Method

The first part of the project was to get information which would help us in creating our final map. The base map of the GVRD and the DEM data were provided by the UBC Department of Geography. The surficial material map was an image which we downloaded off of the Natural Resources Canada website. Before starting our analysis, we first had to digitize the surficial material map.

Surficial Materials

The map was georeferenced to the base map of the GVRD and from there we began the process of digitizing. This was a long process but in the end it produced a map that would be a large part of our analysis. The digitized data was in vector format but for our analysis, it needed to be in raster format because the final map would be produced using the raster calculator. The information on surficial materials is important because it influences how an earthquake will behave in certain areas. Earthquake intensity increases when the waves travel through less dense materials and intensity decreases when traveling through denser materials. It does this because the waves slow down in less dense materials and when it slows down, its amplitude increases which causes an increase in intensity. In addition, the surficial materials also play a role in liquefaction hazards.

GVRD Outline

While not being a major part of our analysis, this layer was used several times to create our final map. More importantly, it was used when we digitized the coast line to create our tsunami hazard map.

Digital Elevation Model (DEM)

The DEM was another very important part of our project. With the DEM we were able to calculate slope and tsunami hazards. Slopes greater than 30 degrees are more prone to landslides with the risk increasing as slope aspect increases. For a landslide to transpire, a trigger mechanism must occur. Often the trigger is rain or gravity, but an earthquake is a very important trigger and when a large 9+ magnitude earthquake does strike, those slopes will not be the safest places to be. The elevation information was used to realize which areas were prone to flooding from a tsunami.

Analysis

After we digitized the soil map, through on-screen digitizing, acquired from Natural Resources Canada, we used the spatial analyst to convert the data from vector to raster. Thereafter, the digitized map was converted into the earthquake intensity map. There were fourteen different categories of surficial materials. Starting from the least dense surficial material such as landfill and silt to the denser granitic rock and foliated volcanic rock. As stated previously, earthquake intensity will increase as the density of the surfacial material decreases. With that in mind, we created our first map, the GVRD Surficial Material Map. The lighter colours indicate the areas with the least dense surface materials while the darker colours indicate more dense surface materials. Therefore, earthquake intensity should be greater in the areas with a lighter colour. The least dense areas were given a hazard value of 3.5 decreasing by a value of .25 for each category of increasing density.

The surficial material layer was also used to generate a map for liquefaction hazards. During intense earthquake, water saturated surface materials such as silt and clay will lose its strength and turn into a liquid. Areas where the surficial materials are landfill, peat, silt, sand, clay, gravel and loam have high liquefaction potential. In our analysis, we selected all of the areas which had these characteristics. From there we generated a map showing liquefaction hazards in the GVRD. Areas which were prone to liquefaction were given a hazard value of 1 and areas not prone to liquefaction were given a value of 0.

A map showing areas which were prone to landslides was created using slope data which we generated from a DEM of the GVRD. The slopes were reclassified to 4 different categories. Slopes between 0-25 degrees were assigned with a value of 0. The other 3 categories 26-35, 36-55 and 56-73+ degrees were given a value of 1, 2 and 3, respectively.

Tsunamis are a great hazard to coastal areas, but the GVRD is somewhat protected by Vancouver Island. However, a tsunami can still potentially occur and since the goal of this project is to create a hazard map for the GVRD, we need to assume the worst case scenarios. According to the North Shore Emergency Management Office, areas below 25 feet and areas within 1.6 km of the coast are most vulnerable to a tsunami. Therefore, we decided to digitize around the coast line of the GVRD.

After digitizing we created a 1.6km buffer around the digitized coastline. From there, we selected all of the areas within the buffer which had an elevation of 25 feet or less and these areas would become the locations which would be vulnerable to a tsunami. They were given a hazard value of 1.