Cascadia Earthquake
At 9 PM on January 26, 1700 one of the world’s largest earthquakes occurred along the west coast of North America. The undersea Cascadia thrust fault ruptured along a 1000 km length, from mid Vancouver Island to northern California in a great earthquake, producing tremendous shaking and a huge tsunami that swept across the Pacific. The Cascadia fault is the boundary between two of the Earth’s tectonic plates: the smaller offshore Juan de Fuca plate that is sliding under the much larger North American plate.
The earthquake shaking collapsed houses of the Cowichan people on Vancouver Island and caused numerous landslides. The shaking was so violent that people could not stand and so prolonged that it made them sick. On the west coast of Vancouver Island, the tsunami completely destroyed the winter village of the Pachena Bay people with no survivors. These events are recorded in the oral traditions of the First Nations people on Vancouver Island. The tsunami swept across the Pacific also causing destruction along the Pacific coast of Japan. It is the accurate descriptions of the tsunami and the accurate time keeping by the Japanese that allows us to confidently know the size and exact time of this great earthquake.
The earthquake also left unmistakeable signatures in the geological record as the outer coastal regions subsided and drowned coastal marshlands and forests that were subsequently covered with younger sediments. The recognition of definitive signatures in the geological record tells us the January 26, 1700 event was not a unique event, but has repeated many times at irregular intervals of hundreds of years. Geological evidence indicates that 13 great earthquakes have occurred in the last 6000 years.
We now know that a similar offshore event will happen sometime in the future and that it represents a considerable hazard to those who live in southwest B.C. However, because the fault is offshore, it is not the greatest earthquake hazard faced by major west coast cities. In the interval between great earthquakes, the tectonic plates become stuck together, yet continue to move towards each other. This causes tremendous strain and deformation of the Earth’s crust in the coastal region and causes ongoing earthquake activity. This is the situation that we are in now. Some onshore earthquakes can be quite large (there have been four magnitude 7+ earthquakes in the past 130 years in southwest B.C. and northern Washington State). Because these inland earthquakes can be much closer to our urban areas and occur more frequently, they represent the greatest earthquake hazard. An inland magnitude 6.9 earthquake in 1995 in a similar geological setting beneath Kobe, Japan caused in excess of $200 billion damage.
