natural Resources Canada researchers. Data collection is relatively straightforward: 1) A small section (~30 cm x 30 cm) of the ground surface is prepared to ensure good contact with the instrument by removing any loose debris such as sticks or thick moss cover. 2) The instrument is placed on the ground surface with its four short (~5 cm) metal prongs inserted into the soil. 3) The instrument is left to record for 30 minutes. Data will be recorded at a minimum of three locations at each site. It is expected that each site can be completed in ~2 hours. The resultant seismic amplification information will be analysed and used to help constrain the geological and geotechnical models used in the computation of the Seismic Hazard Values in the 2020 National Building Code of Canada. This in turn will guide the design of new infrastructure that will be required for the development of resources in the Mackenzie-Beaufort area. Activity 2: For this preliminary study, two field techniques will be employed. 1) Conducting stratigraphic examinations of coastal exposures for evidence of materials that could have been deposited by a tsunami. The work at this stage is mainly descriptive with detailed note-taking, along with photography of the exposures. 2) Taking push-core samples in coastal drained lake basins for subsequent laboratory examination of the sediments to identify potential tsunami-deposits. A push-core is a small diameter (1-2”) probe that is pushed into the soil surface to the point of refusal (likely ~1 m or the maximum active layer thickness). Recognition or absence of a paleo-tsunami record would contribute to the understanding of the potential seismic hazard as well as direct further research. The research team hopes to have an ARI summer student be a field assistant on some of the field days to gain experience conducting geoscience-base field work. Copies of all scientific contributions (scientific papers, maps, and databases) will be provided to the communities in the ISR through the ARI. The research team hopes to have a representative attend the Yellowknife Geoscience Forum in November 2014 to present the preliminary results of this work. The fieldwork for this study will be conducted from July 24, 2014 to August 17, 2014.