Ground conditions

The downtown peninsula and the North Shore slopes sit on rock. Richmond, Delta and the filled edges of False Creek sit on softer ground laid down by the Fraser River. Two nearby addresses can respond differently because of what lies beneath them.

Rock carries earthquake waves differently from deep sediment. Loose, wet delta soil can also lose its strength during strong shaking. These are separate effects, and both matter in Metro Vancouver.

Liquefaction happens when strong shaking raises the pressure of water between grains of loose sand. The sand stops carrying the weight above it and the ground temporarily behaves like a thick liquid. Foundations can tilt, roads can sag and buried pipes can rise or break.

Richmond and Delta have the three conditions liquefaction needs: loose sand, shallow groundwater and strong earthquake shaking. Soil tests found many metres of material that could liquefy, with groundwater generally within a few metres of the surface. [2] The soft deposits beneath the delta range from less than 20 m to more than 200 m thick. [1]

Researchers estimate that the shaking needed to start liquefaction in Richmond and Delta occurs, on average, every 100 to 250 years. [2] That is not an earthquake schedule or an estimate of how often visible damage will occur. Soil can begin to liquefy below ground without leaving damage at the surface.

For a Cascadia earthquake above magnitude 8.9, the estimated chance of liquefaction across the area studied is 31 to 57 per cent. [3] The range applies to the study area, not every property on the delta.

The same mechanism affects more than buildings. It can damage roads, buried services and the ground beneath dikes. The Dikes page covers the sections Richmond has had analysed.

Beneath much of Metro Vancouver is the Georgia Sedimentary Basin, a deep bowl of soft sediment. It amplifies slow earthquake waves, the kind that make tall buildings sway. [4]

At a shaking period of two seconds, the provincial assessment estimates average amplification of 1.7 times where the basin is 1 to 2 km deep and 2.63 times where it is 3 to 4 km deep. At one test site in the City of Vancouver, accounting for the basin raised the expected shaking at that period by 24 per cent. [4] These factors apply to that speed of shaking and those basin depths. They are not general multipliers for every earthquake wave.

The offshore Cascadia earthquake produces the long, slow waves that the basin amplifies. This is one reason it can be especially important for tall buildings, even though a nearby crustal earthquake produces more intense shaking in much of the city.

The sixth-generation Canadian seismic hazard model and the 2020 National Building Code do not explicitly account for basin effects. Detailed local mapping helps show the variation that the national model leaves out. [4]

The Metro Vancouver Seismic Microzonation Mapping Project has published 29 maps for the western part of the region. They show variations in shaking, basin depth, liquefaction and landslide susceptibility at neighbourhood scale. [5]

The maps do not yet cover the whole region. A second phase for Pitt Meadows, Maple Ridge and Langley is scheduled for completion in late 2026. [5]

The official maps are free to read at metrovanmicromap.ca. They are the best place to compare the ground beneath specific neighbourhoods.