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

Abstract

Large tsunamis strike the British Columbia coast an average of once every several hundred years. Some of the tsunamis, including one from Alaska in 1964, are the result of distant great earthquakes. Most, however, are triggered by earthquakes at the Cascadia subduction zone, which extends along the Pacific coast from Vancouver Island to northern California. Evidence of these tsunamis has been found in tidal marshes and low-elevation coastal lakes on western Vancouver Island. The tsunamis deposited sheets of sand and gravel now preserved in sequences of peat and mud. These sheets commonly contain marine fossils, and they thin and fine landward, consistent with deposition by landward surges of water. They occur in low-energy settings where other possible depositional processes, such as stream flooding and storm surges, can be ruled out. The most recent large tsunami generated by an earthquake 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, together with theoretical numerical modelling, indicate wave run-ups of up to 5 m asl along the outer coast of Vancouver Island and up to 15–20 m asl at the heads of some inlets. The waves attenuated as they moved eastward along Juan de Fuca Strait and into Puget Sound and the Strait of Georgia. No deposits 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 British 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, such tsunamis are infrequent — perhaps as few as seven have occurred in the last 3500 yr. Other tsunamis that are much smaller and more localized, although probably more frequent, are caused by local crustal earthquakes and landslides along the British Columbia coast. Two such tsunamis have occurred in British Columbia in recent years, one in 1946 in the Strait of Georgia and another in 1975 at the head of a fiord on the northern mainland coast.