chapter  2
22 Pages

Internal Structure of Earth and Plate Tectonics

Two Cities in Harm’s Way Southwestern British Columbia lies at the western edge of a large tectonic plate (the North American plate). Its two largest metropolitan areasVancouver and Victoria-are located 70 km and 40 km, respectively above the boundary between the North American plate and the oceanic Juan de Fuca plate, which is moving downward beneath the continent ( Figure 2.1 ). The zone between the two plates, which is referred to as the Cascadia subduction zone, extends more than 1000 km southward from Vancouver Island to Northern California. The mega-fault that marks this plate boundary gradually descends eastward from near the ocean fl oor about 220 km west of Victoria to 70 km beneath Vancouver. Rarely-about once every 500 years on average-the fault ruptures and the western edge of North America moves up to several tens of metres westward relative to the ocean fl oor. This sudden slip produces a mega-quake of magnitude 8.5 to 9 that severely shakes the ground surface over an area of tens of thousands of square kilometres in the Pacifi c Northwest. The most recent of these giant earthquake happened in A.D. 1700, prior to European settlement of British Columbia. It spawned a huge tsunami that travelled across the Pacifi c Ocean to Japan, where damaging waves up to several metres high struck the east coast of Honshu. The tsunami was devastating to Native American coastal communities on the Pacifi c coasts of Vancouver Island, Washington, and Oregon.