More than 200 earthquakes are recorded each year on the Lower Mainland and Vancouver Island. Although most are too small to be felt, an earthquake capable of structural damage can be expected to occur somewhere in the region about once every ten years. The surface of the earth is always moving. These movements cause stress to build up within the crust. When the strain becomes too great, the stressed rock breaks suddenly, causing an earthquake.
Earthquakes occur most frequently where plate interact.
Southwest British Columbia overlies a subduction zone , where one crustal plate slides beneath another. The Juan de Fuca subduction zone stretches from a point off the coast of northern Vancouver Island south to California. Pacific plates are separated by the much smaller Juan de Fuca Plate, a fragment of ocean floor that is sliding down under the continental margin. Beneath southwest British Columbia, the Juan de Fuca Plate subducts at a rate of about 4 cm per year. The stresses built up by this subduction process cause frequent earthquakes in the Vancouver Island - Lower Mainland region.