Earthquake Hazard GIS Modelling in the Greater Vancouver Regional District

Methodology

The methodology used in my Relative Earthquake Hazard GIS Mapping Model is broken down into 4 steps:

1. Mapping Amplification of Soil
2. Mapping Earthquake Intensity
3. Mapping Liquefaction Susceptibiliy
4. Mapping Slope Instability

Different factors are used in mapping each step above. The factors are selected according to the information available in my GIS data that are discussed below (Note: the information available are also briefly discussed in the previous Overview and Data Sections). The 4 maps above are then combined to produce the Relative Earthquake Hazard Map in the GVRD using the weighted linear combination model of the multi-criteria analysis (MCE). This MCE spatial anlysis is discussed in the next section Spatial Analysis.

1. Mapping Amplification of Soil

According to Victoria Relative Amplification of Ground Motion Hazard Map by Ministy of Energy and Mines and Liquefaction Hazard and Shaking Amplification Maps of Alameda, and etc.....by USGS, Amplification are highest in the following areas:

• Thick (> 10m) deposits of soft clay
• Peat and organic soils

Note: Amplification is lowest in exposed bedrock.

Thickness.rdc and Descriptin.rdc are reclassified and overlayed to produce a map of thick clay. Descriptin.rdc are again reclassified to produce a relative amplification in different soil texture and it is overlayed with thick clay to produce a final map of soil amplification. See Soil Amplification Cartographic Model for the described processes and the map produced.

2. Mapping Earthquake Intensity

Degree of closeness to fault line is used in mapping the earthquake intensity. To do this, IDRISI Distance Module and Fuzzy Module is applied on the fault lines (faults.rdc) to produce a final map of earthquake intensity. See Earthquake Intensity Cartographic Model for the described processes and the map produced.

3. Mapping Liquefaction Susceptibilty

Two main factors are considered for mapping liquefaction:

• Age of Underlying deposits (It is widely accepted that recent sediments or fills of saturated, cohesionless soils at shallow depths will liquefy in a large magnitude earthquake. Source:Relative Amplification of Ground Motion Hazard Map of Greater Victoria and Analysis of Soil Borings for Liquefaction Resistance). stratage.rdc and lithology.rdc were used in mapping this. Stratage.rdc is reclassified chronologically based on Geological Timescale website. Lithology.rdc is reclassified to Deposit Age according to QUATERNARY GEOLOGY OF SOUTHEASTERN VANCOUVER ISLAND AND GULF ISLANDS by Ministry of Energy and Mines. Then the reclassified stratage.rdc and lithology.rdc are overlayed to produce a map of Age deposits and clipped to the GVRD municaplity boundaries. I've clipped this because stratage.rdc have include values for the ocean which are not needed for this analysis. The clipped map of Age deposits are reclassified to different levels of liquefaction based on Table 7.1 from Analysis of Soil Borings for Liquefaction Resistance.)
  
• Soil Texture (Liquefaction is highest in soils of coarse texture sand and silt. descriptin.rdc (soil texture description data) is reclassified to different levels of liquefaction based on Table 1 from Assessment Of Susceptibility To Soil Liquefaction In Taiwan to produce a map of liquefaction due to soil texture.

The resulted liquefaction maps (based on the above 2 factors) are then overlayed and standardized (to values of 0 to 255) to create a final map of liquefaction susceptibility. See Liquefaction Susceptibility Cartographic Model for the described processes and map produced.

4. Mapping Slope Instability

Three main factors (based on GeoMap Vancouver by Natural Resources Canada, Mapping Ski Hill Avalanche Terrain in the Vancouver, B.C., Area by Earth Observation Magazine, and Avalanche by Glacier Valley Education)are considered for mapping slope instability:

• Bedrock Instability (Resistant to disintegration or forms of weathering)
• Aspect (Leeward slope (northeastern-, eastern-, or southeastern-facing) have more dangerous avalanches than windward slope)
• Slope Degree (Lanslides generally occurred on slopes > 20 degrees in the Vancouver Area)

Bedrock instability is derived by reclassifying bedrock.rdc to different levels of weathering resistance with values between 0-255. gvrd_dem.rdc is used to derive maps of Aspect and Slope Degree. Fuzzy model is then applied indiviually on Aspect and Slope maps to derive two maps: 1) a map with increasing intensity values toward the leeward slope. 2) a map of increasing intensity values toward slopes > 20 degrees. The three factors (Bedrock instability, Aspect intensity, and Slope degree intensity) are then combined using multi-criteria (MCE) analysis weighted-linear combination model using two constraints: Undeveloped areas (derived by reclassifying landuse.rdc) and Highlands with elevation > 5m (derived from gvrd_dem.rdc). I've used these constraints because I've assumed that landslides will only occur in those two areas. See Slope Instability Cartographic Model for the described processes, the weights used in the MCE analysis and the map produced.

Click on spatial analysis to see the MCE analysis on mapping Relative Earthquake Hazards in GVRD.