THE UNDERAPPRECIATED RISK OF LANDSLIDE-TRIGGERED TSUNAMIS IN BRITISH COLUMBIA

Tsunamis triggered by landslides are a significant, but under-documented hazard in British Columbia (B.C.). Much of the B.C’s coast is dissected by fjords with steep rock walls that extend up to hundreds of metres above sea level. The coast is situated along an active plate margin subject to frequent large earthquakes. Large deltas, located at the heads of all B.C. fjords, comprise thick sequences of loose, water-saturated sediments that are prone to mass movements. Historical and prehistoric records, combined with experience from other areas that have similar fjord coastlines, notably Norway and Chile, indicates that rockslides and rockfalls will trigger destructive and potentially deadly tsunamis along the British Columbia coast in the future. Tsunamis are also likely to occur during subaqueous delta-front mass movements, as happened for example at Kitimat in 1975. Landslides from steep mountain slopes into lakes and landslides into major rivers are also a concern. British Columbia has nearly 14,000 large (>10 ha) lakes, including nearly 120 hydroelectric reservoirs. Many of these lakes are bordered by steep, unstable rock slopes. Any rockslide larger than about 1 Mm³ would generate displacement waves with run-ups of metres to tens of metres along the adjacent shorelines. A 2-3 Mm³ rockslide into Chehalis Lake, 75 km east of Vancouver, in December 2007 destroyed forest up to 30 m above the shoreline, as well as three campsites that fortunately were unoccupied at the time. The possibility of a very large landslide (>10 Mm³) damaging or overtopping a hydroelectric dam in B.C., is low, but not zero. A greater concern is the rapidly growing residential and touristic development at the shorelines of B.C. lakes, which is happening without consideration of the possibility of landslide-triggered tsunamis.