Great Wave off the Coast of Kanagawa, by Hokusai, a famous late eighteenth- and early nineteenth-century Japanese artist. Part of The Thirty-Six Views of Fuji series (1823-29), this print, although often used a graphic in tsunami literature, is somewhat misleading in that context because tsunamis do not always manifest themselves as the huge breaking waves depicted in the print.
Plate Tectonic theory is based on an earth model characterized by a small number of lithospheric plates, 70 to 250 km (40 to 150 mi) thick, that float on a viscous underlayer called the asthenosphere. These plates, which cover the entire surface of the earth and contain both the continents and seafloor, move relative to each other at rates of up to ten cm/year (several inches/year). The region where two plates come in contact is called a plate boundary, and the way in which one plate moves relative to another determines the type of boundary: spreading, where the two plates move away from each other; subduction, where the two plates move toward each other and one slides beneath the other; and transform, where the two plates slide horizontally past each other. Subduction zones are characterized by deep ocean trenches, and the volcanic islands or volcanic mountain chains associated with the many subduction zones around the Pacific rim are sometimes called the Ring of Fire.
Tsunamis, also called seismic sea wave or incorrectly tidal waves, are caused generally by earthquakes, less commonly by submarine landslides, infrequently by submarine volcanic eruptions and very rarely by large meteorite impacts in the ocean. To generate a tsunami, the fault where the earthquake occurs must be underneath or near the ocean, and cause vertical movement of the seafloor (up to several meters) over a large area (up to a hundred thousand square kilometers). The amount of vertical and horizontal motion of the sea floor, the area over which it occurs, the simultaneous occurrence of slumping of underwater sediments due to the shaking, and the efficiency with which energy is transferred from the earth's crust to the ocean water are all part of the tsunami generation mechanism.
Where the ocean is over 6,000 m deep, unnoticed tsunami waves can travel at the speed of a commercial jet plane, over 800 km per hour (~500 mi per hour). They can move from one side of the Pacific Ocean to the other in less than a day. This great speed makes it important to be aware of the tsunami as soon as it is generated. Scientists can predict when a tsunami will arrive at various places by knowing the source characteristics of the earthquake that generated the tsunami and the characteristics of the seafloor along the paths to those places. Tsunamis travel much slower in shallower coastal waters where their wave heights begin to increase dramatically.
Pacific Warning Network -- Established in 1949, the Pacific Tsunami Warning Center (PTWC) in Ewa Beach, Hawai`i, provides warnings for teletsunamis to most countries in the Pacific Basin as well as to Hawai`i and all other US interests in the Pacific outside of Alaska and the US West Coast. Tsunami warnings, watches, and information bulletins issued by PTWC and other Regional Centers are disseminated to local, state, national, and international users as well as the media.
As the tsunami wave travels from the deep-water, continental slope region to the near-shore region, tsunami runup occurs. Runup is a measurement of the height of the water onshore observed above a reference sea level. Contrary to many artistic images of tsunamis, most tsunamis do not result in giant breaking waves (like normal surf waves at the beach that curl over as they approach shore). Rather, they come in much like very strong and very fast tides (i.e., a rapid, local rise in sea level). Much of the damage inflicted by tsunamis is caused by strong currents and floating debris. The force of some tsunamis is enormous. Large rocks weighing several tons, along with boats and other debris, can be moved inland hundreds of meters by tsunami wave activity, and homes and buildings destroyed. All this material and water move with great force, and can kill or injure people. The small number of tsunamis that do break often form vertical walls of turbulent water called bores. Tsunamis will often travel much farther inland than normal waves. After runup, part of the tsunami energy is reflected back to the open ocean. In addition, a tsunami can generate a particular type of wave called edge waves that travel back-and forth, parallel to shore. These effects result in many arrivals of the tsunami at a particular point on the coast rather than a single wave. Because of the complicated behavior of tsunami waves near the coast, the first runup of a tsunami is often not the largest, emphasizing the importance of not returning to a beach several hours after a tsunami hits.
July 30, 1995, Chilean Tsunami Model results showing the maximum runup and inundation relative to the normal sea level and shoreline (white line) at Tahauku Bay, Hiva Hoa, in the Marquesas Islands, French Polynesia.
Peru - 2001 A tsunami washed over the low-lying coastal resort region near Camaná, southern Peru, following a strong earthquake on June 23, 2001. The earthquake was one of the most powerful of the last 35 years and had a magnitude of 8.4. After the initial quake, coastal residents witnessed a sudden drawdown of the ocean and knew a tsunami was imminent. They had less than 20 minutes to reach higher ground before the tsunami hit. Waves as high as 8 m came in four destructive surges reaching as far as 1.2 km inland. The dashed line marks the approximate area of tsunami inundation. Thousands of buildings were destroyed, and the combined earthquake and tsunami killed as many as 139 people. This image (ISS004-ESC-6128) was taken by astronauts onboard the International Space Station on 10 January 2002. It shows some of the reasons that the Camaná area was so vulnerable to tsunami damage. The area has a 1 km band of coastal plain that is less than 5 m in elevation. Much of the plain can be seen by the bright green fields of irrigated agriculture that contrast with the light-colored desert high ground. Many of the tsunami-related deaths were workers in the onion fields in the coastal plain that were unwilling to leave their jobs before the end of the shift. A number of lives were spared because the tsunami occurred during the resort off-season, during the daylight when people could see the ocean drawdown, and during one of the lowest tides of the year.
Alaska - 1964 More than 90% of the deaths in Alaska during the 1964 earthquake and subsequent tsunamis were due to the tsunamis. The great Alaskan earthquake of 1964 was the largest earthquake in North America and the second largest ever recorded (largest occurred in Chile in 1960). The earthquake occurred at 5:36pm on March 27, 1964, Alaska Standard Time (or, at 03:36 Universal Time code on March 28, 1964). The epicenter was in the Northern Prince William Sound (61.1N 147.5W) about 75 miles E of Anchorage, or about 55 miles west of Valdez. The reported Richter magnitudes (Ms) for this earthquake ranged from 8.4 to 8.6. The moment magnitude (Mw) is reported as 9.2. The depth, or point where the rupture began was about 14 miles within the earth's crust. The 1964 earthquake caused 115 deaths in Alaska alone, with 106 of these due to tsunamis which were generated by tectonic uplift of the sea floor, and by localized subareal and submarine landslides.
Hokkaido - 1993 The Hokkaido-Nansei-Oki earthquake on July 12 produced one of the largest tsunamis in Japan's history. At 2217 local time (1317 UTC), the Ms-7.8 quake rocked the west coast of Hokkaido and the small, offshore island of Okushiri in the Sea of Japan, generating a major tsunami. Within 2-5 minutes, extremely large waves engulfed the Okushiri coastline and the central west coast of Hokkaido. Extensive damage occurred on the southern tip of Okushiri Island at the town of Aonae.
Hokkaido - 1993 Tsunami vertical runup measurements varied between 15 and 30 m over a 20-km portion of the southern part of Okushiri Island, with several 10-m values on the northern portion of the island. Along the west coast of Hokkaido, no survey values exceeded 10 m, but damage was extensive at several coastal towns. Given the sudden onset of the tsunami and its high energy, it is amazing that more people were not killed. As of 21 July 1993, 185 fatalities were confirmed, with 120 attributed to the tsunami. The death toll is expected to rise, as missing persons are included among the fatalities. Property losses have been estimated at $600 million, due principally to tsunami damage.
Papua New Guinea, 17 July 1998
On the evening of Friday July 17, 1998, a magnitude Ms 7.1 earthquake occurred near the northwest coast of Papua New Guinea 850 km (510 miles) northwest of Port Moresby, the capitol of Papua New Guinea (PNG). The earthquake, which occurred at 6:49 pm local time (08:49 GMT), was followed by a series of three catastrophic tsunami waves that devastated the villages of Sissano, Warupu, Arop (1 and 2) and Malol on the north coast of PNG killing at least 2,182, injuring 1,000, and displacing more than 10,000 people. Looking back toward the coast from about one-fifth km inland near the Arop school site. Tsunami sand was deposited more than 650 m inland at this location. Tsunami deposits were gray colored sand typically overlying brown, rooted soil. The sand particles were larger near the shore. In places, plants were bent over and buried by the sand or removed entirely by the tsunami. (Photo credit: L. Dengler, Humboldt State University.)
Papua New Guinea, 17 July 1998
The sand spit where the two Arop villages once stood. In the foreground are remains of a septic tank. The wave removed almost all other traces of the several hundred houses that stood on the sand spit. (Photo credit: Hugh Davies, University of PNG.)
Papua New Guinea, 17 July 1998 Air photo of the Sissano Lagoon spit near the lagoon mouth. The Arop village of Otto was located near here. The spit averages about 100 m in width and is no higher than a few meters in elevation above sea level. The tsunami overtopped the spit and washed the villages on it into the lagoon. Note debris in the lagoon. Here damage extended further from the coast because lagoon waters helped to transmit tsunami energy as far as 1.3 km from the coast, snapping off mature mangroves one to two meters above the water level. (Photo credit: National Mapping Bureau of Papua New Guinea.)
Papua New Guinea, 17 July 1998
Air photo taken east of the lagoon and Arop villages #1 and #2. Before the tsunami, transportation from the Arop villages to Malol and Aitape was mainly by boat through the marsh areas seen here. The tsunami penetrated about 0.5 km inland in this area. The yellow areas near the edge of the inundation zones are tree trunks. (Photo credit: National Mapping Bureau of Papua New Guinea.)