A Review of Geological Records of Large Tsunamis at Vancouver Island, British Columbia, and Implications for Hazard John J

Home , 1700 Cascadia earthquake, 1964 Alaska earthquake, Alberni Inlet, Port Renfrew

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

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 narrow 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 vertical 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 depositional 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. and lakes. Tsunamis commonly leave extensive thin sheets Tsunami sediments generally have sharp contacts with of coarse sediment some distance inland from the overlying and underlying deposits. In most cases, they shore (Figs. 2 and 3 ). However, the geological consist of massive sand with abundant wood and other preservation potential in most areas is low. Tsunami plant detritus (Clague and Bobrowsky, 1994b). Locally, sediments are generally preserved only in protected, however, the deposits are gravelly or graded, or consist low-energy environments where they may be covered of two or more paired laminae (couplets) of silt and by mud or peat. Favourable environments include sand (Fig. 4). The sediments commonly contain marine 852 J.J. Clague et al. / Quaternary Science Reviews 19 (2000) 849}863

Fig. 5. Simpli"ed diagram showing tsunami sand being deposited on a coseismically subsided marsh surface immediately after an earthquake. The sand is covered by tidal mud soon after the earthquake. Fig. 4. Couplets within a tsunami sand layer at Fair Harbour, British (Modi"ed from Atwater et al., 1995, Fig. 3.) Columbia. Each couplet is composed of a coarse basal layer and an overlying "ner layer; the coarse basal layers are bracketed by pins. The couplets are thought to record separate surges of water in a tsunami wave train. (Photo by Kurt Grimm; reproduced from Quaternary California (Hansen et al., 1966). The main tsunami swept Research with permission of Academic Press.) southward across the Paci"c Ocean at a velocity of about 830 km/h, reaching Antarctica in only 16 h. Waves be- microfossils (Hemphill-Haley, 1996), and thin, "ne, and came larger as they moved up Barkley Sound and Al- rise in a landward direction (Benson et al., 1997). Such berni Inlet on Vancouver Island * they were two and fossils and architecture indicate that the sediments were one-half times higher at Port Alberni, at the head of the deposited by landward surges of water, not by streams. inlet, than at To"no and Ucluelet on the open coast. As In open coastal settings, the deposits of rare large a consequence, Port Alberni was the hardest hit com- storms are commonly di$cult to distinguish from munity in British Columbia; 260 homes were damaged, tsunami deposits. Storm deposits, however, are unlikely 60 extensively (Wigen and White, 1964; Murty and to be found far from the shore in protected tidal inlets Boilard, 1970; Thomson, 1981). because as water levels rise, current velocities in channels Three main waves struck Port Alberni between 12:20 drop below values required to transport enough sand to a.m. and 3:30 a.m. on March 28 (Figs. 6 and 7 ). Most form widespread sheets. Also, some sand sheets consist of people in the town were asleep when the "rst wave rolled two or more "ning-upward beds, suggestive of deposition in. The sea surged up Somass River at a velocity of about by successive tsunami waves rather than a storm surge. 50 km/h and spilled onto the land, inundating entire Other evidence supports the inference that these an- neighbourhoods with chest-deep water. This "rst wave omalous tidal-marsh sand sheets are tsunami deposits. In reached 3.7 m above geodetic datum and knocked out the source area of tsunami-generating earthquakes, sand the Port Alberni tide gauge. The second and most de- sheets may directly overlie marsh or forest soils that structive wave swept into town less than two hours later, subsided suddenly during the earthquakes (Fig. 5). Sub- at 2 a.m. The lights of the waterfront mills went out as the sidence was caused by the sudden release of stress in the water smashed through the facilities. The ground #oor of Earth's crust. The sand may entomb delicate leaves and the Barclay Hotel, 1 km inland, was splintered by the stems of herbaceous plants that were growing on the surging water; guests had to be rescued from an upper marsh before it subsided. Preservation of the delicate #oor by police in boats. Logs and debris crashed into plant remains shows that sand deposition occurred im- buildings, automobiles were cast about, and houses were mediately after subsidence and implies that the two swept o! their foundations and rafted inland. As the events are linked, the only plausible common cause being water subsided, some buildings were dragged seaward; a large earthquake (Atwater and Yamaguchi, 1991). two houses drifted into Alberni Inlet and were never seen again. The second wave left a mark on the tide station at 4.3 m above geodetic datum. The third wave, which ar- 4. The 1964 tsunami rived at about 3:30 a.m., was the largest of all, but because the tide had fallen it crested at 3.9 m and did 4.1. Description of the event little further damage. Other waves oscillated in Alberni

The Alaska earthquake of March 27, 1964, the world's second largest of the twentieth century, triggered Geodetic datum corresponds approximately to mean sea level (zero tsunamis that killed 130 people, some as far away as elevation). J.J. Clague et al. / Quaternary Science Reviews 19 (2000) 849}863 853

the sand, and have obscured or destroyed any original internal structure. Branches, sticks, seeds, and cones are scattered through the sediment and, in places, form a cap of woody detritus. The sand contains rare tests of foraminifera * mainly intertidal species such as Tro- chammina macrescens, Trochamminita irregularis, and Miliammina fusca (Clague et al., 1994). It also contains many well preserved valves of diatoms, chie#y tidal-#at taxa but also the benthic species Mastogloia ellipitica and M. smithii, and the planktonic species Thalassiosira pacixca (Hemphill-Haley, 1996). Photographs of Port Alberni taken soon after the tsunami show large amounts of freshly deposited woody debris and mineral sediment. It thus is a reasonable inference that the near-surface woody sand sheet in the marsh at the mouth of Somass River was deposited Fig. 6. Port Alberni tide gauge record showing the passage of the during this event. This inference was con"rmed through tsunami generated by the 1964 Alaska earthquake. Three main waves Cs analysis of sediments in the marsh (Clague et al., arrived between 12:20 a.m. and 3:30 am on March 28; the second wave was the most destructive. (Modi"ed from Thomson, 1981, Fig. 9.2.) 1994). Cesium-137 values increase markedly just below the sand layer, most likely due to frequent atmospheric testing of nuclear bombs from about 1953 to 1963 Inlet, with decreasing strength, for another two days (Fig. 9). The erratic decrease in Cs concentration from (Fig. 6). the top of the sand to the surface records the reduction The 1964 tsunami caused damage elsewhere on Van- and cessation in atmospheric testing after 1963. couver Island * it sank a "shing boat and damaged log The Cs pro"le supports the contention that the booms at Ucluelet, damaged wharf facilities at To"no, sand was deposited by the 1964 Alaska tsunami. Other breached the municipal water pipeline that crosses the possible explanations, notably deposition during a river sea#oor near To"no, destroyed a village at Hot Springs #ood or storm surge, are unlikely. The former can be Cove, and swept buildings o! their foundations at Zebal- ruled out because the sand is thicker south of the pipe- los. Waves were largest at the heads of some inlets due to line, which is farther from the river, than it is to the north. the focussing of wave energy and resonance e!ects. A storm surge mechanism also is unlikely because the The 1964 tsunami was exceptional in the historical head of Alberni Inlet is distant from the open ocean and period in British Columbia. Only two of the 43 tsunamis is relatively sheltered. Even after extended periods of registered at To"no between 1906 and 1981 produced strong, sustained, southerly winds, waves at the head of waves more than 1 m high * the 1964 tsunami, with the inlet do not exceed 1 m in height (Morris and Leaney, a maximum wave height (trough to crest) of 2.40 m, and 1980). a tsunami in 1960 generated by the M 9.5 Chilean A similar, thin, shallow sand sheet occurs at numerous earthquake, with a wave height of 1.26 m (Table 1). other marshes on Vancouver Island, for example at Fair Harbour, Koprino Harbour, and Neurotsos Inlet 4.2. The deposit (Benson et al., 1997; Fig. 1, Table 2). It is especially conspicuous in marshes at the heads of inlets that The deposit of the 1964 tsunami was "rst documented experienced large waves and damage during the 1964 at Port Alberni in the early 1990s (Clague and Bob- tsunami. At Fair Harbour, for example, the tsunami rowsky, 1994b; Clague et al., 1994), and has since been destroyed two bridges that crossed the marsh and moved found at several other sites on western Vancouver Island buildings at a nearby logging camp (Benson et al., (Benson et al., 1997; Clague et al., 1998; Table 2, Fig. 1). 1997). At most sites where the deposit has been identi"ed, A sheet of sand, averaging 1}2 cm thick, occurs within it is 0.5}2 cm thick and occurs within peat 5}10 cm brown rooty peat at depths of less than 11 cm throughout below the present marsh surface (Benson et al., 1997). a marsh in the Somass River estuary at the head of It typically "nes and thins inland away from tidal chan- Alberni Inlet (Fig. 8). It is thickest (up to 15 cm) just nels and pinches out near the forest edge. The sand south of an elevated water pipeline that supplies water to also contains marine diatoms that are rarely found in a pulp mill at Port Alberni, which suggests that the the modern marsh. These observations indicate that the pipeline impeded the northward transport of the sand. sediment was transported landward from the sea. The The deposit is massive, moderately well sorted, "ne to sand sheet is generally massive, but at some places medium sand with 5}30% disseminated silt. Roots (locally at Fair Harbour, for example) it contains up of living herbaceous plants extend into and through to three beds of coarse to "ne sand and mud. Each of 854 J.J. Clague et al. / Quaternary Science Reviews 19 (2000) 849}863

Fig. 7. Areas inundated by the 1964 Alaska and 1700 Cascadia tsunamis at Port Alberni (1964 area"orange; 1700 area"yellow# orange). Inundation areas are approximate: that of the 1700 tsunami is based on a maximum wave height of 16 m, calculated using a numerical model (Ng et al., 1991; see Fig. 14). (From Clague and Bobrowsky, 1999, Fig. 10; reproduced from Geoscience Canada with permission of Geological Association of Canada.) the beds "nes upward and was probably deposited by more than 20 estuaries and tidal marshes on the Paci"c one wave in the tsunami wave train. coast between central Vancouver Island and northern The 1964 tsunami deposit is thin or absent at many California (Atwater et al., 1995; Clague, 1997). Several marshes on the outer coast of Vancouver Island. At lines of evidence show that the soil subsided during To"no, for example, the deposit, where present, is a dis- a subduction earthquake (Atwater et al., 1995): (1) sub- continuous layer of mud a few millimetres thick (Clague mergence was sudden and typically ranged from a few and Bobrowsky, 1994b). The maximum run-up of the tens of centimetres to perhaps 2 m, similar to the amount 1964 tsunami at To"no was only 0.6 m above the high of subsidence during historic subduction zone earth- tide level (Wigen and White, 1964), much lower than at quakes elsewhere; (2) submergence at sites 90 km apart the heads of "ords such as Alberni and Neroutsos inlets. occurred at the same time (between August 1699 and In general, the tsunami was smaller on the exposed outer May 1700; Jacoby et al., 1997; Yamaguchi et al., 1997); (3) coast of Vancouver Island than at the heads of bays and submergence at sites hundreds of kilometres apart could inlets. also be of this age based on high-precision radiocarbon dating of earthquake-killed plants (Nelson et al., 1995); and (4) submergence coincided with a tsunami. Ana- 5. The 1700 tsunami logues for this earthquake include the great subduction earthquakes in Chile in 1960 and Alaska in 1964, both of 5.1. Description of the event which produced widespread crustal subsidence in the source areas and destructive tsunamis that crossed the The last great earthquake at the Cascadia subduction Paci"c Ocean (Plafker, 1969, 1972; Plafker and Savage, zone occurred in 1700. It is recorded by a buried soil at 1970). J.J. Clague et al. / Quaternary Science Reviews 19 (2000) 849}863 855

Japanese writings provide an exact age for the event. waves were not the result of a local earthquake, and there Tsunami waves up to 5 m high damaged sites along are no accounts of a great earthquake elsewhere in the a 1000-km length of the east coast of Honshu on January Paci"c Ocean that conceivably could be the source of the 27}28, 1700 (Satake et al., 1996; Tsuji et al., 1998). The tsunami. Japanese researchers thus argued, by a process of elimination, that the tsunami was produced by a great Table 1 earthquake o! the west coast of North America. They Twentieth century tsunamis recorded at To"no, British Columbia" further reasoned, from the size of the waves and the distance from the source, that the earthquake had a mo- Date Source area Height of maximum ment magnitude close to 9. After correcting for the travel wave (cm)! time to Japan and the time zone di!erence, they con- cluded that the earthquake occurred on January 26, November 1, 1915 Japan 12 1700, at about 9 p.m. local time. This conclusion agrees May 1, 1917 Kermadec Islands 12 with oral traditions of North American coastal native July 18, 1918 Indonesia 16 September 7, 1918 Kuril Islands 16 peoples who describe the shaking and tsunami of a large April 30, 1919 Tonga 15 earthquake at night in winter (Clague, 1995; McMillan November 11, 1922 Peru 27 and Hutchinson, 2000). February 3, 1923 Kamchatka 27 April 13, 1923 Kamchatka 15 5.2. The event deposit March 7, 1929 Aleutian Islands 11 March 2, 1933 Japan 23 November 30, 1934 Mexico 22 The deposit of the 1700 tsunami is present in many November 10, 1938 Aleutian Islands 27 tidal marshes and some low-elevation coastal lakes on December 7, 1944 Japan 12 Vancouver Island (Fig. 1, Tables 2 and 3). Like the 1964 December 27, 1944 New Hebrides (Vanuatu) 12 deposit, it has a sheet-like form, it thins and "nes land- April 1, 1946 Aleutian Islands 58 March 4, 1952 Japan 12 ward and away from tidal channels, and it commonly November 4, 1952 Kamchatka 58 contains abundant woody detritus and some marine March 9, 1957 Aleutian Islands 52 microfossils (Fig. 2). In most areas, however, the 1700 March 11, 1957 Aleutian Islands 18 deposit is coarser and thicker than the 1964 deposit, November 6, 1958 Kuril Islands 10 suggesting that the older tsunami was the larger of the May 22, 1960 Chile 126 October 13, 1963 Kuril Islands 16 two events at Vancouver Island. March 28, 1964 South-central Alaska 240 At tidal marshes, the 1700 tsunami deposit consists May 16, 1968 Japan 12 mainly of "ne to very coarse sand; gravel is limited to the May 16, 1968 Japan 13 banks of tidal channels facing open water in some marshes. The sand sheet is 1}2 cm thick on average, but !Sum of displacements of maximum wave and successive trough from corresponding tide levels. is locally several tens of centimetres thick. It is commonly "Source: Wigen (1983, Appendix B); only tsunamis with maximum found 40}70 cm below the marsh surface; where the 1964 wave heights of 10 cm or more are included. tsunami deposit is also present, the two are separated by

Table 2 Marshes containing tsunami deposits

Site (location, Fig. 1) Coseismic Tsunami deposits Reference subsidence! AD1964 AD1700 Older

Koprino Harbour (1) ᭿᭿ Benson et al. (1997) Neroutsos Inlet (2) ᭿᭿ Benson et al. (1997) Power Lake (3) ᭿᭿ Clague et al. (1999) Fair Harbour (4) ᭿᭿ Benson et al. (1997) Zeballos (5) ᭿ unpublished Catala Lake (6) Minor? ᭿ Clague et al. (1999) Louie Bay (7) ᭿? ᭿ Clague et al. (1997) Channel Lagoon (8) Minor? ᭿? Clague et al. (1997) Port Alberni (9) ᭿᭿᭿Clague and Bobrowsky (1994a, b) Clague et al. (1994) To"no (10) ᭿ ᭿᭿᭿Clague and Bobrowsky (1994a, b) Ucluelet (11) ᭿ ᭿᭿᭿Clague and Bobrowsky (1994a, b) Port Renfrew (12) Minor ᭿ Clague et al. (1997)

!Coseismic subsidence during AD 1700 earthquake. 856 J.J. Clague et al. / Quaternary Science Reviews 19 (2000) 849}863

the sand occurs within a uniform sequence of intertidal peat which grades down into mud (Benson et al., 1997). Though typically massive or normally graded, the sand sheet locally comprises two or three couplets of basal clean coarse to "ne sand overlain by muddy "ne sand and silt (Fig. 4). As in the case of the 1964 tsunami deposit, the couplets are attributed to the successive waves in a tsunami wave train.

6. Earlier tsunamis

Sand layers attributed to tsunamis older than the AD 1700 event have been found at tidal marshes at To"no and Port Alberni and in some coastal lakes (Fig. 1, Table 2}4; Clague and Bobrowsky, 1994b; Hutchinson et al., 1997; Clague et al., 1999). Evidence for the events is scarce at tidal marshes because (1) most marshes on the west coast of Vancouver Island are less than 1000 yr old Fig. 8. Thickness of the 1964 tsunami deposit at the Port Alberni and (2) there are few suitable marshes farther east at the marsh. Stratigraphy at starred site is shown in Fig. 9. (Modi"ed from heads of bays and "ords. Nevertheless, what has been Clague et al., 1994, Fig. 4.) interpreted to be a tsunami sand at To"no dates to 500}800 years ago, and a possible correlative at Port Alberni is 600}1000 yr old (Clague and Bobrowsky, 1994b). Eight older sand layers were penetrated in a sonic drill hole at the Port Alberni marsh (Clague and Bobrowsky, 1994b). No paleoecological work has been done on these sands, and their subsurface spatial distribution is un- known. However, they resemble the 1964 and 1700 tsunami deposits at Port Alberni and occur within a muddy subtidal sedimentary sequence far from any stream. They are thus tentatively ascribed to tsunamis. Radiocarbon dating has shown that the sand layers range in age from 3500 to 2000 yr old. The potential for preserving deposits of tsunamis that are more than 1000 yr old on Vancouver Island is greater in lakes than tidal marshes. Marsh deposits are destroyed or covered by forest when they are raised out of the intertidal zone, whereas lakes continue to act as sediment traps as the land rises. The challenge is to "nd lakes at appropriate elevations close to the coast, in areas where Fig. 9. Stratigraphy of the upper part of the intertidal sequence at Port tsunami run-up is high and where confounding processes Alberni (starred site in Fig. 8), and graph of Cs concentrations as such as storm surges and stream #ooding can be ruled a function of depth. The two sand layers are tsunami deposits * the out (Hutchinson et al., 1997). upper layer was deposited in 1964 and the lower layer in 1700. Cesium- After a long search, we are only now beginning to 137 concentrations below 27 cm are very low. A radiocarbon age of 360$50 yr BP (GSC-5174) was obtained on a piece of wood at a depth recover sedimentary records of pre-1700 tsunamis from of 70}72 cm in the lower tsunami sand layer. Lenses of bioturbated coastal lakes on Vancouver Island. A tsunami dating to, sand directly overlie this layer. (Modi"ed from Clague et al., 1994, or shortly after, 2800 yr ago left a layer of sand and plant Fig. 5.) debris in Kanim Lake (Hutchinson et al., 1997), and a tsunami deposited sediment in Catala Lake about 1000 yr several tens of centimetres of peat and mud (Fig. 10). The ago (Clague et al., 1999). Deposits of at least two pre-1700 sand abruptly overlies a peaty marsh soil and is overlain tsunamis have been identi"ed at Deserted Lake, which is by mud#at deposits at marshes within the zone of coseis- subject to tidal incursions at very high tides. These mic subsidence of the 1700 earthquake (Fig. 11; Clague tsunamis occurred 2400}2850 and 1530}1950 yr ago and Bobrowsky, 1994a, b). Outside this zone, however, at the time of known, great, Cascadia plate-boundary J.J. Clague et al. / Quaternary Science Reviews 19 (2000) 849}863 857

Table 3 Lakes containing tsunami deposits

Site (location, Fig. 1) Elevation! (m) Area (km) Distance" (km) Tsunami deposits Reference

AD1700 Older

Catala Lake (6) &3 0.1 0.5 ᭿᭿Clague et al. (1999) Deserted Lake (13) 3 0.5 0.5 ᭿᭿Hutchinson et al. (1999) Kanim Lake (14) 6 1.2 0.7 ᭿ Hutchinson et al. (1997) Kakawis Lake (15) &4 0.1 0.2 ᭿ Unpublished

!Datum is mean sea level; subtract approximately 2 m for elevation above high tide level. "Approximate distance of lake outlet from seashore.

Fig. 10. Stratigraphy of the intertidal sequence at Fair Harbour. The peat contains two sand sheets, one deposited in 1964 and another deposited in 1700. Vertical datum is lower low water. (From Benson et al., 1997, Fig. 4; reproduced from Quaternary Research with permission of Academic Press.)

earthquakes (earthquakes N, 2400}2800 yr ago; earth- tsunamis or by prolonged submergence in saline waters, quake S, 1300}1700 yr ago; Hutchinson et al., 2000). or that became established at about the same time as the Diatom and protozoan assemblages in sediments directly earthquakes, have been dated by radiocarbon and de- above each of the Deserted Lake tsunami deposits are ndrochronological (tree-ring counting) techniques (At- more marine than the assemblages below, suggesting that water and Yamaguchi, 1991; Atwater et al., 1991; Nelson the site subsided during the earthquakes. et al., 1995; Jacoby et al., 1997; Yamaguchi et al., 1997). Earthquake ages have also been estimated by measuring thicknesses of clay beds separating seismically triggered 7. Recurrence turbidites in deep-sea cores collected o! the Washington and Oregon coasts (Adams, 1990, 1996). The history of large subduction earthquakes and ac- Previous estimates of average recurrence intervals for companying tsunamis in Cascadia has been inferred by tsunami-generating earthquakes at the Cascadia subduc- dating rapidly buried soils and overlying tsunami sand tion zone range from 200 to 600 yr, but these estimates sheets at many sites along the Paci"c coast from northern have uncertainties that may total many hundreds of California to Vancouver Island (e.g., Atwater and years (Atwater et al., 1995). The most recent and best Hemphill-Haley, 1997). Plants that were killed by the estimate of average recurrence, about 500 yr, comes from 858 J.J. Clague et al. / Quaternary Science Reviews 19 (2000) 849}863

700}1300 yr separates the youngest of these three events from the next recognized earthquake, which dates to sometime between 1700 and 1500 yr ago. Two earthquakes occurred 1350}1130 and 1300}900 years ago; after this, a period of 600 to 1000 yr elapsed before the most recent earthquake 300 yr ago. The sand sheets of the 1700 and 1964 tsunamis are the youngest tsunami deposits found to date on Vancouver Island; in fact, they are the only tsunami deposits less than 500 yr old. At Port Alberni, there are nine older sand and gravel layers that may be tsunami deposits; all of these are younger than about 3500 yr old. The average tsunami recurrence on Vancouver Island, based on these data, is about 350 yr, somewhat less than the average recurrence derived from buried soils in southwestern Washington. This suggests that most tsunamis large enough to leave a geological signature on Vancouver Island are products of subduction earthquakes in Cas- Fig. 11. Representative stratigraphic sections of tidal marsh sediments cadia and Alaska. Many lesser, distant earthquakes pro- near To"no. A layer of tsunami sand abruptly overlies a buried marsh duced tsunamis, but the waves were probably no larger surface that subsided suddenly during a Cascadia plate-boundary than those in 1964. earthquake in 1700 (the peat layer that de"nes this surface is high- lighted). Bioturbated mud at the base of two of the stratigraphic The catalogue of large Cascadia earthquakes may be sections is late Pleistocene in age. Radiocarbon age uncertainties and incomplete. In addition to the seven events recognized in material dated are given in Clague and Bobrowsky (1994). (Modi"ed southwestern Washington, other large earthquakes, from Clague and Bobrowsky, 1994a, Fig. 2.) which are unrecorded in this area, may have occurred on northern and southern segments of the subduction zone. a detailed study of buried soils at estuaries in south- Conversely, some of the southern Washington events western Washington (Atwater and Hemphill-Haley, may not have been large enough to leave geological 1997). Seven large earthquakes have struck this area in imprints in southwestern British Columbia. the last 3500 yr, the last in AD 1700 (Fig. 12). The age ranges of four of the six pre-1700 events have been determined from high-precision (long-count-time) 8. Hazard radiocarbon ages on plants that died or began to grow at about the time of the earthquake. The age ranges for the Historical records and geological data show that large other two events are larger and have been established tsunamis strike British Columbia once every several hun- from conventional radiocarbon ages and stratigraphic dred years on average (Table 4). Most large tsunamis are position. triggered by subduction earthquakes beneath the Paci"c Intervals between successive earthquakes di!er con- Ocean, principally o! southern Alaska and along the siderably (Atwater and Hemphill-Haley, 1997). Three Cascadia subduction zone. The most vulnerable areas to earthquakes occurred between 3500}3320 and future tsunamis of this type are the outer coast and inlets 2800}2400 yr ago; their mean recurrence is thus more of Vancouver Island, where damage to some coastal than 270 yr, but less than 550 yr. An interval of communities would be large (Fig. 13). A large Paci"c

Fig. 12. Ages of inferred great (M 8-9) Cascadia earthquakes in the last 4000 yr in southern coastal Washington (bars indicate estimated 95% con"dence interval; data from Atwater and Hemphill-Haley, 1997). J.J. Clague et al. / Quaternary Science Reviews 19 (2000) 849}863 859

Table 4 are of the order of 10 m (peak-to-trough height). Ampli"- Tsunamis during the last 3000 yr for which evidence has been found on cation, up to a factor of three, is predicted for some inlets # Vancouver Island (Fig. 7 and 14). The simulated tsunami attenuates as it Age (years before Tsunami source! Sites passes through narrow passages connecting Juan de A.D. 2000) Fuca Strait to Puget Sound and the Strait of Georgia, Marshes Lakes and its amplitude is reduced to 1 m or less by the time the waves reach Seattle and Vancouver (Fig. 14). A tsunami ᭿ 36 Alaska would, of course, be superimposed on the tides, which on 300 Cascadia (Y) ᭿᭿ 800" Alaska? ᭿ the south coast of British Columbia have a range of up to 1000" Cascadia (W) ᭿᭿ 5 m. A positive sea level displacement at low tide would 1600" Cascadia (S) ᭿ have much less impact than one at high tide. However, 2700" Cascadia (N) ᭿ a tsunami could persist through a tidal cycle, thus ex- treme water levels are likely to combine tsunami and #Note: As many as ten, poorly dated, pre-1964 tsunamis may be recorded in sediments at Port Alberni. tidal e!ects (Ng et al., 1991). !Letters in parentheses are Cascadia earthquake identi"ers of Atwater The travel time of a tsunami with a source on the and Hemphill-Haley (1996). Cascadia subduction zone is much less than one gener- "Approximate age * average of possible age range. ated elsewhere in the Paci"c Ocean. The "rst wave would reach the outer coast of Vancouver Island in less than 30 min (Hebenstreit and Murty, 1989; Murty, 1992). tsunami would attenuate before entering the Strait of Travel times to Victoria and Vancouver might be as little Georgia and probably would not cause major damage as 1.5 and 3 h, respectively. In contrast, the 1964 Alaska there. tsunami reached the west coast of Vancouver Island Estimates have been made of maximum water levels about 4 h after the earthquake. and velocities on the British Columbia coast for tsunamis Although numerical modelling can provide estimates triggered by hypothetical subduction earthquakes o! of tsunami amplitudes and velocities, the estimates have south-central Alaska, the Aleutian Islands, Kamchatka, large uncertainties. Wave amplitudes are strongly in- and Chile. Dunbar et al. (1989, 1991) made computer #uenced by the magnitude, form, and acceleration of models of tsunamis from each of these source regions to sea#oor displacement during the earthquake, none of study their likely behaviour in 20 British Columbia inlets. which is well known, and by the coastal topography and The model predictions were calibrated to water level o!shore bathymetry, which can be very complex and measurements made at tide gauge stations after the 1964 di$cult to model in detail. Such uncertainties yield pre- Alaska earthquake. Simulated sea#oor displacements dictions that may be in error by a factor of two or more. were based on data from the 1960 Chile and 1964 Alaska Furthermore, tsunami run-up can vary by a factor of 5 or earthquakes. The largest waves in the simulations occur 10 over even short stretches of coast. on the west coasts of Vancouver Island and the Queen Charlotte Islands, and on the central mainland coast. 8.1. Local tsunamis Several sites at the heads of inlets could experience waves up to 9 m high, moving at 3}4 m/s (Dunbar et al., 1989; Paci"c subduction earthquakes produce the largest Murty, 1992). The largest waves on the northern British and potentially most damaging tsunamis in coastal Brit- Columbia coast were generated from the simulated ish Columbia, but tsunamis of other origins are also earthquake o! south-central Alaska. In all other regions, a threat. A local crustal earthquake could generate the largest waves came from the Aleutian source. Based a tsunami in a "ord or an inland waterway such as the on these simulations, major population centres bordering Strait of Georgia. A magnitude-7.3 earthquake on Van- the Strait of Georgia are not at risk from tsunamis couver Island in 1946 (Rogers and Hasegawa, 1978) trig- triggered by distant earthquakes because the waves gered a tsunami with waves up to 1 m high in the Strait of would dissipate as they move through Juan de Fuca Georgia and nearby inlets (Murty, 1977), and a shallow Strait and the Gulf Islands. crustal earthquake near Seattle about 1000 yr ago pro- Even larger waves might be produced by an earth- duced a much larger tsunami in Puget Sound (Atwater quake at the Cascadia subduction zone. A numerical and Moore, 1992). model was used to simulate three tsunamis, involving Simulations have been made of tsunamis from large rupture of di!erent parts of the plate boundary (Ng et al., earthquakes within the North America plate (Murty and 1990, 1991). As might be expected, the west coast of Hebenstreit, 1989; Murty, 1992). The earthquakes used in Vancouver Island was found to be the most strongly the simulations were similar in magnitude and motion to a!ected area in British Columbia. For a simulated M8.5 the above-mentioned 1946 Vancouver Island earth- earthquake on a northern section of the subduction zone, quake, but were centred in Juan de Fuca Strait near modelled maximum wave amplitudes on the outer coast Victoria, in Puget Sound near Seattle, and in the Strait of 860 J.J. Clague et al. / Quaternary Science Reviews 19 (2000) 849}863

Fig. 13. Generalized tsunami hazard map for south-coastal British Columbia based on the distribution of tsunami deposits and numerical modelling of wave heights. The map shows four generalized hazard zones, as well as sites with evidence for large prehistoric tsunamis and maximum wave heights of the 1964 Alaska tsunami. Details on three important recent tsunamis are given at the bottom.

Georgia near Vancouver. In all cases, the resulting Strait of Georgia. It should be noted, however, that tsunamis are small (largest waves"0.5}1 m) and would a local crustal earthquake of magnitude 8, which is much have only a local e!ect. Little wave energy passes larger than the 1946 event, might produce a tsunami with through the islands between Juan de Fuca Strait and the waves 3 m or more in height in the Strait of Georgia J.J. Clague et al. / Quaternary Science Reviews 19 (2000) 849}863 861

earthquakes. The locally generated tsunamis, however, a!ect smaller areas and thus are potentially less destructive than subduction-zone tsunamis. The region of Brit- ish Columbia at greatest risk from large Cascadia tsunamis is western Vancouver Island. Other parts of the coast, including the Strait of Georgia and mainland "ords, are more likely to be a!ected by smaller tsunamis generated by local earthquakes or landslides of British Columbia.

9. Conclusion

Geological evidence of recent large tsunamis has been found in tidal marshes and coastal lakes on western Vancouver Island. The evidence includes sheets of sand and gravel preserved in stratigraphic sequences that are otherwise dominated by peat and mud. These sheets thin and "ne landward and contain marine fossils. The largest and most destructive tsunamis on Cana- Fig. 14. Sea-level displacements generated by a hypothetical M8.5 da's west coast are generated by earthquakes at the earthquake at the northern part of the Cascadia subduction zone: (a) Cascadia subduction zone, where the oceanic Juan de Vancouver, Seattle, and selected sites on Vancouver Island; (b) Port Fuca plate moves beneath North America. Geological Alberni and Barkley Sound. Resonance in Alberni Inlet is responsible evidence indicates that large tsunamis from this source for the approximately threefold ampli"cation of waves between Barkley have an average recurrence of about 500 yr. Numerical Sound and Port Alberni. (Modi"ed from Ng et al., 1990, Figs. 4 and 5.) modelling suggests that the waves of these tsunamis may reach up to 5 m asl along the outer coast of Vancouver Island and up to 15}20 m asl at the heads of some "ords. (Murty and Hebenstreit, 1989). A tsunami of this size When such a tsunami next occurs, the "rst waves will probably would cause widespread #ooding in low-lying strike the west coast of Vancouver Island less than areas and would severely damage shoreline development. 30 min after the earthquake, thus there will be little time No geological evidence has yet been found, however, for to evacuate low-lying areas. Waves will attenuate as they such a tsunami in the Strait of Georgia. move eastward through Juan de Fuca Strait and the A submarine landslide could also generate a destruc- narrow waterways separating the Gulf Islands, and they tive tsunami. As an example of what is possible, the 1964 will probably be no higher than 1 m at Vancouver. Alaska earthquake triggered a large submarine slump Destructive tsunamis can also be produced by great near Valdez, Alaska, which produced a local tsunami earthquakes at other subduction zones around the North that destroyed waterfront facilities and the "shing #eet Paci"c Ocean, including those o! south-central Alaska, (Coulter and Migliaccio, 1966). The slide and accom- the Aleutian Islands, Kamchatka, and Japan. The panying tsunami were responsible for the loss of 30 lives, tsunami of the 1964 Alaska earthquake, for example, nearly 25% of all the casualties of the earthquake. caused about $40 million damage to communities on A non-seismically triggered submarine landslide near the Vancouver Island. Tsunamis from these distant sources head of a "ord on the northern British Columbia coast in arrive at the British Columbia coast hours after the 1975 produced a tsunami that caused about $1,500,000 earthquake, thus people have time to evacuate threat- (year-2000 US dollars) to shore installations (Campbell ened areas. and Skermer, 1975; Murty, 1979). Other, more localized tsunamis are triggered by crustal Local tsunamis generated by landslides a!ect relative- earthquakes and landslides. A magnitude-7.3 earthquake ly small areas, although the near-"eld waves can be very on Vancouver Island in 1946 caused a small tsunami in large. A large and rapid landslide is required to generate the Strait of Georgia, and a submarine landslide in a signi"cant tsunami. The landslide can be entirely sub- a "ord on the northern British Columbia coast in 1975 marine or it can enter the sea from a source on land. The produced waves that damaged nearby coastal works. failure can occur in bedrock or in thick Quaternary Such locally generated tsunamis are more common that sediments, as in a "ord-head delta. larger tsunamis produced by earthquakes at the Cas- In southwestern British Columbia, tsunamis triggered cadia subduction zone, but no studies have yet been done by local crustal earthquakes and landslides are probably of the tsunami hazard posed by strong crustal earth- more frequent than those produced by great subduction quakes and landslides in British Columbia. 862 J.J. Clague et al. / Quaternary Science Reviews 19 (2000) 849}863

Acknowledgements Clague, J.J., Bobrowsky, P.T., Hutchinson, I., Mathewes, R.W., 1998. Geological evidence for past large earthquakes in southwest British The work on which this paper is based was funded by Columbia. In: Current Research 1998-A. Geological Survey of Ca- nada, Ottawa, Ontario, pp. 217}224. the Geological Survey of Canada, the British Columbia Clague, J., Harper, J.R., Hebda, R.J., Howes, D.E., 1982. Late Quater- Geological Survey, and the United States National nary sea-levels and crustal movements, coastal British Columbia. Earthquake Hazards Reduction Program. Helpful re- Canadian Journal of Earth Sciences 19, 597}618. views by Brian F. Atwater and Norman D. Smith are Clague, J.J., Hutchinson, I., Mathewes, R.W., Patterson, R.T., 1999. gratefully acknowledged. Evidence for late Holocene tsunamis at Catala Lake, British Colum- bia. Journal of Coastal Research 15, 45}60. Coulter, H.W., Migliaccio, R.R., 1966. E!ects of the earthquake of March 27, 1964 at Valdez, Alaska. U.S. Geological Survey Profes- References sional Paper 542-C. Darienzo, M.E., Peterson, C.D., 1990. Episodic tectonic subsidence of Adams, J., 1990. Paleoseismicity of the Cascadia subduction zone late Holocene salt marshes, northern Oregon coast, central Cas- } evidence from turbidites o! the Oregon-Washington coast. Tec- cadia margin. U.S.A., Tectonics 9, 1}22. tonics 9, 569}583. Darienzo, M.E., Peterson, C.D., Clough, C., 1994. Stratigraphic evid- Adams, J., 1996. Great earthquakes recorded by turbidites o! the ence for great subduction-zone earthquakes at four estuaries in Oregon-Washington coast. In: Rogers, A.M. Walsh, T.J., Kockel- northern Oregon. Journal of Coastal Research 10, 850}876. man, W.J., Priest, G.R., (Eds.), Assessing Earthquake Hazards and Dunbar, D., LeBlond, P.H., Murty, T.S., 1989. Maximum tsunami Reducing Risk in the Paci"c Northwest. U.S. Geological Survey amplitudes and associated currents on the coast of British Colum- Professional Paper 1560, pp. 147}158. bia. Science of Tsunami Hazards 7, 3}44. Atwater, B.F., 1987. Evidence for Holocene earthquakes along the outer Dunbar, D., LeBlond, P.H., Murty, T.S., 1991. Evaluation of tsunami coast of Washington state. Science 236, 942}944. amplitudes for the Paci"c coast of Canada. Progress in Oceanogra- Atwater, B.F., 1992. Geologic evidence for earthquakes during the past phy 26, 115}177. 2000 yr along the Copalis River, southern coastal Washington. Friele, P.A., Hutchinson, I., 1993. Holocene sea-level change on the Journal of Coastal Research 97, 1901}1919. central west coast of Vancouver Island, British Columbia. Canadian Atwater, B.F., Hemphill-Haley, E., 1997. Recurrence intervals for great Journal of Earth Sciences 30, 832}840. earthquakes of the past 3500 years at northeastern Willapa Bay, Hansen, W.R., Eckel, E.B., Schaem, W.E., Lyle, R.E., George, W., Washington. U.S. Geological Survey Professional Paper 1576. Chance, G., 1966. The Alaska earthquake March 27, 1964: "eld Atwater, B.F., Moore, A.L., 1992. A tsunami about 1000 yr ago in Puget investigations and reconstruction e!ort. U.S. Geological Survey Sound Washington. Science 258, 1614}1617. Professional Paper 541. Atwater, B.F., Yamaguchi, D.K., 1991. Sudden, probably coseismic Hebenstreit, G.T., Murty, T.S., 1989. Tsunami amplitudes from local submergence of Holocene trees and grass in coastal Washington earthquakes in the Paci"c Northwest region of North America; Part state. Geology 16, 706}709. 1: The outer coast. Marine Geodesy 13, 101}146. Atwater, B.F., Nelson, A.R., Clague, J.J., Carver, G.A., Yamaguchi, Hemphill-Haley, E., 1996. Diatoms as an aid in identifying late-Holo- D.K., Bobrowsky, P.T., Bourgeois, J., Darienzo, M.E., Grant, W.C., cene tsunami deposits. The Holocene 6, 439}448. Hemphill-Haley, E., Kelsey, H.M., Jacoby, G.C., Nishenko, S.P., Hutchinson, I., Clague, J.J., Mathewes, R.W., 1997. Reconstructing the Palmer, S.P., Peterson, C.D., Reinhart, M.A., 1995. Summary of tsunami record on an emerging coast: a case study of Kanim Lake, coastal geologic evidence for past great earthquakes at the Cascadia Vancouver Island British Columbia, Canada. Journal of Coastal subduction zone. Earthquake Spectra 11, 1}18. Research 13, 545}553. Atwater, B.F., Stuiver, M., Yamaguchi, D.K., 1991. Radiocarbon test of Hutchinson, I., Guilbault, J.-P., Clague, J.J., Bobrowsky, P.T. (2000). earthquake magnitude at the Cascadia subduction zone. Nature Tsunamis and tectonic deformation at the northern Cascadia mar- 353, 156}158. gin: a 3000 year record from Deserted Lake, Vancouver Island, Benson, B.E., Grimm, K.A., Clague, J.J., 1997. Tsunami deposits be- British Columbia. The Holocene, in press. neath tidal marshes on northwestern Vancouver Island British Jacoby, G.C., Bunker, D.E., Benson, B.E., 1997. Tree-ring evidence for Columbia. Quaternary Research 48, 192}204. an A.D. 1700 Cascadia earthquake in Washington and northern Campbell, D.B., Skermer, N.A., 1975. Report to British Columbia Oregon. Geology 25, 999}1002. Water Resources Service on investigation of seawave at Kitimat, McMillan, A. D., and Hutchinson, I., 2000. When the mountain-dwarfs B.C. Golder Associates, Vancouver danced: paleoseismic events re#ected in aboriginal traditions in Clague, J.J., 1995. Early historical and ethnographic accounts of large northern Cascadia. Ethnohistory, in press. earthquakes and tsunamis on western Vancouver Island In: Current Morris, S., Leaney, A.J., 1980. The Somass River estuary; status of Research 1995-A. Geological Survey of Canada, Ottawa, Ontario, environmental knowledge to 1980. Canada Department of Fisheries pp. 47}50. and Oceans and Canada Department of Environment, Paci"c and Clague, J.J., 1997. Evidence for large earthquakes at the Cascadia Yukon Region, Special Estuary Series, no. 9. subduction zone. Reviews of Geophysics 35, 439}460. Murty, T.S., 1977. Seismic sea waves } tsunamis. Fisheries Research Clague, J.J., Bobrowsky, P.T., 1994a. Evidence for a large earthquake Board of Canada Bulletin 198. and tsunami 100}400 years ago on western Vancouver Island, Murty, T.S., 1979. Submarine slide-generated water waves in Kitimat Inlet British Columbia. Quaternary Research 41, 176}184. British Columbia. Journal of Geophysical Research 84, 7777}7779. Clague, J.J., Bobrowsky, P.T., 1994b. Tsunami deposits beneath tidal Murty, T.S., 1992. Tsunami threat to the British Columbia coast. In: marshes on Vancouver Island, British Columbia. Geological So- Geotechnique and Natural Hazards. BiTech Publishers, Van- ciety of America Bulletin 106, 1293}1303. couver, B.C., pp. 81}89. Clague, J.J., Bobrowsky, P.T., 1999. The geological signature of great Murty, T.S., Boilard, L., 1970. The tsunami in Alberni Inlet caused by earthquakes o! Canada's west coast. Geoscience Canada 26, 1}15. the Alaska earthquake of March 1964. In: Mans"eld, W.M. (Ed.), Clague, J.J., Bobrowsky, P.T., Hamilton, T.S., 1994. A sand sheet Tsunamis in the Paci"c Ocean; Proceedings, International Sympo- deposited by the 1964 Alaska tsunami at Port Alberni, British sium on Tsunamis and Tsunami Research, Honolulu, 1969. East- Columbia. Estuarine, Coastal and Shelf Science 38, 413}421. West Center Press, Honolulu, pp. 165}187. J.J. Clague et al. / Quaternary Science Reviews 19 (2000) 849}863 863

Murty, T.S., Hebenstreit, G.T., 1989. Tsunami amplitudes from local Plafker, G., Savage, J.C., 1970. Mechanism of the Chilean earthquake of earthquakes in the Paci"c Northwest region of North America; Part May 21 and 22, 1960. Geological Society of America Bulletin 81, 2: Strait of Georgia, Juan de Fuca Strait, and Puget Sound. Marine 1001}1030. Geodesy 13, 189}209. Rogers, G.C., Hasegawa, H.S., 1978. A second look at the British Nelson, A.R., Atwater, B.F., Bobrowsky, P.T., Bradley, L.-A., Clague, Columbia earthquake of 23 June, 1946. Bulletin of the Seismological J.J., Carver, G.A., Darienzo, M.E., Grant, W.C., Krueger, H.W., Society of America 68, 653}676. Sparks, R., Sta!ord Jr., T.W., Stuiver, M., 1995. Radiocarbon evid- Satake, K., Shimazaki, K., Tsuji, Y., Ueda, K., 1996. Time and size of ence for extensive plate-boundary rupture about 300 years ago at a giant earthquake in Cascadia inferred from Japanese tsunami the Cascadia subduction zone. Nature 378, 371}374. records of January 1700. Nature 379, 246}249. Ng, M.K.-F., LeBlond, P.H., Murty, T.S., 1990. Numerical simulation Thomson, R.E., 1981. Oceanography of the British Columbia coast. of tsunami amplitudes on the coast of British Columbia due to local Canada Department of Fisheries and Oceans, Canadian Special earthquakes. Science of Tsunami Hazards 8, 97}127. Publication of Fisheries and Aquatic Sciences no. 56. Ng, M.K.-F., LeBlond, P.H., Murty, T.S., 1991. Simulation of tsunamis Tsuji, Y., Ueda, K., Satake, K., 1998. Japanese tsunami records from the from great earthquakes on the Cascadia subduction zone. Science January 1700 earthquake in the Cascadia subduction zone. Zisin 51, 250, 1248}1251. 1}17 [in Japanese with English abstract, "gures, and "gure cap- Peterson, C.D., Darienzo, M.E., 1996. Discrimination of climatic, tions]. oceanic, and tectonic mechanisms of cyclic marsh burial. In: Rogers, Wigen, S.O., 1983. Historical study of tsunamis at To"no, Canada. In: A.M., Walsh, T.J., Kockelman, W.J., Priest, G.R., (Eds.), Assessing Iida, K., Iwasaki, T. (Eds.), Tsunamis * Their Science and Engin- Earthquake Hazards and Reducing Risk in the Paci"c Northwest. eering. Terra Scienti"c Publishing Co, Tokyo, pp. 105}119. U.S. Geological Survey Professional Paper 1560, pp. 115}146. Wigen, S.O., White, W.R., 1964. Tsunami of March 27}29, 1964, west Plafker, G., 1969. Tectonics of the March 27, 1964, Alaska earthquake. coast of Canada. Canada Department of Mines and Technical U.S. Geological Survey Professional Paper 543}I. Surveys, Ottawa, Ontario. Plafker, G., 1972. Alaskan earthquake of 1964 and Chilean earthquake Yamaguchi, D.K., Atwater, B.F., Bunker, D.E., Benson, B.E., Reid, of 1960 * implications for arc tectonics. Journal of Geophysical M.S., 1997. Tree-ring dating the 1700 Cascadia earthquake. Nature Research 77, 901}925. 389, 922.