Quaternary Science Reviews 19 (2000) 849}863 A review of geological records of large tsunamis at Vancouver Island, British Columbia, and implications for hazard John J. Clague! " *, Peter T. Bobrowsky#, Ian Hutchinson$ !Depatment of Earth Sciences and Institute for Quaternary Research, Simon Fraser University, Burnaby, BC, Canada V5A 1S6 "Geological Survey of Canada, 101 - 605 Robson St., Vancouver, BC, Canada V6B 5J3 #Geological Survey Branch, P.O. Box 9320, Stn Prov Govt, Victoria, BC, Canada V8W 9N3 $Department of Geography and Institute for Quaternary Research, Simon Fraser University, Burnaby, Canada BC V5A 1S6 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 Paci"c 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 "ne landward, consistent with deposition by landward surges of water. They occur in low-energy settings where other possible depositional processes, such as stream #ooding 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 Paci"c 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 "rst 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 "ord on the northern mainland coast. ( 2000 Elsevier Science Ltd. All rights reserved. 1. Introduction Western British Columbia is located at the edge of the North America lithospheric plate along the Paci"c `Ring Tsunamis are ocean waves generated by underwater of Firea and is vulnerable to tsunamis generated by disturbances of the sea#oor or by surface impacts. They earthquakes beneath the Paci"c Ocean. The largest are triggered by earthquakes and, less commonly, by tsunamis in British Columbia result from great (moment * landslides, volcanic eruptions, and meteorite impacts. magnitude M 8) earthquakes at the Cascadia sub- Earthquake-triggered tsunamis are also called seismic duction zone where the oceanic Juan de Fuca plate sea waves and, erroneously, tidal waves. They are imper- moves underneath North America (Fig. 1). Although the ceptible on the open ocean, where they have amplitudes Cascadia subduction zone has not produced a great of less than 1 m and move at velocities of up to earthquake in the historical period (i.e., the last 200 yr), 1000 km/h, but commonly reach to heights of 5}10 m as a variety of geological data suggest that many such they come ashore. events have occurred during late Holocene time (Atwater et al., 1995; Atwater and Hemphill-Haley, 1997; Clague, 1997). * Corresponding author. Fax: 001 604 291-4198. Evidence supporting this conclusion has come from E-mail address: [email protected] (J.J. Clague). studies of sediment sequences in estuaries from northern 0277-3791/00/$- see front matter ( 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 9 9 ) 0 0 1 0 1 - 8 850 J.J. Clague et al. / Quaternary Science Reviews 19 (2000) 849}863 Fig. 1. Vancouver Island and its tectonic setting. California to southern Vancouver Island. Deposits in the hours, reached the outer coast of British Columbia, caus- high intertidal zone of the estuaries display repetitive ing about $40 million damage (year-2000 US dollars), sequences of peat sharply overlain by tidal mud. The peat mainly to the Vancouver Island communities of Port layers accumulated slowly in intertidal marshes and Alberni, Hot Springs Cove, and Zeballos. fringing forests, whereas the overlying mud layers were This paper reviews the physical evidence for large deposited in lower intertidal environments following tsunamis of late Holocene age on the coast of Vancouver coseismic subsidence. In many cases, contacts between Island and discusses implications for tsunami hazard the buried marsh peat and overlying mud are marked by assessment. We summarize and discuss the geological thin sand and gravel layers, inferred to have been depos- research on tsunamis done by ourselves and our col- ited by tsunamis (Atwater, 1987, 1992; Darienzo and leagues on the west coast of Canada over the last 10 yr. Peterson, 1990; Atwater and Yamaguchi, 1991; Clague We start with a brief description of the Vancouver Island and Bobrowsky, 1994a, b; Darienzo et al., 1994; Peterson coast, where we have focused our research e!ort, then and Darienzo, 1996). summarize the characteristics and distribution of British Columbia is also a!ected by tsunamis of more tsunami deposits that we have found there. Next we distant Paci"c earthquakes. The largest tsunami to strike discuss two large, well documented tsunamis * the 1964 British Columbia in this century was generated by the event mentioned above and the tsunami of a great earth- great (M 9.2) Alaska earthquake of March 27, 1964 quake at the Cascadia subduction zone in AD 1700. (Wigen and White, 1964; Murty and Boilard, 1970). Evidence for older tsunamis is reviewed next. We con- A series of waves radiated outward from the earthquake clude with a discussion of tsunami hazards in southern rupture area o! south-central Alaska and, within a few British Columbia. J.J. Clague et al. / Quaternary Science Reviews 19 (2000) 849}863 851 2. Setting The west coast of Vancouver Island is located at the north end of the Cascadia subduction zone, close to the source of great tsunami-producing earthquakes. For these reasons, this area is a good place to look for tsunami deposits. Western Vancouver Island is rugged and rocky. A nar- row coastal plain is backed by a mountain range in- dented by narrow, steep-sided "ords. Patches of intertidal marsh, which have good potential for recording tsunamis, are restricted to small areas at the heads of sheltered inlets and other embayments, and to the fore- shores of "ord-head deltas. Some of these areas, however, are not ideal for our type of research. Fiord-head deltas, for example, are high-energy #uvial systems, and tsunami deposits in these environments are rapidly destroyed through channel migration and avulsion. Western Vancouver Island has risen relative to the sea Fig. 2. General characteristics of Vancouver Island tsunami deposits. at a net rate of about 1 m ka\ over the last several Arrows are directed landward. (Modi"ed from Benson et al., 1997, thousand years (Clague et al., 1982; Friele and Hutchin- Fig. 11.) son, 1993). Intertidal marshes on this coast occupy a ver- tical range of about 1 m and thus have a life span of about 1000 yr before they emerge from the intertidal zone. Present-day marshes are therefore likely to record only those tsunamis that have happened in the last mil- lenium. Uplifted marshes are rapidly colonized by forest, and their tsunami deposits are obscured or destroyed by bioturbation and erosion. To document older events on an emerging coast like western Vancouver Island, one must investigate depos- itional sites above the limit of tides. Suitable sites include low-lying bogs and lakes, in particular lakes with low #uvial inputs. 3. Characteristics of tsunami deposits Photographs and eyewitness accounts of historical Fig. 3. Drainage ditch exposing a single sheet of tsunami sand within tsunamis in Chile, Hawaii, Indonesia, Japan, and else- an intertidal peat sequence, Cultus Bay, Washington (Atwater and where indicate that large amounts of sediment are trans- Moore, 1992; see Fig. 1 for location). The sand sheet has sharp upper ported and deposited by the turbulent, landward-surging and lower boundaries, thins and "nes landward, and contains marine waves. Most of the sediment is derived from loose near- microfossils. The tsunami that deposited the sand was triggered by shore deposits, generally sand and silt, but in some cases a large earthquake near Seattle about 1000 yr ago. Shovel handle is 0.5 m long. (Photo by J.J. Clague.) gravel. The sediment is carried by the turbulent water in suspension and traction. It is deposited as sheets in the upper part of the intertidal zone and in low-lying areas beyond the tidal limit as the waves lose energy or begin to protected tidal marshes and low-elevation coastal bogs recede.