Paleoseismology and seismic hazards, southwestern British Columbia

Abstract

There have been nine magnitude 6-7 earthquakes in southwestern British Columbia and northwestern Washington State since the late 1800s, and geological evidence indicates that even larger earthquakes, with no precedent in the historical period, have occurred in the recent past. Holocene coastal stratigraphy on western Vancouver Island and in Washington State and Oregon records episodic sudden submergence during late Holocene time. The submergence is inferred to be the result of subsidence during great (M8-9) earthquakes on the nearby Cascadia subduction zone. These earthquakes have occurred, on average, once every several hundred years during the late Holocene, most recently about 300 years ago. They were accompanied by large tsunamis that deposited distinctive sheets of sand and gravel in low-lying coastal areas. Abrupt changes in vegetation and sedimentation in coastal wetlands near Vancouver and Victoria about 3600 and 1900 years ago record large earthquakes centred on the Cascadia subduction zone or within the North America or Juan de Fuca plate. Ground shaking during the 1900-year-old earthquake and possibly during younger seismic events generated sand and silt dykes and "blows" on the Fraser River delta and along Serpentine River south of Vancouver. Seismic hazards include ground motion, ground rupture, coseismic subsidence and flooding, tsunamis and seiches, liquefaction, and landsliding. Seismic shaking may directly damage buildings and other structures, and induce destructive secondary phenomena such as liquefaction, landsliding, and fire. Large tsunamis can be triggered by earthquakes on the Cascadia subduction zone or by distant plate-boundary earthquakes elsewhere in the North Pacific Ocean, and are most likely to damage coastal communities on western Vancouver Island. Although an M8-9 subduction earthquake would affect a much larger area than an M6-7 crustal or subcrustal event, its probability of occurrence is about 50 times less. Furthermore, recent earthquakes at Los Angeles (1994, M6.7, U.S. $26 billion damage) and near San Francisco (1989, M7.2, U.S. $10 billion) show how destructive earthquakes of the same size as those that occur regularly in the Pacific Northwest can be. In view of this, a greater effort should be made to better understand the causes of moderate and large earthquakes within the North America and Juan de Fuca plates.