GY3011 - The Active Earth

Web Page Assignment

Catherine Howe

A tsunami is a very long ocean wave which is generated by a sudden displacement of the sea floor. The term is derived form a Japanese word meaning "harbour wave". Tsunamis can occur with little or no warning, bringing death and massive amounts of destruction to coastal communities.

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INTRODUCTION

Any event which causes a significant displacement of the sea floor, also causes the displacement of an equivalent volume of water. This is the basic mechanism behind which tsunamis are generated (Murck et al., 1996). Although most tsunamis are produced from earthquakes, they can also be caused by volcanic activity, submarine landslides and occasionally by human activity.

Tsunamis pose a significant threat to over twenty countries in the circum-Pacific region. Over the past one hundred years, it has been estimated that approximately 50,000 coastal residents have lost their lives (Smith, 1996).

Tsunami forecasting and warning systems are well established. In 1948, a tsunami warning system was set up for the Pacific whereby seismograph stations around the Pacific relay their information to a Warning Centre near Honolulu, Hawaii (Lockridge, 1985). This international monitoring network currently relies upon approximately thirty seismic stations and seventy tide stations throughout the Pacific basin. Following the catastrophic tsunaimigenic earthquake of 1964 in Alaska, the Alaska Tsunami Warning Centre was established in 1967 to provide more localsied warning. In 1982, this regional responsibility was extended to British Columbia, Washington, Oregon and California. Project THRUST is a regional program, installed to aid those countries with little or no form of adequate warning system against tsunamis. It is a reliable, low cost form of protection and has proved to be very successful in less economically developed countries such as Chilie.

 

THE CREATION OF TSUNAMIS

The diagram below illustrates the countries affected by tsunamis as a direct result of seismic activity.

Most tsunamis are produced by earthquakes. The primary causes of wave generation are the release of energy and associated crustal deformation resulting from the earthquake (Murck et al., 1996). Any earthquake which produces a tsunami is known as a tsunamigenic earthquake.

The magnitude of the earthquake does not dictate whether or not a tsunami will be produced or its size, it is decided by the type of fault from which the earthquake is generated. For example, the San Andreas fault in California, is characterised by a horizontal, i.e., strike-slip, motion. There is no vertical displacement of the sea floor, therefore no tsunami was generated by the great 1906 earthquake of the area. By contrast, when the sea floor undergoes vertical deformation, it acts like a huge paddle, pushing huge volumes of displaced water outward from the zone of deformation.

The major Alaskan earthquake of 1964 was the largest in North America and the second largest ever recorded. The earthquake caused 115 deaths, 106 of these were due to the resultant tsunami. The tsunami was generated as a result of tectonic uplift and by localised submarine landslides (http://wcatwc.gov/64quake.htm).

An example of a tsunamigenic earthquake is the 1946 Unimak Island, Aleutian Islands in Alaska earthquake, which caused a tsunami that travelled at a velocity of 800km/h, innundating Hilo, Hawaii, 4.5 hours later with a run up of 18 metres higher than normal high tide.

Toward the lower left hand corner of the photo, it is possible to make out a picture of a man. He was swept away by the wave. The photo was taken by a sailor on the Brigham Victory.

(Picture taken from Dangerous Earth, published by Wiley, 1996)

 

PREDICTION AND MONITORING

The prediction of tsunamis centres around efforts on attempting to understand the mechanisms through which they are generated (Smith, 1996). Particular attention is given to tsunami earthquakes where, as aforementioned, the magnitude of the earthquake is not the sole deteminant of the magnitude of a tsunami, the disposition of crustal deformation is the crucial variable to consider.

THE PACIFIC TSUNAMI WARNING CENTRE

In 1948, a tsunami warning system was established for the Pacific whereby seismograph stations around the Ocean relay information to a Warning Centre near Honolulu, Hawaii. The Pacific Tsunami Warning Centre (PTWC) is based in Ewa Beach, Hawaii and includes the majority of nations bordering the Pacific. Between the thirty seismic and seventy tide stations throughout the area, an efficient operation is conducted, by operating seismic, tidal, communication and dissemination facilities to:

  1. Detect and locate major earthquakes in the Pacific basin;
  2. Determine whether a tsunami has been generated;
  3. Provide timely and effective information and warnings to minimize tsunami effects.

Tsunami information is provided from tidal stations which register characteristic tsunami waves and their heights. Seismic information is immediate once an earthquake has struck, whereas tsunami data is slower, as it is dependant upon the arrival of the wave at the tsunami stations.

This is a diagram illustrating the location of the seismic and tidal stations situated throughout the Pacific basin.

(Taken from http://walrus.wr.usgs.gov/nwtsunami)

 

At present, tsunami monitoring is operated on two levels.

The first level of cover provides warnings to all Pacific nations of large, destructive tsunamis which are Pacific-wide. Following a high magnitude earthquake (M=>7.0), local tide stations near the epicentre are alerted to watch for unusual wave activity. If this is detected, a tsunami warning is issued. Warnings are distributed in the form of Tsunami Warning Bulletins, issued when positive data is received from tidal stations indicating that a potentially destructive tsunami exists. Warnings are issued on the basis of seismic information only. When an earthquake of significant magnitude occurs in a tsunamigenic area, a bulletin is issued without waiting for any confirmation from tidal stations. Data are relayed from the seismic stations to tsunami warning stations via NOAA satelites. The aim is to alert all coastal populations at risk within a one hour time period or beyond 750km from the source, about the arrival time of the first wave with an accuracy of +/- 10 minutes.

Diagram of NOAA satelite early warning system

The second level of cover is regional only, based on warning systems serving specific tsunami prone areas, including Japan, Alaska and French Polynesia. Local tsunamis pose a greater threat than Pacific-wide events as they can strike very quickly (Smith, 1996). Regional systems rely on local data obtained in real-time via telephone lines or dedicated communication links. They aim to issue a regional warning within minutes, for areas within 100-750kms from the epicentre. For example, the Japanese Meteorological Agency has maintained its own warning service since 1952, designed to issue a warning within 20 minutes of a tsunamigenic earthquake occurring within 600kms off the Japanese coast.

Japan case study

 

PROJECT THRUST

An example of a regional based system is project THRUST (Tsunami Hazard Reduction Utilising Systems Technology). Project THRUST has been developed due to the fact that many coastal areas still remain unprotected from tsunami activity. In the less economically developed countries, a need for a reliable, low cost warning system has been recognised. The project is "a comprehensive approach to risk mitigation, based on rapid warning via satelite communication links" (Bernard et al., 1988). This system has been trialled for Valparaiso, Chilie, where the worst case scenario would imply a wave arrival within minutes. The system is activated instantly by the triggering of water level sensors which communicate messages via NOAA's GOES West satelite to the Valparaiso Tsunami Warning Centre, and also to the PTWC. Less than 3 minutes after an earthquake has triggered the system, information is available locally for the implementation of emergency action. Since the hardware costs for the THRUST project are minimal ($15, 000), and the GOES satelite covers the entire Pacific basin, the scheme appears to offer potential for rapid tsunami warning in the future.

THE ALASKA TSUNAMI WARNING CENTRE

1964 was the year of the Great Alaskan Earthquake and Tsunamis. More than 90% of the deaths from this event were due to the tsunamis (Murck et al., 1996). The potential death and devestation from tsunamis make the coastal areas of Alaska extremely vulnerable and necessitate continuous 24hr earthquake monitoring for each day of the year by the Alaska Tunami Warning Cente (ATWC), located in Palmer, Alaska. The 1964 events alerted State and federal officials to respond to the need for an effective and efficient tsunami and earthquake warning facility for Alaska and the Northern Pacific. The ATWC was established in 1967 and provides tsunami warnings for Alaska, Canada, Washington, Oregon and British Columbia. The ATWC has 16 remote seismic stations and 8 tide gauge stations in Alaska, 11 seismic stations and 9 tide gauge stations along the West Coast of Canada and the United States.

Although Alaska's seismic and tsunami history is only 200 years old, the region is extremely seismically active through the subduction of the Pacific plate under the North American plate. The vertical crustal movements in this area result in vertical sea floor displacements which, as has been described earlier, makes the area highly tsunamigenic. The past three tsunamis that were generated in Alaska have resulted in Pacific-wide destruction (http://wcatwc.gov/64quake.htm).

The ATWC operated in a slightly different fashion to the PTWC in that it relies heavily on regional stations which provide the earliest possible warning to the immediate vicinity of the epicentre by issuing warnings based on earthquake information only. Once a significant earthquake occurs in any tsunamigenic area, the ATWC issues immediate Warning and Watch bulletins, placing certain portions of the coast in warning status, and others in watch status. The threshold magnitude for issuance of bulletins varies from area to area, ranging from earthquake magnitudes of 6.75 - 7.1. The length of coastline warned, depends upon the magnitude and location of any particular earthquake.

Diagram above shows computer simulation of the tsunamis resulting from the Great Alaskan Earthquake of 1964. The crustal deformation that accompanied the earthquake included 3 metres of uplift and 1 - 2 metres of subsidence. Tsunami propogation after (A) 1 hour, (B) 3 hours, (C), 4 hours.

 

DISSEMINATION

Several dissemination agencies are used to disseminate warning bulletins and take appropriate action for the area they serve. The Provincial Emergency Program (PEP) is the main agency used for this purpose in the Alaskan area. Its information is received via telephone from the ATWC, the PTWC and the NOAA Weather Wire Service. Subsequent bulletins are issued to areas at risk and these regions are placed on various levels of watch.

 

CONCLUSION

The success of any early warning system depends heavily upon the access to timely information about earthquake occurrence and subsequent water levels. Other important factors are the ability of local authorities to assess the danger, agencies to disseminate the information, and the education of the public to respond quickly and appropriately in the event of a tsunami emergency. The dramatic reduction in death rates, for example in Japan, illustrates the effectiveness of early warning systems against tsunamis. Continuing research into the mechanisms behind the types of faults which produce these seismec waves ensures the future success of both regional and local tsunami warning centres.

 

REFERENCES

WEBSITES

http://wcatwc.gov - Home page of the Alaska Tsunami Warning Centre

http://walrus.wr.usgs.gov/nwtsunami - Western Region Coastal and Marine Geology tsunami page

 

USEFUL LINKS

NOAA - The National Oceanographic and Atmospheric Administration (http://www.noaa.gov)

 

CO-OPS - Centre for Operational Oceanographic Products and Services (http://www.co-ops.nos.noaa.gov)

CO-OPS collects,analyses and distributes historical and real-time observations and predictions of water levels, coastal currents and other meteorological and oceanographic data.

 

Western Region Coastal and Marine Geology Home Page (http://walrus.wr.usgs.gov)


This page has been created and is maintained by Catherine Howe ). This page was last updated 20/11/99.

All information and images used on this page have been cited where possible. If any of the sources of this information are incorrect or in breach of copyright, please contact the author.