INTRODUCTION TO TSUNAMI

A.Prof. Ted Bryant HomePage
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The Hollow of the Deep-Sea Wave off Kanagawa (Kanagawa Oki Uranami), a colour wood cut, No. 20 from the series "Thirty-six Views of Fuji" circa 1831, by Katsushika Hokusai. Many people depict this wave as a tsunami wave, but it is a wind-generated wave. It has a special shape called an N-wave characterised by a deep leading trough and a very peaked crest. Many tsunami emulate this form close to shore

Introduction

Tsunami are long water waves generated by offshore earthquakes, explosive volcanism near the surface of the ocean, submarine slides or a meteorite hit with the ocean. Tsunami can occur in oceans, bays, lakes or reservoirs.

The term tsunami is Japanese for a harbour (tsu) wave (nami), because such waves often develop as resonant phenomena in harbours after offshore earthquakes. Note that in Japanese, both the singular and plural of tsunami are the same.

The characteristic that makes tsunami so dangerous along a coast is their exceptional wavelength compared to their height. In the open ocean, even the largest tsunami rarely exceeds 0.5 m in height. However, the spacing between tsunami wave crests can be hundreds of kilometres. These characteristics allow a tsunami’s wave height to increase substantially in the last 10-20 m depth of water before shore. The wave then can override the coast without breaking or loss of energy. This allows tsunami to run up on dry land to heights beyond that of any conceivable storm wave or surge.

Facts about tsunami

Tsunami are most frequent in the Pacific Ocean (53.3% of all events) and the East Indies (20.3%). However, this incidence is deceptive. One of the most widespread tsunami recorded occurred in the Atlantic Ocean following the Lisbon earthquake on 1 November 1755. The Atlantic Ocean accounts for less than 2% of events historically, and yet the Lisbon wave reached heights of 7 m above sea level in the West Indies and even affected the coast of The Netherlands in the North Sea.

Tsunami caused by landslides have also killed over 200 people in the narrow fjords of Norway. Historically, they have swept the coastlines of inland seas such as the Black Sea and Sea of Marmara in Turkey. On 14 September 1509, one such tsunami overtopped 6 m high seawalls surrounding Istanbul.

Earthquakes account for the greatest number of tsunami and the greatest death toll (82% of events and 85% of deaths in the Pacific Ocean). Both volcanoes and submarine slides account for 5% of tsunami, although volcanoes are associated with a higher death toll (51,643 deaths for volcanoes versus 14,661 for slides in the Pacific Ocean).

The biggest recorded tsunami was produced by the Great Kamchatka Earthquake of 17 October 1737. Its run up reached more than 60 m above sea level on the North Kurile Islands. The Kamchatka Peninsula has the greatest frequency of tsunami, about one event every 12 years.

The largest death toll recorded for a tsunami was over 50,000 people on the island of Taiwan on 22 May 1782, followed by 36,417 deaths caused by tsunami associated with the eruption of Krakatau in the Sunda Straits of Indonesia on 27 August 1883. There are probably larger death tolls, but they are unsubstantiated. For example, the eruption of Santorini around 1670 BC had an important effect on Minoan civilisation in the Greek Islands and on the Island of Crete, but the death toll remains unknown.

Submarine slides and meteorite impacts

Until recently, submarine slides and meteorites were underrated as important hazards generating tsunami. The 17 July 1998 event along the north coast of Papua New Guinea jolted scientists’ awareness of this type of hazard. The coast was struck during the early evening by a moderate earthquake measuring 7.1 on the Richter scale. Twenty minutes later a tsunami roared in submerging some parts of the coast under 15 m of water. The earthquake was too small to generate such a large wave, and recent bathymetric mapping has shown evidence for a submarine slide on the steep slopes that flank the coast.

A cursory examination of a bathymetric map of the globe shows similar topography scattered throughout the oceans—from steep continental slopes to volcanic islands and seamounts—that are just as prone to submarine slides. In the geological past, huge chunks of the Hawaiian and Canary Islands have slipped into the ocean generating tsunami waves that might have had heights of 20 m or more in the open ocean. Even chunks of the Norwegian continental slope have collapsed as recently as 8000 years ago, sending waves more than 5 m in height across the North Atlantic.

These waves are enormous when compared to the fact that most recorded tsunami rarely exceed a height of 0.5 m in the open ocean. Meteor impacts with the ocean are just as underrated. While no historical event has been recorded, native peoples in many countries, including Australia and New Zealand, have legends about comet and meteorite affects. They also have numerous legends about tsunami.

Historical tsunami in Australia:

The fact that legends on tsunami exist in Australia is not unusual. After all, Australia is an island continent and lies exposed to active seismic and volcanic zones to the north. The problem in Australia is that the historical record is not that long and does not contain very large events.

The largest tsunami measured on the Sydney tide gauge had a height of only 1.07 m. It was generated by the Arica, Chile earthquake of 10 May 1877. The Chilean tsunami of 22 May 1960 measured less than 0.8 m on this gauge, but produced run up of 4.5 m above sea level along some parts of the coast.

The Northwest Coast of Australia, however, has the greatest frequency of tsunami and the largest events because it lies closest to the Sunda Arc of Indonesia. An earthquake there on 19 August 1977 generated a tsunami that measured 1.5 and 2.5 m on tide gauges at Port Hedland and Dampier. Both the Krakatau eruption in 1883 and an earthquake on 3 June 1994 generated tsunami that ran up over 4 m above sea level along this coast. The Krakatau tsunami moved boulders 2 m in diameter 100 m inland at Northwest Cape opposite gaps in the Ningaloo Reef.

Our fieldwork indicates that the Australian coastline contains a wealth of geological evidence for tsunami on a scale much greater than this historical record. These tsunami have been repetitive with the most recent events overlapping in time European exploration of the coast.

Prehistoric tsunami in Australia

Identification of a range of features or signatures imprinted upon the landscape by large tsunami has allowed us to construct a record of palaeo- (prehistoric) tsunami. These signatures include sedimentary deposits and erosional bedrock features.

Depositional and erosional signatures of tsunami

For example, there are numerous piles of boulders aligned like fallen dominoes along the top of cliffs at Jervis Bay, New South Wales. The waves that overwashed these cliffs also deposited shelly sand that can be radiocarbon dated around AD 1500. This event produced one wave that overran the 130 m high headland flanking the south of Jervis Bay.

The photographs in the linked windows show this evidence. At Gum Getters Bay, boulders as large as a boxcar were moved. They are stacked parallel with each other to the top of a 30 m high cliff. In this regard, cliff collapse did not produce the deposit. Rather the boulders were deposited from a wave that overwashed the cliff.

More dramatic are features carved into bedrock that render much of Australia’s rocky coast so scenic. Multiple vortices in fast moving water can sculpture solid bedrock into many distinct features such as stacks, arches, and sea caves. Our web pages show many photographs of these features around the Australian coastline. Many of these features have been attributed, without substantiation, to long-term erosion by wind-generated waves. However, some features lie beyond the reach of normal or high-energy wave attack and bear similarities to features carved by enormous floods that were last present on Earth during the Last Ice Age. Stacks such as Cathedral Rocks near Kiama on the New South Wales South Coast, and giant whirlpools on the sides of headlands were carved by vortices in tsunami flow within a few minutes.

 

Inverted keel-like forms at Cathedral Rocks, ninety kilometres south of Sydney, Australia. The stack is part of a cluster. They are aligned towards Bombo Headland in the background. A mega-tsunami approached the coastline from the southeast, overrode the headland, dropped into the sheltered embayment on the leeside, and then cut into the cliffs at Cathedral Rocks. Horizontal, eggbeater like vortices gouged out the stacks.. A sea cave was then bored into the cliffs by one of the vortices landward of the main stack.
Whirlpool bored into bedrock on the south side of Atcheson Rock south of Wollongong. The potholes around the floor of the whirlpool indicate the presence of smaller multiple vortices embedded in the main vortex. The tsunami carved this feature within a few minutes as it overtopped a 20-25 m high headland.

Some of the above events would be devastating were they to recur today near inhabited coastline. Fortunately, only two large events have been identified along the New South Wales coast over the past 10000 years. The size of the evidence is beyond the capability of earthquake-generated tsunami and alludes to a meteorite or comet hit with the ocean. The figure below presents a record of comet and meteorites for the last 2000 years.

 

Incidence of comets and meteorites between 0- AD 1800. The meteorite record sums Chinese, Japanese, and European observations. The comet record is only from Asia. The calibrated radiocarbon dates under the Mystic Fires of Tamaatea are from wood and peats burnt on the South Island of New Zealand. The Mystic Fires of Tamaatea refers to a Maori legend where fire from the sky scorched the Earth killing many people. The radiocarbon dates of prehistoric tsunami events in Australia are based on shell collected from tsunami deposits along the New South Wales coast and from Lord Howe Island.

Also shown is the frequency of corrected radiocarbon ages from tsunami deposits along the New South Wales coast and Lord Howe Island. The panel entitled Fires of Tamaatea plots the frequency of corrected radiocarbon dates on burnt wood and peat from the South Island of New Zealand. This dates a Maori legend about the descent of fire from the sky that destroyed the landscape. The AD 1500 tsunami along the east coast of Australia corresponds to a peak in the comet and meteorite observations, and to the timing of Maori legends describing a comet hit. Dates are now coming from tsunami deposits on the east coast of New Zealand confirming this age. Astronomers believe that a comet came into the inner solar system within the last 20,000 years and fragmented. The Earth periodically passes through the most concentrate part of the debris trail at 500-1500 year intervals. The evidence suggests that one such fragment might have struck the Australian-New Zealand region 500 years ago. Except for random meteorites, the Earth will not intercept this comet trail again until AD 3000.

While our east coast is not threatened by another comet- or meteorite-induced tsunami, tsunami generated by submarine slides cannot be ruled out. Radiocarbon dating suggests that an important tsunami occurs along the New South Wales coast every 1000 years or less. The source of these events is not difficult to envisage. There are over 170 slide scars on the continental shelf edge along the New South Wales coast. They have not been dated, but they are geologically recent. Any of them could cause a localised tsunami. Some slides are large. One off northern Wollongong measures 20 km in length and 10 km in width. It would have generated a tsunami that affected a considerable length of coast south of Sydney. There is no detection system in place in Australia to warn against tsunami generated by nearby submarine slides. Even if there were, people would have only 20- 30 minutes to evacuate to safety. They could do it though.

People fleeing the third and highest tsunami wave that flooded the seaside commercial area of Hilo, Hawaii following the Alaskan earthquake of 1 April 1946.

Where do you flee

Not all coastlines are exposed to tsunami. On the open coast, beaches, headlands, and cliffs are unsafe. Beaches are swamped while headlands that jut out onto the shelf receive the full force of any tsunami. Cliffs—even 100 m ones—also pose no barrier to a tsunami, because their height is puny compared to the 100 km wavelength of the tsunami. The back corner of an embayment is the safest place to flee to if a tsunami approaches one of our beaches. Islands are also dangerous. They tend to sit further out on the continental shelf. More important, tsunami will wrap around islands and become higher on the lee side. Here, the centre of the island is the safest place to seek refuge.

If one is on a boat, one should never come into shore, shelter behind a headland, or enter a harbour following a tsunami warning. Tsunami increase dramatically in height in water depths shallower than 20 m depth. They also increase in size inside harbours where resonance can operate.

Rivers leading from bays are also vulnerable. Tsunami can travel up a river to the tide limit, undergoing enhancement as the river narrows, and eventually spilling over the banks swamping floodplains.

Coastal floodplains within a few meters of sea level are also at risk. Once a tsunami gets onto a floodplain, it moves inland—depending on the type of obstructions—as if it was moving through shallow water. A ten-metre high wave could conceivably travel 8-10 km inland on a delta covered in pasture. On a forested delta, the same wave would only penetrate 500 m inland. The fifteen-metre high tsunami in Papua New Guinea in 1998 never travelled more than 600 m inland through trees.

If houses cover the floodplain, the same sized wave would only travel a few hundred meters. However, all buildings, including those made of reinforced concrete, would be destroyed. If the wave were only 3-4 m high, all buildings except wooden ones could survive. This offers an alluding possibility for safety in an urban coastal area if you were standing on a beach such as Bondi, and had about twenty minutes warning of a tsunami. You could simply turn and run to the nearest multi-storied building, making certain that it was not a block of flats with a secure entrance. You would then take the elevator to the top floor or run up the stairs one-to-two floors. Not only would you be positioned above flood level for most tsunami, but the building would also escape destruction.

These safety points are universal. They are worth remembering on your next holiday to any shoreline whether it is Waikiki Beach, Hawaii, the windswept coast of Northern Scotland, or the banks of Warragamba Dam. A tsunami will happen again, sometime soon, on a shoreline near you—on a reservoir, a lake, a sheltered sea, inside a coral barrier reef, in the lee of an island or along an open coast.

Visit the attached web links to see some of our evidence. I hope you enjoy what you see and that the photographs will excite you as much as it did us when we visited these sites and realised what was happening.

Further Reading

Bryant, E.A. 2001 Tsunami: The underrated hazard. Cambridge University Press, Cambridge, 320p.

Published Papers

Bryant, E.A. and Nott, J.A. 2001 Geological Indicators of large tsunami in Australia. Natural Hazards v. 24 No. 3 pp. 231-249.

Bryant, E.A.; Young, R.W.; Price, D.M.; Wheeler, D. and Pease, M.I. 1997 The impact of tsunami on the coastline of Jervis Bay, southeastern Australia. Physical Geography v. 18 No. 5 p. 441-460.

Bryant, E.A., Young, R.W. and Price, D.M. 1996 Tsunami as a major control on coastal evolution, Southeastern Australia. Journal of Coastal Research v. 12 No. 4 p. 831-840.

Bryant, E.A. and Young, R.W. 1996 Bedrock-Sculpturing by tsunami, South coast New South Wales, Australia. Journal of Geology v. 104 p. 565-582.

Young, R.W.; Bryant E.A., and Price D.M. 1996 Catastrophic wave (tsunami?) transport of boulders in southern New South Wales, Australia. Zeitschrift fÝr Geomorphologie v. 40 No. 2 pp. 191-207.

Young, R.W. and Bryant, E.A. 1993 Coastal rock platforms and ramps of Pleistocene and Tertiary age in Southern New South Wales, Australia. Zeit fur Geomorph N.F. v. 37 No. 3 p. 257-272.

Bryant, E.A.; Young, R.W. and Price, D.M. 1992 Evidence of tsunami sedimentation on the southeastern coast of Australia. Jour. Geology v. 100 No. 6 p. 753-765.

Bryant, E.A.; Young, R.W.; Price, D.M. and Short, S.A. 1992 Evidence for Pleistocene and Holocene raised marine deposits, Sandon Point, New South Wales. Australian Journal of Earth Science v. 39 p. 484-493.

Young, R. and Bryant, E. 1992 Catastrophic wave erosion on the southeastern coast of Australia: Impact of the Lanai tsunami ca. 105 ka? Geology v. 20 p. 199-202.

Young, R. and Bryant, E. 1992 Catastrophic wave erosion on the southeastern coast of Australia: Impact of the Lanai tsunami ca. 105 ka?: Reply Geology v. 20 p. 1151.

Papers

Bellambi: A barrier constructed by tsunami

Photos of tsunami features around Australia

Evidence for mega-tsunami around Australia and New Zealand

Thumbnail catalog of photos taken of tsunami signatures near Wollongong (Russian link)

 

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