Natural Hazards 28: 433–461, 2003.
© 2003 Kluwer Academic Publishers. Printed in the Netherlands.
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Tsunami Hazard and Risk in Canada
JOHN J. CLAGUE
Department of Earth Sciences, Simon Fraser University, Burnaby, B.C., Canada V5A 1S6, Canada;
and Geological Survey of Canada, 101–605 Robson St., Vancouver, B.C., Canada V6B 5J3,
Canada, E-mail: jclague@sfu.ca
ADAM MUNRO
Environment Waikato, P.O. Box 4010, Hamilton East, New Zealand, E-mail:
adam.munro@ew.govt.nz
TAD MURTY
Baird & Associates, 1145 Hunt Club Rd., Suite 1, Ottawa, Ontario, Canada K1V 0Y3, Canada,
E-mail: tmurty@baird.com
(Received: 19 June 2001; accepted in final form: 4 October 2001)
Abstract. Tsunamis have occurred in Canada due to earthquakes, landslides, and a large chemical
explosion. The Pacific coast is at greatest risk from tsunamis because of the high incidence of
earthquakes and landslides in that region. The most destructive historical tsunamis, however, have
been in Atlantic Canada – one in 1917 in Halifax Harbour, which was triggered by a catastrophic
explosion on a munitions ship, and another in 1929 in Newfoundland, caused by an earthquaketriggered landslide at the edge of the Grand Banks. The tsunami risk along Canada’s Arctic coast and
along the shores of the Great Lakes is low in comparison to that of the Pacific and Atlantic coasts.
Public awareness of tsunami hazard and risk in Canada is low because destructive tsunamis are rare
events.
Key words: tsunami, hazard, risk, Canada
1. Introduction
Tsunamis have been responsible for considerable damage and loss of life in many
coastal areas, particularly around the Pacific Ocean. The cataclysmic eruption of
Krakatoa in Indonesia in 1883, for example, generated a tsunami with waves up to
30 m high that claimed about 36,000 lives. More recently, in 1998, a seismically
triggered submarine landslide produced a tsunami that killed over 2200 people in
Papua New Guinea.
Canada has experienced many tsunamis, but few have caused damage. The only
tsunami disaster in Jones’ (2000) exhaustive compendium of Canadian disasters
is the tsunami triggered by the Grand Banks earthquake of November 18, 1929,
which claimed 28 lives, all except for one on Newfoundland’s Burin Peninsula.
Interestingly, Jones excludes from his catalogue the tsunami caused by the Halifax
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Harbour explosion of December 6, 1917, even though many people drowned during
this catastrophe (Halifax Herald, December 7, 1917).
Canada experienced a third, large, destructive tsunami in 1964 on the west coast
of Vancouver Island following a great earthquake in Alaska. Several other, smaller
tsunamis have occurred on both the Atlantic and Pacific coasts, and it is possible
that much larger tsunamis than those in 1917, 1929, and 1964 could strike Canada
in the future. A tsunami larger than any of the historical period could be triggered
by a great earthquake at the Cascadia subduction zone, which underlies the sea
floor of the eastern North Pacific Ocean (Atwater et al., 1995; Clague, 1997),
or by a large landslide from the submerged flank of one of the volcanoes on the
Hawaiian Islands (McMurtry et al., 1999). A landslide anywhere along a populated
reach of the British Columbia coast also could generate a destructive tsunami. A
large landslide at the Atlantic continental margin, or collapse of the flank of a
volcano on the Canary Islands coast (Carracedo et al., 1999; Elsworth and Day,
1999; Ward and Day, 2001), might trigger a tsunami that would devastate Canada’s
east coast. These very large tsunamis are rare events, but if one were to occur, it
would have catastrophic effects on coastal infrastructure and might kill and injure
large numbers of people.
This paper summarizes what is known about tsunami hazard and risk in Canada
(“risk” is defined as hazard multiplied by exposure). The paper begins with a brief
description of the tsunami process, followed by a review of the different types of
tsunamis. Tsunami hazard is discussed next, first in a general way and then with
specific reference to Canada. Major tsunamis on the Pacific and Atlantic coasts
are described, and the socio-economic vulnerability of these coasts to tsunamis
is then discussed. The next section of the paper deals with tsunami preparedness
and mitigation. It is followed by conclusions and recommendations, and a list of
tsunami papers relevant to this paper.
2. Physical Basis for the Hazard
Tsunami is a Japanese word meaning ‘harbour wave’. The word has been generally
adopted worldwide for long-period gravity waves generated by a sudden displacement of the water surface (Figure 1). The cause of this sudden displacement or
release of energy is usually a submarine earthquake, but tsunamis may also be
triggered by landslides, volcanic eruptions, human-made explosions, and the impact of bolides in the ocean (Murty, 1977; Mofjeld et al., 1999, and references
therein). In deep water, tsunamis have amplitudes of less than 1 m, wavelengths
of hundreds of kilometres, and move at velocities of up to 1000 km h−1 . Due to
their long period, they behave as shallow water gravity waves (González, 1999). A
tsunami traveling into shallower water slows down and increases in height. Tsunamis may reach heights of 30 m or more in shallow water, although most tsunamis
arrive at the shore less than 1 m high.
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Figure 1. A large earthquake displaces the ocean floor, producing a tsunami that propagates
outward from the source. The velocity of the tsunami is proportional to the square root of the
ocean depth at any given point – as shown in this figure, the velocity of a wave in 5500 m of
water is over 800 km h−1 , but in 20 m of water the velocity is only 50 km h−1 . The waves
become higher, and their spacing shorter, as they approach the shore. (Clague, 2001, figure 1.)
3. Tsunami Types
Canadian tsunamis can be subdivided into three groups for the purpose of hazard
assessment. Teletsunamis are generated beyond the Canadian continental shelf,
have periods of up to 1 hour, and persist for several days. They affect a large section
of the coastline. Local tsunamis are generated within the Canadian continental
shelf, have shorter periods, and do not last long. They affect only a small section
of the coastline. A third group, which is less important than the other two, includes
tsunamis in lakes and rivers triggered by large landslides.
3.1. EARTHQUAKE - GENERATED TSUNAMIS
Earthquakes generate most tsunamis, generally by displacement of the sea floor.
Subduction earthquakes are particularly effective in producing tsunamis. The west
coast of Canada is vulnerable to this type of tsunami due to the presence of both
local and distant subduction zones at the margins of the Pacific basin (Thomson,
1981; Bernard, 1997, 1998).
3.2. LANDSLIDE - GENERATED TSUNAMIS
Landslides entering water may generate large waves that are extremely destructive
near their source. For example, an earthquake in Alaska in 1958 triggered a rockslide (31 × 106 m3 ) that fell into Lituya Bay in Alaska (Miller, 1960). The rock
mass pushed water to a height of 525 m above sea level on the opposite shore of
the fiord and generated a gravity wave that moved down the bay to its mouth at a
speed of between 43 and 58 m s−1 .
Submarine landslides can also produce large tsunamis. For example, an
earthquake-triggered landslide generated a devastating tsunami that killed more
than 2200 people in Papua New Guinea in July 1998 (González, 1999; McSaveney,
1999; Tappin, 2001; Tappin et al., 2001). Tsunami-producing submarine landslides
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can be triggered by a variety of mechanisms, including earthquakes, undersea
volcanic eruptions, and construction activity. Gases trapped under layers of sediment at the edge of a continental shelf could cause the sediments to fail and slide
downslope, triggering giant waves. Landslides can generate tsunamis in lakes and
rivers (see Section 5.4). Although these tsunamis can be very destructive, their
effects are localized.
3.3. VOLCANIC - GENERATED TSUNAMIS
Almost a quarter of the deaths caused by volcanic eruptions are the result of the
tsunamis generated by the eruptions (de Lange and Healy, 1999). Volcanogenic
tsunami mechanisms include earthquake-induced deformation, mass movement
(pyroclastic flows, avalanches, lahars, and lava flows), cratering (submarine explosions and caldera collapse), and phase coupling (basal surges, shock waves,
and atmospheric pressure waves). While there are presently no active volcanoes
in Canadian waters, the threat from Hawaii or another distant volcano merits
attention.
3.4. IMPACT- GENERATED TSUNAMIS
The Earth receives a constant rain of material from outer space, approximately
100,000 tonnes a−1 (de Lange and Healy, 1999). Most of this material is small
dust particles, but it includes 100-300 bolides greater than 1 kg. The larger objects impact the Earth’s surface with considerable force. Bolides larger than a few
hundred metres in diameter may produce sizeable tsunamis. A very large bolide,
many kilometres in diameter, could produce a tsunami with wave heights of several
kilometres (de Lange and Healy, 1999), although such an event is likely to occur,
on average only once every hundred million years or so.
3.5. TSUNAMIS PRODUCED BY EXPLOSIONS
Human-made explosions, such as that in Halifax in 1917, can generate large
displacement waves. These waves dissipate more rapidly than their natural counterparts due to their short periods (tens of seconds to minutes). Damage is also
confined to the immediate blast area.
4. Tsunami Hazard
A popular misconception about tsunamis is that a single, very large wave breaks
onto the shore. A tsunami, however, is not a single breaker, but rather a series of
waves separated by minutes to an hour or more (González, 1999). Generally, the
second or third wave is the largest. The waves commonly are turbulent, onrushing
surges and not simple breakers. When one wave overtakes another, however, a steep
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wall of turbulent water, or “bore”, may be created. A bore moves forward through
the transfer of momentum to the still waters trapped in front of it. Damage can be
considerable because of the high horizontal and vertical turbulence of the bore. In
some areas, a tsunami may be less energetic, with water levels rising rapidly, but
no strong landward surge of water.
Tsunami hazard is normally evaluated by the maximum wave run-up. Run-up
can be measured as either the elevation reached by the water, or the horizontal
distance the wave floods inland (inundation). Because inundation depth depends
on the height of the wave at the shore and local topography, vertical run-up is used
most often to quantify tsunami hazard. Hazard obviously increases with increasing
vertical run-up. Any run-up exceeding 1 m is considered dangerous.
The shallow water dynamics of tsunamis are complex and not fully understood
(Liu et al., 1991; Shuto, 1991). Wave amplitude and run-up are dependent on offshore bathymetry, shoreline orientation and shape, and onshore topography. Gentle
bottom slopes are conducive to wave breaking, whereas steep slopes produce strong
wave reflection with little or no breaking (Synolakis, 1991; Synolakis and Skjelbreia, 1993). Other topographic effects include focusing, attenuation, formation
of edge waves, formation of bores, and resonance in harbours, bays, and inlets
(Mofjeld et al., 1999, and references therein). Because topography varies along
most coasts, run-up may differ considerably over even short distances, making
it difficult to assess local hazard. For example, the 1993 Hokkaido Nansei-Oki
tsunami ranged in height from 5 to 30 m over a 500 m length of coast on Okushiri
Island in Japan (Satake and Tanioka, 1995). Estimates of local tsunami risk are,
nevertheless, necessary for populated areas and areas of new construction. In general, run-up is small on steep shorelines and is higher at the heads of shallow broad
bays and some long inlets, and along straight shorelines fronting broad shallow
offshore areas.
The threat may also vary over time. Successive tsunamis in any area are rarely
generated from exactly the same source or in the same way. Run-up and tsunami
behaviour, therefore, may be different in successive tsunamis.
4.1. IMPACTS
Tsunami death and damage result mainly from inundation, forces exerted by
flooding and receding waves, impact of floating debris, and contamination.
Tsunamis can generate current velocities of many metres per second due to
large differences in water level. Most drownings associated with tsunamis have
been people swept into deep water by the return flow. The high current velocities
also make tsunamis very erosive.
During the 1993 Hokkaido-Nansei-Oki tsunami, most deaths (71%) and injuries
were caused by the impact of floating debris (de Lange and Healy, 1999). Studies
have shown that debris carried by tsunamis can generate very high forces. Floating
wood pushed by tsunami bores, for example, may exert impulsive forces of more
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Figure 2. Aftermath of the tsunami produced by the great 1960 Chilean earthquake in
Hilo, Hawaii. Parking meters were bent by the force of debris carried by the turbulent waters of the tsunami. (Photo by U.S. Navy; provided by National Oceanic and Atmospheric
Administration, U.S. Department of Commerce.)
than 9 tonnes, higher than most structures can withstand (Figure 2; de Lange and
Healy, 1999). The return flow may also carry debris with the same potential for
injury and damage as an advancing tsunami bore.
Tsunamis may also spread liquid contaminants, which is of particular concern
in port areas where combustible materials such as fuel oil, gasoline, and other hazardous chemical compounds are common. In Japan, fishing vessels and other boats
swept inland by tsunami waves have been a major cause of fires associated with the
tsunamis. The fires are commonly sparked by overturned gas cooking appliances.
Public health can also be compromised if the tsunami damages sewage and other
wastewater services or water supply lines. Untreated wastewater may mix with the
water supply, spreading highly infectious diseases.
4.2. SOCIO - ECONOMIC VULNERABILITY
Humans have inhabited tsunami-prone shores for thousands of years, perhaps even
tens of thousands of years. Since the advent of the industrial era in the 1700s,
population and property in coastal areas susceptible to tsunamis have increased
by several orders of magnitude. Some of these people and their economic infrastructure are at risk because tsunamis cannot be prevented. Canada is fortunate
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because the number of people and amount of property at greatest risk along its
most tsunami-prone coast (the Pacific) are relatively small. The main population
centres in this region are located around the Strait of Georgia, where the risk is
relatively low. A small tsunami did occur in the Strait of Georgia in 1946 following
a large earthquake centered on central Vancouver Island, but it caused little damage.
Tsunami risk on the Atlantic coast of Canada is also low, although by no means
nil, as indicated in the next section.
Loss of life and property damage are closely related to tsunami size. Large
teletsunamis, with waves more than 10 m high, can be cataclysmic but fortunately
are rare. More common, smaller tsunamis, however, may still be destructive. Landslides can produce very large waves, although their impacts are more localized than
those of teletsunamis.
5. Tsunami Risk in Canada
5.1. PACIFIC COAST
The west coast of Canada is located along the Pacific “Ring of Fire” and is vulnerable to tsunamis generated by earthquakes beneath the Pacific Ocean (Figure
3; Lander et al., 1993; Lander, 1996; González, 1999). The largest tsunamis in
British Columbia result from great (magnitude 8 or larger) earthquakes at the Cascadia subduction zone where the oceanic Juan de Fuca plate moves underneath
North America (Clague, 1997; Clague et al., 1999a). The British Columbia coast
is also affected by tsunamis of more distant Pacific earthquakes. In fact, the largest
tsunami to strike British Columbia in historical time was generated by the great
(magnitude 9.2) Alaska earthquake of March 27, 1964 (Plafker, 1969; Lander,
1996). A series of waves radiated outward from the earthquake rupture area off
south-central Alaska and, within a few hours, reached the outer coast of British
Columbia, causing about $10 million damage (1964 Canadian dollars), mainly to
the Vancouver Island communities of Port Alberni, Hot Springs Cove, and Zeballos
(Thomson, 1981).
Tsunamis of more local extent, triggered by landslides and shallow, crustal
earthquakes also pose a hazard to people and property on Canada’s west coast,
especially in steep-walled fiords and inlets that indent the coast. The 1964 Alaska
earthquake triggered a large (∼75 000 000 m3 ) submarine slump near Valdez,
Alaska, which produced a local tsunami that destroyed waterfront facilities and the
fishing fleet (Coulter and Migliaccio, 1966). The slide and accompanying tsunami
were responsible for the loss of 30 lives, nearly 25% of all the casualties of the
earthquake. More recently, in November 1994, a submarine landslide at the head
of Taiya Inlet, just outside Skagway, Alaska, triggered a local tsunami with waves
up to 9–11 m high at the shoreline (Kulikov et al., 1996; Rabinovich et al., 1999;
Thomson et al., 2001). The tsunami killed one person and destroyed more than 300
m of a cruise ship dock that was under construction at the time. The landslide and
accompanying tsunami caused over $1 million damage to Skagway’s ferry terminal
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Figure 3. Most large tsunamis in the Pacific Ocean have been triggered by great earthquakes at
the subduction zones (black bands) shown in this figure. Circles locate the sources of tsunamis
larger than 10 cm in height that were recorded at the Tofino tide gauge on the west coast of
Vancouver Island between 1905 and 1981. (Clague, 2001, figure 3.)
and boat harbour. The replacement cost for the cruise ship dock has been estimated
to be $15–20 million (1994 U.S. dollars).
Fortunately, most people on the west coast of Canada live around the Strait
of Georgia, an area of low tsunami risk (Figure 4). The greatest risk is to small
communities on western Vancouver Island (e.g., Tofino, Ucluelet, Port Alberni)
(Figure 4; Clague et al., 1999a). The total population at risk in these communities
is about 5000, which is much smaller than the number of people living within the
tsunami inundation zone on the Pacific coasts of Oregon and Washington.
5.2. ATLANTIC COAST
Crustal earthquakes and large submarine landslides at the edge of the Atlantic
continental shelf may generate tsunamis that strike the east coast of Canada.
Most Atlantic tsunamis are destructive only near their source, but a distant, huge
submarine landslide could generate a very large tsunami with wide impact.
There have been only two destructive historical tsunamis in Atlantic Canada
– one in 1917 in Halifax Harbour, and the other in 1929 in Newfoundland. The
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Figure 4. Tsunami hazard map, southwestern British Columbia. The map shows four generalized hazard zones, as well as sites with evidence for large prehistoric tsunamis, the location of
the 1975 Kitimat tsunami (inset), and maximum wave heights of the 1964 Alaska tsunami. Details on four important recent tsunamis are given at the bottom. Southwestern British Columbia
is the only part of the Canadian coast where sufficient information exists to produce a map like
this. (Clague et al., 1999, figure 13.)
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Halifax Harbour tsunami was triggered by a massive chemical explosion and
thus was not a natural event. An earthquake-triggered landslide caused the 1929
tsunami. It claimed many more human lives than any other natural historical
tsunami, or for that matter earthquake, in Canada (see Section 6.2). The recurrence
interval for an earthquake of the size of the 1929 event is probably between a few
hundred years and 1000 years. Even if an earthquake of this size were to occur
off Canada’s east cost, it might not trigger a tsunami unless it vertically displaced
a large area of the sea floor. There is a greater risk that such an earthquake could
indirectly generate a destructive tsunami by triggering a submarine landslide, as
happened in 1929. A large tsunami-generating landslide could also occur independently of an earthquake, although there is no historical precedent for such an
event.
5.3. ARCTIC COAST
Little is known about the tsunami hazard in Arctic Canada. No significant tsunamis
have been reported in this region, and no geological evidence has been found for
prehistoric tsunamis. Although moderate and large earthquakes occur in some parts
of the Arctic, for example Baffin Bay and Beaufort Sea, the presence of extensive
sea ice greatly reduces the possibility of a large tsunami anywhere in the region.
5.4. INTERIOR WATERWAYS
Landslides can generate destructive waves in lakes and rivers bordered by steep,
unstable slopes and in lakes containing actively growing deltas. Although relatively rare, these events can cause much damage and loss of life. In August
1905, for example, Pleistocene glaciolacustrine sediments slid into the Thompson
River at Spences Bridge in southwestern British Columbia (Evans, 2001). The
landslide produced a wave 5–6 m high that rushed more than 1.5 km upstream,
destroying twenty buildings and drowning fifteen people. Three years later, in
1908, a landslide occurred suddenly on the Liève River in western Quebec (Evans,
2001). It generated a displacement wave that overwhelmed part of the village of
Nortre-Dame-de-la Salette, killing 27 people.
5.5. SPATIAL VARIABILITY
A surprising lesson of the 1964 Pacific tsunami is that, outside the source area in
Alaska, the largest wave amplitude, about 5.5 m, was not on the open ocean coast,
but at Port Alberni at the head of an inlet about 100 km from the coast (Wigen and
White, 1964; Thomson, 1981). Murty and Boilard (1970) explained this apparent
paradox as due to resonance amplification of the tsunami in Alberni Inlet. The
same phenomenon was invoked by Murty (1977) to account for the large waves on
Burin Peninsula following the 1929 Grand Banks earthquake. It is now known that
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tsunami height and run up depend, not only on the distance from the source, but
also on possible resonance amplification in inlets and bays.
Other factors affect tsunami velocity, wave height, and run up. Tidal height
at the time of the tsunami, shoreline configuration, and the gradient of the sea
floor just off the coast are some of the most important of these factors. Numerical
modeling by Chasse et al. (1993) demonstrated that the maximum amplitude of a
tsunami in the St. Lawrence Estuary would be about 1.5 m. The tsunami, however,
could reach up to 4.5 m asl at high tide or only to 0.9 m asl at low tide. Also,
the tsunami would travel much faster along the north side of the estuary, due to
its greater water depth, than along the south side. Similar dependence of speed
on propagation direction was noted during the 1929 Grand Banks tsunami. It took
much longer for the tsunami to reach Nova Scotia than Newfoundland, even though
Nova Scotia was closer to the tsunami source (Berninghausen, 1968).
6. Damaging Historical Tsunamis in Canada
6.1. HALIFAX , 1917
Shortly before 9 a.m. on December 6, 1917, the munitions ship Mont Blanc collided with the relief ship Imo in the Narrows of Halifax Harbour. The Mont Blanc
was carrying the equivalent of about 2630 tonnes of TNT. A fire broke out on the
Mont Blanc, and the crew abandoned the ship, which then drifted onto the Halifax
side of the harbour and grounded. A short time later, the cargo exploded, devastating a large section of the city. Nearly 2000 people were killed and 9000 injured
(Jones, 2000). Some of the deaths and injuries are attributable to the tsunami that
immediately followed the explosion (Greenberg et al., 1993, 1994; Ruffman et al.,
1995).
There was no operational tide gauge in Halifax Harbour at the time of the catastrophe, but survivors reported large waves and extreme water levels (Ruffman et
al., 1995). The explosion occurred near the time of low tide, which was about 0.5
m below mean water level.
Greenberg et al. (1993) reconstructed the tsunami using a finite element model
covering the area of Halifax Harbour from its entrance to the head of Bedford
Basin. The model was initialized with a set-up derived using an empirical formula
for waves resulting from explosions in water. The model results, together with
anecdotal reports, give a consistent picture of the tsunami’s progression and amplitude (Figure 5). At the site of the explosion, waves may have been as high as 10
m, but the tsunami diminished rapidly away from the source.
6.2. NEWFOUNDLAND , 1929
On Monday, November 18, 1929, a magnitude – 7.2 earthquake occurred about
20 km beneath the sea floor at the southern edge of the Grand Banks, 250 km
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Figure 5. (a) Travel times, in minutes, and (b) maximum heights, in metres, of the tsunami of
December 6, 1917. (Data from Greenburg et al., 1994.)
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south of Newfoundland (Heezen and Ewing, 1952; Piper et al., 1988). The shaking triggered a huge submarine slump. The slump, in turn, set off a tsunami that
propagated across the Atlantic Ocean, registering on tide gauges as far away as
South Carolina and Portugal. The tsunami arrived in Bermuda about two hours
after the earthquake, travelling at an average velocity of just over 700 km h−1 .
In contrast, it took about 2.5 hours to travel the much shorter distance to Burin
Peninsula of Newfoundland because of the drag exerted by the shallow continental
shelf north and west of the earthquake epicentre.
Burin Peninsula was hardest hit by the tsunami (Figure 6; Whelan, 1994).
Three main waves thundered up narrow channels and into bays over a half-hour
period on the evening of November 18. The first wave was preceded by an unusual
withdrawal of water, which left anchored boats in coastal communities high-anddry. In some areas, this withdrawal was followed by a turbulent onrush of water,
probably a bore. One eyewitness, then a young boy, described the wave coming
in smooth and fast; with the bright moon shining on its crest, it looked like a car
with its headlights beaming, driving up the harbour. In other areas, where there was
relatively deep water near the shore, the sea rose very rapidly with little turbulence.
The tsunami lifted small boats and schooners 5 m high, snapping anchor chains and
tossing the craft onshore or engulfing them. Houses floated from their foundations;
some were splintered, whereas others were swept back and forth by the flooding
and ebbing waters (Figure 7).
The tsunami damaged more than 40 coastal communities on Burin Peninsula
and claimed 27 lives in Newfoundland and one in Nova Scotia. Property damage,
including the cost of repairing the undersea cables, was $400 000 (1929 dollars).
Damage was made worse by the fact that the tsunami arrived near the peak of a very
high tide. Maximum wave heights on Burin Peninsula are not known; estimates at
the time of the disaster ranged from 9 to 15 m, but recent field investigations (A.
Ruffman, personal communication, 1999) suggest lower values: about 4.6 m in
Burin and Port au Bras, 3 m in Lamaline and Point au Gaul, and perhaps 7.5 m in
Taylor’s Bay.
6.3. VANCOUVER ISLAND , 1964
The Alaska earthquake of March 27, 1964, the second largest of the twentieth
century (magnitude 9.2), triggered tsunamis that killed 130 people, some as far
away as California (Soloviev and Go, 1975). The main tsunami swept southward
across the Pacific Ocean at a velocity of about 830 km h−1 , reaching Antarctica in
only 16 hours.
The tsunami caused extensive damage on Vancouver Island (Wigen and White,
1964; Murty and Boilard, 1970; Thomson, 1981) – it sank a fishing boat and
damaged log booms at Ucluelet, damaged wharf facilities at Tofino, breached the
municipal water pipeline that crosses the sea floor near Tofino, destroyed an Indian
village at Hot Springs Cove, and swept buildings off their foundations at Zeballos.
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Figure 6. Extent of damage from the 1929 tsunami on Burin Peninsula, Newfoundland.
(Clague, 2001, figure 9; modified from Whelan, 1994.)
Figure 7. Coastal communities on Burin Peninsula bore the brunt of the 1929 tsunami.
This photograph shows buildings in Lord’s Cove that were tossed and smashed by the
tsunami. (Photo by Harris M. Mosdell, from the collection of W.M. Chisholm; provided by A.
Ruffman.)
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Damage was greatest, however, at Port Alberni, located at the head of Alberni
Inlet. Three main waves struck Port Alberni between 12:20 a.m. and 3:30 a.m.
on March 28 (Figure 8). The sea surged up Somass River at a velocity of about
50 km h−1 and spilled onto the land, inundating whole neighbourhoods with chestdeep water. This first wave reached 3.7 m above geodetic datum and knocked out
the Port Alberni tide gauge. The second and most destructive wave swept into town
less than two hours later, at 2 a.m. The lights of the waterfront mills went out as the
water smashed through the facilities. The surging water splintered the ground floor
of the Barclay Hotel, 1 km inland; guests had to be rescued from an upper floor
by police in boats. As the water subsided, some buildings were dragged seaward;
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 arrived 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 little further damage. Other waves oscillated in
Alberni Inlet, with decreasing strength for another two days. Two hundred and
sixty homes in Port Alberni were damaged by this tsunami, sixty extensively, and
the total economic loss in the town was about $5 million (1964 Canadian dollars)
(Thomson, 1981).
6.4. KITIMAT, 1975
Several submarine landslides occurred in Kitimat Inlet (Figure 4), a fiord on the
northern British Columbia coast, between 1952 and 1968 and also in 1971 and
1974. A submarine landslide on October 17, 1974, produced a tsunami with an
amplitude of 2.8 m, but the largest historic tsunami in the area occurred on the
morning of April 27, 1975 (Brown, 1975; Murty, 1979; Prior et al., 1982, 1984;
Johns et al., 1986).
Two or possibly three large waves were generated by the 1975 landslide; the
largest had a peak height from crest to trough of 8.2 m. The waves moved rapidly
to the head of the inlet and into connecting bays and inlets. Brown (1975) mentions
that the first wave arrived at Kitimat at 10:05 a.m., followed by a rapid withdrawal
of water. The entire event lasted only about 1 hour. Wharf facilities and boats at
Kitimat suffered the greatest damage, but some damage was recorded up to 8 km
from the site of the slide at Bish Creek.
The landslide occurred 53 minutes after low tide, suggesting that an ephemeral
decrease in buoyancy and yield stress triggered the event (Kulikov et al., 1999).
Construction of a new dock in the area may also be implicated in the failure.
The underlying cause of the landslide, however, is the presence of unstable, late
Quaternary glaciomarine and deltaic sediments at the sides and head of Kitimat
Inlet. The 1975 event involved two major failures, a landslide in sensitive glaciomarine clay on the west wall of the fiord and a larger failure of sediments on
the slope of the Kitimat River delta. The failed, fiord-wall and deltaic sediments
(estimated volume 10.5 × 106 m3 ) coalesced and flowed down Kitimat Inlet, mo-
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Figure 8. Tidal records for Port Alberni, British Columbia, March 27–29, 1964. The three
main waves of the Alaska tsunami arrived between 12:20 and 3:30 a.m. on March 28, forcing
the tide gauge off scale. Normal tidal heights are shown by the sine curve. (Clague et al., 1999,
figure 6; modified from Thomson, 1981, figure 9.2.)
bilizing an additional 25 × 106 m3 of fiord-bottom sediments. The debris flow
travelled over 5 km from the delta front on slopes of less than 0.4◦ to a water depth
of 210 m (Prior et al., 1984). Similar unstable sediments are present in most fiords
on the British Columbia coast; thus local, landslide-triggered tsunamis are possible
in many areas other than Kitimat Inlet.
6.5. PREHISTORIC TSUNAMIS
Sand sheets deposited by prehistoric tsunami have been found in mud and peat
deposits in protected tidal marshes at many sites on western Vancouver Island,
Washington, and Oregon (Clague, 1997; Clague et al., 1999a). They are evidence
that large tsunamis have struck the Pacific coast repeatedly during the last few
thousand years. The sand sheets thin, fine, and rise in a landward direction, and
commonly contain marine microfossils, indicating that they were deposited by
landward surges of water. Some of the sand sheets directly overlie buried peaty
or forest soils that subsided during large earthquakes (Atwater et al., 1995; Clague,
1997; Clague and Bobrowsky, 1999). The earthquakes that generated the tsunamis
TSUNAMI HAZARD AND RISK IN CANADA
449
occurred on the large thrust fault separating the subducting oceanic Juan de Fuca
plate and the overriding continental North America plate (a region referred to as
the Cascadia subduction zone). Analogues for these prehistoric subduction earthquakes include the great earthquakes in Chile in 1960 and Alaska in 1964, both of
which produced widespread crustal subsidence in the source areas and destructive
tsunamis that crossed the Pacific Ocean.
Recently, tsunami deposits have been found in coastal lakes in Oregon and
on western Vancouver Island (Kelsey et al., 1994; Hutchinson et al., 1997, 2000;
Clague et al., 1999b; Nelson et al., 2000). Tsunamis enter these lakes by surging
up outlet streams or by crossing sand dunes that lie between some of the lakes and
the sea. The most complete record found to date is from Bradley Lake in Oregon,
where there are 14 tsunami layers in a sequence spanning the last 7500 years.
So far, evidence for prehistoric tsunamis has been found only on Canada’s west
coast, but similar evidence possibly exists in Atlantic Canada. A widespread layer
of 8000-year-old sand, which has been found in coastal areas of eastern Scotland,
western Norway, and Iceland, was deposited by a large tsunami triggered by a
submarine landslide in the North Atlantic Ocean (Dawson et al., 1988; Bondevik
et al., 1997). On the basis of the known extent and character of the deposit, the
tsunami may have been large enough to leave a trace on the far side of the Atlantic,
in Canada. It is also possible that other submarine landslides or large earthquakes,
like the Lisbon, Portugal, earthquake of AD 1755, triggered large tsunamis that
have struck Atlantic Canada.
7. Preparedness and Mitigation
Tsunamis cannot be prevented, but the damage they cause can be reduced through
a variety of actions (Tsuchiya and Shuto, 1995; Bernard, 1997, 1998). Hazard
reduction measures are of two types: non-structural, which include land-use controls (zoning, relocation, and property acquisition), emergency preparedness, and
public education; and structural, which include dyking, barrier construction, flood
proofing, and tsunami-resistant construction. An essential first step in any tsunami
hazard reduction program is an assessment of the nature and level of risk for each
coastal community.
7.1. TSUNAMI HAZARD ASSESSMENT
Tsunami prediction is not possible, but risk to coastal communities can be assessed
by estimating the frequency, size, and probable impacts of tsunamis (Mofjeld et al.,
1997, 1999, 2000). Historical records and geological data provide information on
tsunami frequency and size. Maximum possible tsunami heights along a reach of
the coast are estimated using numerical models (e.g., Carrier, 1971; Shuto, 1991;
Whitmore, 1993; Bernard and González, 1994; Bernard et al., 1994; Geist and
Yoshioka, 1996) and the distribution of historic and prehistoric tsunami deposits.
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Maps can then be produced showing areas likely to be inundated by tsunamis with
various return periods (Bernard et al., 1994). These maps may be used to guide or
restrict development in tsunami-prone areas and to educate people living in these
areas about the risk they face. Numerical models also provide estimates of tsunami
arrival times, currents, and forces on structures (Mofjeld et al., 1999).
7.2. LAND - USE CONTROLS
The first objective of any hazard reduction program is protection of life. Public
safety may require that certain uses of land in high-hazard areas be restricted
through zoning regulations or by relocating property owners to higher ground;
however, as a rule, these types of action are strongly resisted by residents who
would be affected by them.
7.3. WARNING SYSTEMS
Protection of life also requires that there be an effective system in place to warn
people of an approaching tsunami and to provide timely evacuation. The principle
objective of a tsunami warning system is to provide accurate and timely notification of an impending tsunami in order to protect life and property (Bernard, 1997,
1998). A secondary objective is to inform and educate people living within tsunami
risk zones so that they are prepared to respond when warnings are issued.
Warning systems, however, are useful only when the source of the tsunami is far
from populated shores. The travel times of far-field tsunamis are sufficiently long
that threatened low-lying areas in British Columbia could be evacuated following
alerts. Where the source of the tsunami is less than about 100 km away, there is
insufficient time to warn and safely evacuate people, and no warning is possible
when local landslides trigger near-source tsunamis. In any case, property within the
inundation zone that cannot be quickly moved is likely to be damaged or destroyed.
There are three types of warming systems for tsunamis in the Pacific Ocean: a
Pacific-wide system (the Pacific Tsunami Warning Center) located in Hawaii and
operated by the U.S. Government; regional systems, including the West Coast and
Alaska Tsunami Warning System, located in Alaska; and local systems in Chile and
Japan (Bernard et al., 1988; Bernard, 1998). All three systems use earthquake magnitude and location to trigger warnings and coastal tidal stations to verify tsunamis.
False alarms are common because most warnings are based on earthquake data,
rather than direct tsunami measurements, and not all earthquakes trigger tsunamis.
The Provincial Emergency Program (PEP) has been designated by the British Columbia Government to disseminate tsunami bulletins issued by the Pacific
Tsunami Warning Center and the West Coast and Alaska Tsunami Warning Center.
PEP is responsible for evaluating tsunami information received from the warning
centres and deciding on appropriate follow-up action in British Columbia. The Canadian Hydrographic Service (CHS) provides PEP with information on tsunamis in
TSUNAMI HAZARD AND RISK IN CANADA
451
real time based on data transmitted from its three warning stations (Langara Island,
Winter Harbour, and Tofino) and from the Pacific and Alaska tsunami warning
centers.
CHS also provides forecast information based on numerical modeling. Maximum wave heights and current velocities have been estimated for tsunamis originating from Alaska, Chile, the Aleutian Islands (Shumagin Gap), and Kamchatka for
185 sites on the British Columbia coast (Dunbar et al., 1989, 1991). The simulated
Alaska tsunami produced the largest waves on the north coast. At all other sites,
the largest waves were generated by Shumagin Gap tsunamis. Wave amplitudes
of up to about 9 m and velocities of 3–4 m s−1 are predicted at several sites. The
most vulnerable regions are the outer coast of Vancouver Island, the west coast
of Graham Island (part of the Queen Charlotte Islands), and the central mainland
coast.
PEP contacts CHS whenever an earthquake of magnitude 6.5 or greater occurs anywhere in the Pacific. Most of these earthquakes do not lead to tsunami
warnings or watches, but the communication tests are extremely useful. PEP and
the governments of Alaska, Washington, Oregon, and California continue to work
closely with the West Coast and Alaska Tsunami Warning Center to improve
tsunami response in the Northeast Pacific. Regional responses to earthquakes,
tsunamis, and other emergencies are coordinated, in part, through meetings of the
B.C. Earthquake and Tsunami Working Group and the B.C. Regional Emergency
Telecommunications Committee.
Although useful, tsunami warning systems have rarely provided reliable measurements of wave heights (Hebenstreit, 1997). A large local earthquake and
tsunami can damage tide gauges or communication systems, limiting the usefulness
of real-time tsunami observing systems. In the case of Pacific-wide tsunami warnings, it is difficult to tell from a few tide gauge recordings what the wave heights
will be at other, more distant locations. Depending on their locations with respect to
the earthquake source, tide gauges may provide misleading information about the
tsunami. A tsunami in Japan in October 1997 produced high waves near the source,
leading to a warning for the entire Pacific coast of North America. The waves that
reached British Columbia, Washington, Oregon, and California, however, proved
to be insignificant. The costs of such false alarms can be considerable; for example,
a tsunami warning that triggered the evacuation of Honolulu on May 7, 1986, cost
Hawaii more than $30 M in lost salaries and business revenues (González, 1999).
How well tide gauge data represent a tsunami depends on the design and location of the gauges (Mofjeld et al., 1999). The stilling wells of tide gauges filter out
the wind waves and swell that commonly ride on top of tsunami waves. They also
limit the highest and lowest tsunami heights that can be measured by the gauge.
Many tide gauges are located in harbours where waves may be attenuated or amplified. Ideally, a tsunami-observing network should include gauges that record the
full range of possible waves, are shock-hardened, independently powered, report
in real time, and are located at exposed coastal and offshore sites.
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In recent years, interest has increased in using wave measurement devices to
record the passage of tsunamis in deep water (Okada, 1995; González, 1999).
NOAA is developing a network of deep-ocean reporting stations that can track
tsunamis and report them in real time, a project known as Deep-Ocean Assessment
and Reporting of Tsunamis (DART). The network comprises a number of sensitive
ocean-bottom sensors that detect the increased pressure from the additional volume
of water produced when a tsunami wave passes overhead. The sensors transmit
the pressure measurements to a buoy at the ocean surface, which then relays the
information to a ground station via satellite.
7.4. EDUCATION
Even the most reliable tsunami warning system is ineffective if people do not respond appropriately. Because tsunamis are infrequent, their recollection, as with
any rare natural hazard, fades with time, leading some communities into a false
sense of security. Education is therefore essential if communities are to become
more resilient to tsunamis. In this context, public awareness of the nature of tsunamis, the chance of one occurring, areas likely to be affected, and correct response
to tsunami warnings are important.
A public education program should provide tsunami information at regular intervals, perhaps annually, and should include instructions on how to get information
during an alert, where to go, and what things to take. Educational initiatives should
be entrenched into school curricula to ensure that future generations understand
the hazards and potential impacts of tsunamis. Education about tsunamis should
not be limited to only those living on or near the coast, but to all communities
because people from inland regions often travel to tsunami-prone areas.
A range of educational initiatives can be undertaken in coastal communities.
Activity sheets containing graphics, pictures, data, questions, and other relevant
information can be used in schools to educate students about tsunami hazards.
Tsunami evacuation routes can be publicized and marked by signs, as has been
done in Japan, the United States, and New Zealand (Figure 9). Citizens should
be consulted about land use in the tsunami inundation zone before decisions are
made about siting or relocation of critical facilities such as hospitals and police
stations, high-occupancy buildings such as schools, and petroleum-storage tank
farms. Tsunami information can be printed in newspapers and telephone books,
along with phone numbers of local emergency service offices. All telephone directories in British Columbia coastal communities, for example, contain information
on earthquakes and tsunamis. Citizens should also be regularly informed about
local warning systems.
TSUNAMI HAZARD AND RISK IN CANADA
453
Figure 9. Two examples of tsunami warning signs.
7.5. STRUCTURAL AND OTHER MEASURES
Structural hazard reduction measures provide protection from tsunamis either by
preventing them from inundating low-lying areas or by lessening their impact on
buildings. Dykes and walls can be constructed to prevent waves from reaching
threatened residential and commercial areas, although these are expensive and
should be built to the highest possible level that can be reached by a tsunami.
In some cases, offshore barriers can deflect tsunami waves or lessen their energy
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before they reach the shore. Again, these are expensive structures and may provide
only limited protection. They are economically feasible only where large populations are at risk, where the threatened shoreline is at the head of a bay or inlet, and
only in wealthy developed countries.
In areas of high tsunami hazard, buildings can be designed or protected to reduce water damage. Elevation of buildings and other types of flood proofing (e.g.,
installation of seals for basement windows, bolting houses to their foundations)
provide protection where water depths are 1 m or less. Structures can be elevated
to higher levels to provide greater protection, but costs may be prohibitive. Some
houses near the shoreline on the Hawaiian Islands, for example, have been built on
piers, their floors elevated 2–3 m above ground level.
7.6. RESPONSE AND RECOVERY
With adequate warning, emergency personnel will evacuate people in areas
threatened by an approaching tsunami. It is questionable, however, whether people
with little or no understanding of tsunamis will respond to an order to evacuate. Past
experience with tsunamis suggests that many people remain in, or return to, their
homes in order to remove possessions, only to be caught in the onrushing waters
(Atwater et al. 1999). In any case, some tsunamis reach the shore with insufficient
advance warning to effect an evacuation.
Unlike earthquakes, tsunami damage is restricted to the coastal zone. Aid thus is
quick to arrive in affected communities served by roads and airports. The response
may be much slower in remote communities, especially in developing countries
with little capacity to deal with natural disasters. Damaged areas may be reoccupied
and rebuilt with little or no appreciation of the fact that similar tsunamis may recur
in the future. Where the loss of life is large, however, governments may restrict or
prevent development in tsunami inundation zones.
8. Needs and Gaps
8.1. RESEARCH
Geological and hydrodynamic research is needed to further define tsunami hazard
and risk in Canada. The distribution of historical and prehistoric tsunami deposits
on the Pacific and Atlantic coasts should be documented to provide information
on local tsunami run-up. Some research of this type has been done on Vancouver
Island (Hutchinson et al., 1997, 2000; Clague et al., 1999b), but much more is
needed both there and in Atlantic Canada. More numerical modeling of tsunamis
produced by earthquakes and landslides is also required. Tsunamis can be simulated with high-speed computers using wave theory and digital models of real
coastlines (Figure 10; Dunbar et al., 1989, 1991; Hebenstreit and Murty, 1989;
Murty and Hebenstreit, 1989; Ng et al., 1990, 1991; Kulikov et al., 1996, 1999;
Rabinovich et al., 1999; Thomson et al., 2001). Results of the geological and
TSUNAMI HAZARD AND RISK IN CANADA
455
Figure 10. Sea level displacements generated by a hypothetical, M8.5 earthquake on the
northern Cascadia subduction zone: (a) Vancouver, Seattle, and selected sites on Vancouver
Island; (b) Port Alberni and Barkley Sound. The first tsunami wave reaches the west coast
of Vancouver Island about one-half hour after the earthquake. Much smaller waves arrive at
Vancouver and Seattle almost four hours later. Resonance in Alberni Inlet is responsible for
the approximately threefold amplification of waves between Barkley Sound and Port Alberni.
(Clague et al., 1999, figure 14; modified from Ng et al., 1990, figs. 4 and 5.)
hydrodynamic research provide the basis for preparation of tsunami inundation
maps, which can be used for education and to prepare communities for tsunamis
(Priest, 1995; Wang and Priest, 1995).
Research of other kinds is also required. There are significant gaps in our ability
to accurately predict tsunami amplitudes from seismic data. Some earthquakes have
generated much stronger tsunamis than were expected from the magnitudes of the
earthquakes (Abe, 1995; Hatori, 1995; Tanioka and Satake, 1996). The size of a
tsunami is related to fault rupture characteristics, such as the extent and speed of
rupture, which generally cannot be obtained from seismic data alone. Although
beyond the scope of this paper, engineering research is also needed to design and
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build, in a cost-efficient manner, tsunami-resistant buildings and lifelines. Social
and political studies may be required to develop appropriate strategies to prepare
communities for tsunamis and other natural disasters.
8.2. INSTITUTIONAL CAPABILITIES
Progress has been made at the municipal, provincial, and federal levels in Canada
in preparing for natural disasters. Unfortunately, emergency preparedness officials
at all three levels are largely unfamiliar with tsunamis and thus may be poorly
prepared to deal with them when the occasion arises. In particular, existing warning
systems for local tsunamis, such as one generated by a large earthquake at the
Cascadia subduction zone off British Columbia’s coast, are inadequate. In addition,
development is allowed in coastal areas without a consideration of tsunami hazards.
8.3. EDUCATION
Information on natural hazards, including tsunamis, is rarely provided in primary
and secondary schools in Canada. Such information should be part of primary
school curricula in British Columbia and perhaps Nova Scotia, New Brunswick,
and Newfoundland-Labrador. The information should include instruction on what
to do in the event of an earthquake or tsunami.
Coastal municipalities also have a responsibility to inform citizens of possible
tsunami hazards. Information can be provided through various media (e.g., newspapers, television, and radio) or through discussions centered on land-use planning
within the tsunami inundation zone. Agencies responsible for land-use permitting
have a responsibility to explain to the public why certain types of structures may
be proscribed in areas that could be inundated during a tsunami.
Although all these measures are laudable, they, unfortunately, are difficult to
implement. Convincing people that they should inform themselves about a natural
process that has a low probability, much less take any action to mitigate the hazard,
is a “hard sell”.
9. Conclusions and Recommendations
Tsunami hazard evaluation is needed for coastal management and to aid warning
centres during tsunami emergencies. Patterns of tsunami inundation and maximum
wave heights are derived from numerical simulations and field surveys of historical
and prehistoric tsunamis.
Tsunami risk in Canada ranges from very low to moderate, on both a regional
and local scale. The tsunami threat is greatest on the Pacific coast and lowest in
the Arctic. Tsunami risk differs considerably at the local scale due to differences in
exposure to tsunami-generating earthquakes, shoreline morphology, and offshore
bathymetry.
TSUNAMI HAZARD AND RISK IN CANADA
457
High priorities for an improved assessment of tsunami hazard are hydrodynamic
research on the generation and propagation of tsunamis and their behaviour in
shallow water; numerical modeling; improvement in real-time observing systems
deployed in the open ocean and in coastal waters; production of maps showing
inundation zones in populated coastal areas; and rapid and accurate predictions of
maximum tsunami wave heights during emergencies. Understanding how coastal
and offshore topography, friction, and non-linear processes affect tsunamis is especially important. Inundation maps can be used to improve public awareness of
tsunamis and for planning purposes (i.e., to reduce or avoid the risk).
The onus is on local authorities to implement disaster management plans,
control land use, and oversee public education and awareness. They are also responsible for maintaining contact lists and warning systems, and initiating frequent
emergency planning exercises.
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