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At 9PM 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.