A review of geological records of large tsunamis at Vancouver Island, British Columbia, and implications for hazard

Large tsunamis strike the British Columbia coast an average of once every s everal hundred years. Some of the tsunamis, including one from Alaska in 19 64, are the result of distant great earthquakes. Most, however, are trigger ed by earthquakes at the Cascadia subduction zone, which extends along the Pacific coast from Vancouver Island to northern California. Evidence of the se tsunamis has been found in tidal marshes and low-elevation coastal lakes on western Vancouver Island. The tsunamis deposited sheets of sand and gra vel now preserved in sequences of peat and mud. These sheets commonly conta in marine fossils, and they thin and fine landward, consistent with deposit ion by landward surges of water. They occur in low-energy settings where ot her possible depositional processes, such as stream flooding and storm surg es, can be ruled out. The most recent large tsunami generated by an earthqu ake at the Cascadia subduction zone has been dated in Washington and Japan to AD 1700. The spatial distribution of the deposits of the 1700 tsunami, t ogether with theoretical numerical modelling, indicate wave run-ups of up t o 5 m asl along the outer coast of Vancouver Island and up to 15-20 m asl a t the heads of some inlets. The waves attenuated as they moved eastward alo ng Juan de Fuca Strait and into Puget Sound and the Strait of Georgia. No d eposits of the 1700 event or, for that matter, any other tsunami, have yet been found in the Strait of Georgia, suggesting that waves were probably no more than 1 m high in this area. If a tsunami like the 1700 event were to occur today, communities along the outer Pacific coast from southern Britis h Columbia to northern California would be severely damaged. There would be little time to evacuate these communities because the tsunami would strike the outer coast within minutes of the first ground shaking. Fortunately, s uch tsunamis are infrequent - perhaps as few as seven have occurred in the last 3500 yr. Other tsunamis that are much smaller and more localized, alth ough probably more frequent, are caused by local crustal earthquakes and la ndslides along the British Columbia coast. Two such tsunamis have occurred in British Columbia in recent years, one in 1946 in the Strait of Georgia a nd another in 1975 at the head of a fiord on the northern mainland coast. ( C) 2000 Elsevier Science Ltd. All rights reserved.