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Tsunami: Where they Happen and Why
From: Cambridge University Press
| By:
Edward Bryant |
EDITOR'S INTRODUCTION |
 Since 1990, almost a dozen tsunami events have caused major devastation and loss of life throughout the world.These giant waves are closely linked with subaquatic seismic and volcanic activity, landslides, meteorites falling to earth and even weather conditions. This phenomenon, whose name is derived from the Japanese for harbor (tsu) and wave (nami), has been recorded for thousands of years. In this feature, extracted from his book Tsunami: The Underrated Hazard (available through Fathom), geoscientist Edward Bryant identifies the areas of the world in which tsunami occur and the disasterous impact which follows in their wake. |
 ccounts of tsunami extend back almost 4,000 years in China, 2,000 years in the Mediterranean--where the first tsunami was described in 479 B.C.--and about 1,300 years in Japan. However, many important tsunamigenic regions have much shorter documentation. For example the Chile-Peru coastline, which is an important source of Pacific-wide tsunami, has records going back only 400 years to 1562, while those from Alaska have only been documented since 1788. Tsunami records in Hawaii, which is a sentinel for events in the Pacific Ocean, exist only from 1813 onwards. Few records exist along the West Coast of Canada and the contiguous United States. The Southwest Pacific Ocean records are sporadic and almost anecdotal in reliability. Only in the last ten years have records been compiled from Australia and New Zealand, with historical documentation extending back no further than 150 years. |
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The regional distribution of major tsunami is tabulated in Table 1.1. Only the South Atlantic appears to be immune from tsunami. The North Atlantic coastline also is virtually devoid of tsunami. However, the Lisbon earthquake of 1 November 1755, which is possibly the largest earthquake known, generated a 15-m-high tsunami that destroyed the port at Lisbon. It also sent a wall of water across the Atlantic Ocean that raised tide levels 3-4m above normal in Barbados and Antigua in the West Indies. Tsunami also ran up and down the West Coast of Europe, and along the Atlantic Coast of Morocco. The Spanish port of Cádiz and Madeira in the Azores were also hit by waves 15m high, while a 3- to 4-m-high wave sunk ships along the English Channel. The continental slope off Newfoundland, Canada, is seismically active and has produced tsunami that have swept onto that coastline. The Burin Peninsula tsunami described earlier reached Boston, where it registered a height of 0.4m. By far the most susceptible ocean to tsunami is the Pacific Ocean region, accounting for 52.9% of all events. The following sections describe the more noted areas for tsunami. |
Mediterranean Sea
The Mediterranean Sea has one of the longest records of tsunami. Over three hundred events have been recorded since 1300 B.C. Large tsunami originate in the Eastern Mediterranean, the Straits of Messina of southern Italy, or southwest of Portugal. About 7% of known earthquakes in this region have produced damaging or disastrous tsunami. Around Greece, 30% of all earthquakes produce a measurable seismic wave, and seventy major tsunami have been recorded. Around Italy, there have been sixty-seven reliably reported tsunami over the past 2,000 years. The majority of these have occurred in the last 500 years, as records have become more complete. Of these, forty-six were caused by earthquakes and twelve by volcanoes. By far the most destructive tsunami followed an earthquake on 28 December 1908 in the Messina Strait region. A small proportion of the 60,000 people killed during this event were drowned by the tsunami, which flooded numerous coastal villages and reached a maximum run-up exceeding 10m in elevation. |
Caribbean Sea
The Caribbean--including the South Coast of the United States--is particularly prone to tsunami as the Caribbean plate slides eastward relative to the North American plate at a rate of 2cm yr-1, producing strong seismic activity in the Puerto Rico Trench. Unfortunately, the threat here has been overshadowed by the more frequent occurrence of tropical cyclones or hurricanes. The record of tsunami is one of the longest in North America, beginning on 16 April 1690 with an event near St. Thomas in the Virgin Islands. Here the sea retreated vertically 16-18m. The devastating Port Royal, Jamaica, tsunami, which drowned 3,000 people, followed this event two years later in June 1692. An earthquake that sent much of the city sliding into the sea triggered the tsunami. Ships standing in the harbour were flung inland over two-storey buildings. An earthquake in the Anegada trough between St. Croix and St. Thomas produced another significant tsunami on 18 November 1867. The resulting tsunami reached 7-9m at St. Croix, 4-6m high at St Thomas, 3m at Antigua, and 1-6m in Puerto Rico. Run-ups of 1.2-1.5m were common elsewhere throughout the southern Caribbean. Other notable events have occurred in 1842, 1907, 1918, and 1946. Of these, two bear mention--the tsunami of 25 October 1918 and 4 August 1946. The former event had a maximum run-up height of 7m at Frederiksted, St. Croix, and was recorded at Galveston, Texas. The latter event followed a magnitude 8.1 earthquake off the Northeast Coast of the Dominican Republic. Locally, the tsunami penetrated several kilometres inland and drowned about 1,800 people. It also was observed at Daytona Beach, Florida. |
Pacific Ocean
Figure 1A plots the distribution of 1,274 observations of tsunami reported along the coastlines of the Pacific Ocean since 47 B.C. The size of the circles is proportional to the number of observations per degree square of latitude and longitude. The data are biased in that the same event can be recorded at more than one location. The map excludes 217 observations that cannot be precisely located. The distribution of all observations is tabulated by region in Table 1.2. Because some countries have better observation networks than others, smaller events are overemphasised. This is true of the West Coast of North America, which is overrepresented in the modern record, despite having records of tsunami for only the last 200 years. Some countries are underrepresented in Figure 1A. For example, over a hundred observations from Australia are not included. The coastline of Japan has the longest historical record of tsunami, with 22.1% of all events originating here. Two other regions also stand out as having a high preponderance of tsunami--the coast of South America with 18.6% of events and Indonesia with 12.3%. A few small areas are highly prone to tsunami. These areas include Northern California, Hawaii, Southwest Chile, and the Chile-Peru border region. Destructive tsunami have inundated the Chilean Coast at roughly thirty-year intervals in recorded history. |
Tsunamigenic earthquakes with surface magnitudes greater than 8.2 on the Richter scale affect the entire Pacific Ocean once every twenty-five years. Figure 1B plots the source region of oceanwide events. For completeness, the map also includes some events, such as the eruption of Krakatau in 1883, not technically lying in the Pacific Ocean. Major events are also listed in Table 1.3. Significant events have increased in frequency in the twentieth century. Earthquakes in southern Chile, Alaska, and the Kamchatka Peninsula have the greatest chance of generating oceanwide tsunami in the Pacific. The West Coast of the United States provides a fourth source; however, the last Pacific-wide event originating here before European settlement occurred 300 years ago on 26 January 1700. The 22 May 1960 Chilean event is the most significant historical tsunami. It is a benchmark for tsunami in the twentieth century. A series of tsunami waves spread across the Pacific over a period of twenty-four hours, taking over 2,500 lives. The tsunami significantly affected such diverse places as Hawaii, Pitcairn Island, New Guinea, New Zealand, Japan, Okinawa, and the Philippines. |
The most tsunamigenic coastline in the world is that of the Kamchatka Peninsula, Russia. Between 1737 and 1990, the region experienced almost 8,000 earthquakes of which 96 generated localised tsunami. Volcanic eruptions here have also produced six tsunami, while four events have an unknown source. During the same period, the region was subject to fifteen tsunami from distant sources. A significant tsunami floods alluvium plains along the peninsula every 12.3 years. The most destructive tsunami, penetrating up to 10km inland, occurred in 1737, 1841, 1923, 1937, 1952, and 1969. The largest event followed the Great Kamchatka Earthquake of 17 October 1737. Tsunami run-up heights reached 60m above sea level in the North Kurile Islands. The second largest event occurred on 4 November 1952, with run-up heights of 20m in the same area. This latter tsunami was also a Pacific-wide event. |
Local tsunami are also common on the islands of the South Pacific; however, no significantly sized, earthquake-induced tsunami has propagated outside this region. Because many islands drop off into deep water, locally generated tsunami can travel at their maximum velocity, up to 1,000km hr-1, and reach the adjacent shore in 5-10 minutes. The Papua New Guinea-Solomon Islands region has experienced seventy-eight tsunami in the period between 1768 and 1983. Volcanism has caused one eighth of all tsunami. The largest event occurred on 13 March 1888, when the Ritter Island volcano off the North Coast of Papua New Guinea collapsed, generating a 15-m-high tsunami. The most recent event of significance occurred in the early evening of 17 July 1998 along the Sissano-Aitape Coast of Northern Papua New Guinea. One of the stories earlier referred to this event. Eleven tsunami have struck Fiji in the one-hundred-year period 1877-1977, averaging one tsunami every ten years. Tsunami are a more frequent hazard here than tropical cyclones. Many small islands in the South Pacific are vulnerable to tsunami because populations are concentrated around coastlines and perceive the hazard as being rare. |
Over the past 2,000 years there have been 462,597 deaths attributed to tsunami in the Pacific region. Of these deaths, 95.4% occurred in events that killed more than a thousand people each. The number of deaths are tabulated in Table 1.4 for each of the main causes of tsunami, while the events with the largest deathtolls are presented in Table 1.5. Tectonically generated tsunami account for the greatest death toll, 84.5 %, with volcanic eruptions accounting for 11.2%--mainly during two events, the Krakatau eruption of 26-27 August 1883 (36,417 deaths) and the Unzen, Japan, eruption of 21 May 1792 (14,524 deaths). The number of fatalities has decreased over time and is slightly concentrated in Southeastern Asia, including Japan. The biggest tsunami of the twentieth century occurred in Moro Gulf, Philippines, on 16 August 1976, where 8,000 people died. The largest total death toll is concentrated in the Japanese Islands where 211,300 fatalities have occurred. Two events affected the Nankaido region of Japan on the 28 October 1707 and 20 September 1498, killing 30,000 and 26,000 people respectively. The Sanriku Coast of Japan has the misfortune of being the heaviest populated tsunami-prone coast in the world. About once per century, killer tsunami have swept this coastline, with two events striking within a forty-year time span between 1896-1933. On 15 June 1896 a small earthquake on the ocean floor, 120km southeast of the city of Kamaishi, sent a 30-m wall of water crashing into the coastline, killing 27,122 people. The same tsunami event was measured 10.5 hours later in San Francisco on the other side of the Pacific Ocean. In 1933, disaster struck again when a similarly positioned earthquake sent ashore a wave that killed 3,000 inhabitants. Deadly tsunami have also affect Indonesia and the South China Sea. In the South China Sea, recorded tsunami have killed 77,105 people, mainly in two events in 1762 and 1782. Indonesia has experienced a comparable death toll (69,420 deaths) over this period--the largest following the eruption of Krakatau in 1883. |
New Zealand and Australia
Australia and New Zealand are not well represented in any global tsunami database. This is surprising for New Zealand, because it is subject to considerable local tectonic activity and lies exposed to Pacific-wide events. At least thirty-two tsunami have been recorded in this latter country since 1840. The largest event occurred on 23 January 1855 following the Wellington earthquake. The run-up was 9-10m high within Cook Strait and 3m high at New Plymouth, 300km away along the open West Coast. However, the highest recorded tsunami occurred following the Napier earthquake of 2 February 1931. The earthquake generated little in the way of a tsunami; however, it triggered a rotational slump in the Waikare estuary that swept water 15.2m above sea level. The most extensive tsunami followed the 13 August 1868 Arica earthquake in Chile. Run-up heights of 1.2-1.8m were typical along the complete East Coast of the islands. At several locations, water levels dropped 4.5m before rising an equivalent amount. Subsequent earthquakes in Chile in 1877 and 1960 also produced widespread effects. New Zealand has the distinction of recording two tsunami generated by submarine mud volcanism associated with diapiric intrusions. These occurred near Poverty Bay on the East Coast of the North Island. The largest wave had a run-up of 10m elevation. |
In Australia, forty-three tsunami events have been recorded, beginning with the 1868 Arica, Chile, event. The closest sources for earthquake-generated tsunami lie along the Tonga-New Hebrides trench, the Alpine Fault on the West Coast of the South Island of New Zealand, and the Sunda Arc south of the Indonesian islands. The Alpine Fault is an unproven source because it last fractured around 1455, before European settlement. It has the potential to produce an earthquake with a surface magnitude, Ms, of 8.0 on the Richter scale, with any resulting tsunami reaching Sydney within two hours. The largest tsunami to be recorded on the Sydney tide gauge was 1.07m following the Arica, Chile, earthquake of 10 May 1877. However, the Chilean tsunami of 22 May 1960 produced a run-up of 4.5m above sea level. In Sydney and Newcastle harbours, this tsunami tore boats from their moorings and took several days to dissipate. The Northwest Coast is more vulnerable to tsunami because of the prevalence of large earthquakes along the Sunda Arc, south of Indonesia. The largest run-up measured in Australia is 6m, recorded at Cape Leveque, Western Australia, on 19 August 1977 following an Indonesian earthquake. Waves of 1.5m and 2.5m height were measured on tide gauges at Port Hedland and Dampier respectively. Another tsunami on 3 June 1994 produced a run-up of 4m at the same location. The Krakatoa eruption of 1883 generated a tsunami run-up in Geraldton, 1,500km away, that obtained a height of 2.5m. This tsunami moved boulders 2m in diameter 100m inland and more than 4m above sea level opposite gaps in the Ningaloo Reef protecting the Northwest Cape. South of the Northwest Cape, tsunami heights decrease rapidly because the coastline bends away to the east. At present, Indonesia is the only known source for tsunami in the Indian Ocean. |
Bays, fjords, inland seas, and lakes
Tsunami are not restricted to the open ocean. They can occur in bays, fjords, inland seas, and lakes. The greatest tsunami run-up yet identified occurred at Lituya Bay, Alaska, on 9 July 1958. The steep slope on one side of the bay failed following an earthquake, sending 0.3km3 of material cascading into a narrow arm of the bay. A wall of water swept 524m above sea level on the opposite shore, and a 30- to 50-m-high tsunami propagated down the bay, killing two people. Steep-sided fjords in both Alaska and Norway are also subject to similar slides. In Norway, seven tsunami-genic events have killed 210 people. The heights of these tsunami ranged between 5 and 15m, with run-ups surging up to 70m above sea level. |
Inland seas are also prone to tsunami. There have been twenty observations of tsunami in the Black Sea in historical records. In Bulgaria, maximum probable run-up heights of 10m are possible. One of the earliest occurred in the first century B.C at Karvarna. In A.D. 853, a tsunami at Varna swept 6.5km inland over flat coastal plain and travelled 30km up a river. Last century, on 31 March 1901, a 3-m-high tsunami swept into the port of Balchik. Bulgarian tsunami originate from earth-quakes on the Crimean Peninsula or from the eastern shore in Turkey. Submarine landslides are also likely sources because the Black Sea is over 2,000m deep with steep slopes along its eastern and southern sides. The Anatolian Fault Zone that runs through Northern Turkey and Greece has produced many tsunami in the Black Sea and the Sea of Marmara to the west. At least ninety tsunami have been recorded around the coast of Turkey since 1300 B.C. A tsunami flooded Istanbul on 14 September 1509, overtopping seawalls up to 6m high. At least twelve major tsunami have occurred historically in the Sea of Marmara, mainly in Izmit Bay. The most recent occurred on 17 August 1999. This tsunami appears to have been caused by submarine subsidence during an earthquake. Maximum run-up was 2.5m along the northern coast of the bay and 1.0-2.0m along the southern shore. Ten tsunami generated by earthquakes or landslides have been recorded in the Caspian Sea. Seven of these occurred on the West Coast and three on the East Coast. However, the risk is small, as run-ups for the 1:100 event do not exceed 1m. |
Finally, tsunami can be generated even in small lakes. The Krakatoa eruption of 27 August 1883 sent out a substantial atmospheric shock wave that induced a 0.5- m-high, 20-minute oscillation in Lake Taupo situated in the middle of the North Island of New Zealand. Burdur Lake in Turkey has had numerous reports of tsunami although the lake is only 15km long. On 1 January 1837 an earthquake-generated tsunami swept its shores and killed many people. Tsunami have washed up to 300m inland around this lake. |
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