10 TSUNAMI (tsoo-nah-mee)
What does "tsunami" mean?
Tsunami is a Japanese word with the English translation, "harbor wave." Represented by two characters, the top character, "tsu," means harbor, while the bottom character, "nami," means "wave." In the past, tsunamis were sometimes referred to as "tidal waves" by the general public, and as "seismic sea waves" by the scientific community. The term "tidal wave" is a misnomer; although a tsunami's impact upon a coastline is dependent upon the tidal level at the time a tsunami strikes, tsunamis are unrelated to the tides. Tides result from the imbalanced, extraterrestrial, gravitational influences of the moon, sun, and planets. The term "seismic sea wave" is also misleading. "Seismic" implies an earthquake-related generation mechanism, but a tsunami can also be caused by a nonseismic event, such as a landslide or meteorite impact.
How do tsunamis differ from other water waves?
Tsunamis are unlike wind-generated waves, which many of us may have observed on a local lake or at a coastal beach, in that they are characterized as shallow-water waves, with long periods and wave lengths. The wind-generated swell one sees at a California beach, for example, spawned by a storm out in the Pacific and rhythmically rolling in, one wave after another, might have a period of about 10 seconds and a wave length of 150 m. A tsunami, on the other hand, can have a wavelength in excess of 100 km and period on the order of one hour.
As a result of their long wave lengths, tsunamis behave as shallow-water waves. A wave becomes a shallow-water wave when the ratio between the water depth and its wave length gets very small. Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s/s) and the water depth - let's see what this implies: In the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s, or over 700 km/hr. Because the rate at which a wave loses its energy is inversely related to its wave length, tsunamis not only propagate at high speeds, they can also travel great, transoceanic distances with limited energy losses.
This animation shows the propagation of the earthquake-generated 1960 Chilean tsunami across the Pacific. Note the vastness of the area across which the tsunami travels - Japan, which is over 17,000 km away from the tsunami's source off the coast of Chile, lost 200 lives to this tsunami. Also note how the wave crests bend as the tsunami travels - this is called refraction. Wave refraction is caused by segments of the wave moving at different speeds as the water depth along the crest varies.
What happens to a tsunami as it approaches land?
As a tsunami leaves the deep water of the open ocean and travels into the shallower water near the coast, it transforms. If you read the "How do tsunamis differ from other water waves?" section, you discovered that a tsunami travels at a speed that is related to the water depth - hence, as the water depth decreases, the tsunami slows. The tsunami's energy flux, which is dependent on both its wave speed and wave height, remains nearly constant. Consequently, as the tsunami's speed diminishes as it travels into shallower water, its height grows. Because of this shoaling effect, a tsunami, imperceptible at sea, may grow to be several meters or more in height near the coast. When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore.
Tsunamis rarely transform into the great, towering breaking waves many of us imagine. In video footage taken in Japan during the 1983 Sea of Japan tsunami, we see the tsunami attack as a very fast-rising flood. The bystanders on the sea wall who can barely outrun the on-rushing water.
Sometimes a tsunami may break far offshore, out in deeper water than the wind-generated swell typically breaks.
As a tsunami propagates from open water into an abruptly shallower bay or river, it may form into a bore. A bore is a step-like wave with a steep breaking face connecting the undisturbed water in front of the bore with the deeper water behind it.
What happens when a tsunami encounters land?
As a tsunami approaches shore, we've learned in the "What happens to a tsunami as it approaches land?" section that it begins to slow and grow in height. Just like other water waves, tsunamis begin to lose energy as they rush onshore - part of the wave energy is reflected offshore, while the shoreward-propagating wave energy is dissipated through bottom friction and turbulence. Despite these losses, tsunamis still reach the coast with tremendous amounts of energy. Tsunamis have great erosional potential, stripping beaches of sand that may have taken years to accumulate and undermining trees and other coastal vegetation. Capable of inundating, or flooding, hundreds of meters inland past the typical high-water level, the fast-moving water associated with the inundating tsunami can crush homes and other coastal structures. Tsunamis may reach a maximum vertical height onshore above sea level, often called a runup height, of 10, 20, and even 30 meters.
During the 1993 Hokkaido tsunami, the village of Aonae, located at the southern tip of Okushiri Island, Japan, was devastated by waves that swept across the exposed peninsula.
1957 Aleutian Tsunami
On March 9, 1957, at 14:22 GMT, an earthquake occurred south of the Andreanof Islands, in the Aleutian Islands of Alaska. A Pacific-wide tsunami was triggered by the earthquake, which had a surface-wave magnitude of 8.3, an epicenter of 51.5° N, 175.7° W, and a focal depth of 33 km. Even though no lives were lost, the Hawaiian Islands suffered the greatest with damage costs approximately $5 million (1957 dollars).
First photo in a series of three sequential photos show the arrival of a major wave at Laie Point on the Island of Oahu, Hawaii about 3,600 km from the source. The Island of Kauai, Hawaii, was hit twice as hard by this tsunami than by the Aleutian Islands tsunami in 1946. Houses were washed out and destroyed at Wainiha and Kalihiwai. At Haena, the waves reached heights of 16 m. In addition to that bridges were destroyed and sections of highways were flooded. At Hilo, Hawaii, the run-up was reached 3.9 m and damaged buildings. In Hilo Bay, Cocoanut Island was covered by 1 m of water and the bridge connecting it to shore was destroyed.
Second photo in a series of three sequential photos show the arrival of a major wave at Laie Point on the Island of Oahu, Hawaii. Although the northwest side of the Hawaiian Islands received high levels of water, the rest of the islands only received elevated water levels on average of 2 to 3 m. Both the 1946 and 1957 tsunamis occurred in the same general location (the Aleutian Islands). Even though the 1957 earthquake released more energy than the earthquake of 1946. The tsunami generated by this 1957 event caused less damage than the tsunami of 1946. This uncertainty of the potential destructive power of a tsunami forces Pacific Tsunami Warning System to issue warnings even when a tsunami may have little or no effect.
1964 Prince William Sound Tsunami
On March 28, 1964, at 03:28 GMT, an earthquake occurred in Prince William Sound of Alaska triggering a Pacific-wide tsunami. The earthquake had a surface-wave magnitude of 8.4, an epicenter of 61.1° N, 147.5° W, and a depth of 23 km. The earthquake, local tsunamis due to landslides, and the regional tsunami were responsible for taking the lives of more than 122 people and causing over $106 million in damage. The Surge wave left a 2 x 12 in. (5.2 x 31 cm) plank in a truck tire at Whittier, Alaska. Whittier incurred $10 million in property damage. One of the waves, probably the same one that caused the major damage in Whittier, reached a height of 31.7 m above low tide. At Whittier the waves destroyed two saw mills; the Union Oil Company tank farm, wharf and buildings; the Alaska Railroad depot; numerous frame dwellings; and the railroad ramp handling towers at the army pier. They also caused great damage to the small boat harbor. The tsunami killed thirteen people at Whittier, then a community of 70 people. The greatest amount of damage suffered by any location was Alaska. In Alaska the death toll was 106 and there was $84 million in damage. Among Alaskan areas the run-up measurements varied from 24.2 m at Blackstone Bay, 27.4 m at Chenega, 9.1 m at Valdez, and 6.1 m at Kodiak. Outside Alaska it took 5.4 hrs for the first wave to arrive at Hilo, Hawaii, where the run-up was measured at 3.0 m. Another city outside Alaska that received measurable run-up was Crescent City, CA, where a 4.3 m run-up was recorded 4.1 hrs after the tsunami was triggered. Even though the regional tsunami was very destructive the local tsunamis also caused significant damage. The local tsunamis were generated by landslides, which were triggered by the earthquake. At the Valdez Inlet a large landslide was triggered by the earthquake generated a tsunami that had a run-up measured at 67.0 m in the inlet. In areas where local tsunamis were generated by landslides nearby cities were given no warning of the oncoming waves. The inability to properly warn the Alaska region prompted the creation of the Alaska Tsunami Warning Center. The warning center can quickly warn towns of any threat of local tsunamis.
1975 Hawaiian Tsunami
On November 29, 1975, at 14:48 GMT, an earthquake occurred off the coast of the Island of Hawaii. A locally felt tsunami was triggered by the earthquake, which had a surface-wave magnitude of 7.2, an epicenter of 19.3° N, 155.0° W, and a focal depth of 8 km. The greatest lost was at Halape, a beach park at the base of a large cliff, on the Island of Hawaii.
At Halape, of the 32 campers 19 suffered injuries and 2 died. It was the sounds of the falling rocks from the cliff and the trembling that caused the campers to awake and a few moved to a coconut grove that was closer to the ocean. The campers were awaken by a second quake that sent large boulders down the cliff and forced the rest of the campers to flee toward the sea. However, these campers were forced back to cliffs when the campers at the coconut grove fleeing the rising ocean with cries of tsunami. The first wave that alarmed the campers was only 1.5 m. The second wave, however, was 7.9 m carried campers into a ditch near the base of cliff where they remained until the ordeal ended. Two of the campers were not so lucky and died. The coconut grove that a few campers took shelter in received permanent subsidence between 3.0 and 3.5 meters.
The largest recorded run-up was 14.3 m at Keauhou Landing, Hawaii Island. Also on the Island of Hawaii in the small bay of Punaluu the run-up reached 7.6 m. At Punaluu houses were swept off their foundations and properties were damaged. By the time local authorities could sound the coastal sirens the first wave had already arrived. As in the 1964 in Alaska the best warning to the possible danger of a local tsunami is the trembling from the earthquake that triggers it.
In the case of earthquake-generated tsunamis, the water column is disturbed by the uplift or subsidence of the sea floor. Submarine landslides, which often accompany large earthquakes, as well as collapses of volcanic edifices, can also disturb the overlying water column as sediment and rock slump downslope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can create an impulsive force that uplifts the water column and generates a tsunami. Conversely, supermarine landslides and cosmic-body impacts disturb the water from above, as momentum from falling debris is transferred to the water into which the debris falls. Generally speaking, tsunamis generated from these mechanisms, unlike the Pacific-wide tsunamis caused by some earthquakes, dissipate quickly and rarely affect coastlines distant from the source area. Lituya Bay, Alaska, after a huge, landslide-generated tsunami occurred on July 9, 1958. The earthquake-induced rockslide, shown in upper right-hand corner of this image, generated a 525 m splash-up immediately across the bay, and razed trees along the bay and across LaChausse Spit before leaving the bay and dissipating in the open waters of the Gulf of Alaska.
In June and October 1994 two major undersea earthquakes occurred, the first near Indonesia and the second near Japan. Both generated tsunamis, or seismic sea waves. In both cases reports of water running up onto land to heights of three to five metres were common (1 m is about 3.3 ft). In Indonesia many villages near river inlets were destroyed, and at least 200 people lost their lives. Tsunamis have been a recurring natural hazard throughout history. The Minoan civilization on Crete in the Mediterranean Sea was shaken by the combined effects of a volcanic eruption and a tsunami in the 2nd millennium BC, and Lisbon was devastated by a tsunami in 1755.
The Tsunami Warning System:An international effort to save lives and protect property
Overview of the Tsunami Warning System
The Tsunami Warning System (TWS) in the Pacific, comprised of 26 participating international Member States, has the functions of monitoring seismological and tidal stations throughout the Pacific Basin to evaluate potentially tsunamigenic earthquakes and disseminating tsunami warning information. The Pacific Tsunami Warning Center (PTWC) is the operational center of the Pacific TWS. Located near Honolulu, Hawaii, PTWC provides tsunami warning information to national authorities in the Pacific Basin.
Tsunami Warning Centers
As part of an international cooperative effort to save lives and protect property, the National Oceanic and Atmospheric Administration's (NOAA) National Weather Service operates two tsunami warning centers. The Alaska Tsunami Warning Center (ATWC) in Palmer, Alaska, serves as the regional Tsunami Warning Center for Alaska, British Columbia, Washington, Oregon, and California.
The Pacific Tsunami Warning Center in Ewa Beach, Hawaii, serves as the regional Tsunami Warning Center for Hawaii and as a national/international warning center for tsunamis that pose a Pacific-wide threat. This international warning effort became a formal arrangement in 1965 when PTWC assumed the international warning responsibilities of the Pacific Tsunami Warning System (PTWS). The PTWS is composed of 26 international Member States that are organized as the International Coordination Group for the Tsunami Warning System in the Pacific.
Tsunami Watch and Warning Determination
The objective of the PTWS is to detect, locate, and determine the magnitude of potentially tsunamigenic earthquakes occurring in the Pacific Basin or its immediate margins. Earthquake information is provided by seismic stations operated by PTWC, ATWC, the U.S. Geological Survey's National Earthquake Information Center and international sources. If the location and magnitude of an earthquake meet the known criteria for generation of a tsunami, a tsunami warning is issued to warn of an imminent tsunami hazard. The warning includes predicted tsunami arrival times at selected coastal communities within the geographic area defined by the maximum distance the tsunami could travel in a few hours. A tsunami watch with additional predicted tsunami arrival times is issued for a geographic area defined by the distance the tsunami could travel in a subsequent time period.
If a significant tsunami is detected by sea-level monitoring instrumentation, the tsunami warning is extended to the entire Pacific Basin. Sea-level (or tidal) information is provided by NOAA's National Ocean Service, PTWC, ATWC, university monitoring networks and other participating nations of the PTWS. The International Tsunami Information Center, part of the Intergovernmental Oceanographic Commission, monitors and evaluates the performance and effectiveness of the Pacific Tsunami Warning System. This effort encourages the most effective data collection, data analysis, tsunami impact assessment and warning dissemination to all TWS participants.
Tsunami Warning Dissemination
Tsunami watch, warning, and information bulletins are disseminated to appropriate emergency officials and the general public by a variety of communication methods.
* Tsunami watch, warning and information bulletins issued by PTWC and ATWC are disseminated to local, state, national and international users as well as the media. These users, in turn, disseminate the tsunami information to the public, generally over commercial radio and television channels.
* The NOAA Weather Radio System, based on a large number of VHF transmitter sites, provides direct broadcast of tsunami information to the public.
* The US Coast Guard also broadcasts urgent marine warnings and related tsunami information to coastal users equipped with medium frequency (MF) and very high frequency (VHF) marine radios.
* Local authorities and emergency managers are responsible for formulating and executing evacuation plans for areas under a tsunami warning. The public should stay-tuned to the local media for evacuation orders should a tsunami warning be issued. And, the public should NOT RETURN to low-lying areas until the tsunami threat has passed and the "all clear" is announced by the local authorities.