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Keywords searched: 15

  1. What was the highest tsunami?
  2. Are tsunamis more (or less) dangerous on islands or on normal coasts? Is Hawaii hit so often because it's an island or because it's "in the way" of most tsunamis in the Pacific?
  3. Why are tsunamis so dangerous?
  4. Where do tsunamis most often occur in the world?
  5. How do tsunamis affect the life of fish and marine animals?
  6. How are tectonic plates associated with tsunamis?
  7. Do you think that the Tsunami Warning System should be improved?
  8. Do you think it was possible to decrease the number of death in the Sumatra earthquake?
  9. How do tsunamis differ from other water waves?
  10. How likely is it that within the next 50 or so years, we might face a tsunami on the Washington Coast?
  11. How does ocean bottom pressure affect sea surface height?

  1. What was the highest tsunami?

    The highest, reliably measured tsunami on record occurred in Lituya Bay, Alaska. This unusual event was caused by a massive landslide that fell into the bay on July 9, 1958. The resulting wave surged up the slope on the opposite side of the narrow bay to a height of 518 m(1,700 ft). Some scientists believe that even higher tsunamis have occurred a long time ago when asteroids, or large meteors, fell into the ocean. Two areas where studies are underway to look for evidence of such tsunamis are Hawaii and the coast of the Gulf of Mexico.

    Usually, a tsunami is generated when an offshore earthquake moves the ocean bottom in the vertical direction. The waves then propagate towards the coast, growing larger as the water becomes more shallow. Measurements in the last 10 years have documented a 32 m maximum wave height in Okushiri, Japan, and 26 m height on Flores Island Indonesia. These were exceptionally high values due to topographic and bathymetric situations that were somewhat special. More typically, the heights of 10 destructive tsunamis in the Pacific since 1990 ranged from about 3 to 15 m; these claimed more than 4000 lives.

    If you want to explore the issue in more detail, I invite you to visit NOAA's National Geophysical Data Center site. The site offers a historical tsunami database that can be searched on-line.

    Authority: NOAA Center for Tsunami Research

  2. Are tsunamis more (or less) dangerous on islands or on normal coasts? Is Hawaii hit so often because it's an island or because it's "in the way" of most tsunamis in the Pacific?

    I read somewhere that the most dangerous tsunamis for Hawaii are those generated by local earthquakes (on the islands itself). I don't understand: wouldn't the tsunami flee the island if it's generated by it? The article seemed to suggest that an earthquake under one of the islands implied a violent tsunami on hawaiian beaches shortly after.

    Because Hawaii is in the middle of the North Pacific and because this ocean is surrounded by a many earthquake/tsunami generating regions, Hawaii tends to receive many trans-oceanic tsunamis. Also, volcanic islands tend to have steep, unstable slopes where landslides can occur. The southeastern coast of the Island of Hawaii (with active volcanoes and ground movement) has had two major landslides in the past 150 years that have generated dangerous tsunamis.

    While most of the tsunami energy does radiate out to sea, some remains near the coast. There are two reasons for this. The first is that tsunami waves tend to turn toward shallow water and can be trapped near the coast in the form of 'edge waves'; these can propagate right around an island. The second is the reflection of tsunami waves that occurs when the wave encounter the sharp change in water depth between the shallow areas just off the coast and the deep ocean water farther away from the island.

    The reason that the landslide tsunamis reach the beach so quickly is that they have only a short distance to propagate away from the landslide area before they reach the beach.

    Authority: Dr. Frank Gonzalez, interviews in 1998, NOAA Center for Tsunami Research

  3. Why are tsunamis so dangerous?

    Tsunamis cause the water level and currents to rise rapidly, sometimes high enough to drown or injury people who have not escaped away from the shore to high ground. Dangerous waves can follow the first tsunami wave, trapping people who returned to the danger area because they thought the tsunami was over. Also, people can be caught unaware if they don't know the natural tsunami signs (earthquake shaking, water receding rapidly from the beach, a loud noise like a freight train coming from the ocean) or they are places where there are no tsunami warning systems. Strong tsunamis damage ports and harbors, as well as tourist areas, thereby damaging relief efforts and the economy of the communities.

    Authority: Dr. Hal Mofjeld, interview in February 25, 2005, NOAA Center for Tsunami Research

  4. Where do tsunamis most often occur in the world?

    Tsunamis occur most often in the Pacific Ocean and Indonesia because the Pacific Rim bordering the Ocean has a large number of active submarine earthquake zones. However, tsunamis have also occurred recently in the Mediterranean Sea region and are expected in the Caribbean Sea as well.

    Authority: Dr. Hal Mofjeld, interview in March 13, 2005, NOAA Center for Tsunami Research

  5. How do tsunamis affect the life of fish and marine animals?

    Tsunami currents increase strongly in shallow water where weaker corals can be broken by the force of the tsunami. Fish and marine animals are sometimes stranded on the land after they are carried by the currents to shore. The currents also move sand from the beach onto nearby coral reefs, burying low lying corals. However, the damage often varies greatly from place to place and with distance away from the shore.

    Authority: Dr. Hal Mofjeld, interview in March 13, 2005, NOAA Center for Tsunami Research

  6. How are tectonic plates associated with tsunamis?

    When one tectonic plate subducts under another, it does so in a series of sharp events that often cause earthquakes. One result of this movement is that the ocean bottom is very quickly moves upward in some locations and downward in other nearby locations. This happens so rapidly that the water surface is also up or down by the same amount; this wave pattern is then the initial waveform of the newly created tsunami that then propagates away from the source area. The earthquakes can also trigger submarine landslides that either generate tsunamis on their own or enhance the tsunami generated by the tectonic bottom movement.

    Authority: Dr. Hal Mofjeld, interview in March 13, 2005, NOAA Center for Tsunami Research

  7. Do you think that the Tsunami Warning System should be improved?

    There are always ongoing efforts to improve the tsunami warning system. These are being accelerated in response to the tsunami in the Indian Ocean. The U.S. warning system will be expanded to include the East Coast, Gulf of Mexico and Puerto Rico. Tsunami detection buoys will be placed there and the buoy array in the Pacific will be expanded. In addition, a new tsunami wave height forecasting capability is being developed and should be installed at the warning centers soon. The United States is also talking with other countries about installing a tsunami warning system in the Indian Ocean, like the international systems that exists now in the Pacific. Also important are more extensive tsunami education and improved communication systems for tsunami warnings.

    See also:



    Authority: Dr. Hal Mofjeld, interview in March 15, 2005, NOAA Center for Tsunami Research

  8. Do you think it was possible to decrease the number of death in the Sumatra earthquake?

    We always look back at horrible events like the one in the Indian Ocean and wish we or someone else would help a region prepare for such events. The one that contributed to the loss of life last December was that large tsunamis are so rare in the Indian Ocean. Now that we know they happen, many people are working hard to reduce the effects of the next one.

    Authority: Dr. Hal Mofjeld, interview in March 13, 2005, NOAA Center for Tsunami Research

  9. How do tsunamis differ from other water waves?

    Tsunami waves are shallow-water waves with long periods and wave lengths. (A wave is classified a shallow-water wave when the ratio between the water depth and its wavelength gets very small. The speed of a shallow-water wave is equal to the square root of the product of the acceleration of gravity (32ft/sec/sec or 980cm/sec/sec) and the depth of the water.) Shallow water waves are different from wind-generated waves (the waves many of us have observed on the beach). Wind-generated waves usually have period (time between two succesional waves) of five to twenty seconds and a wavelength (distance between two successional waves) of about 50 to 600 feet (15 to 200 meters) A tsunami can have a period in the range of 10 minutes to 1 hour and a wavelength in excess of 700 km (430 miles).

    Authority: NOAA Center for Tsunami Research

  10. How likely is it that within the next 50 or so years, we might face a tsunami on the Washington Coast?

    Scientists in the U.S. Geological Survey estimate that there is a 10-15% chance of a great earthquake on the Cascadia Subduction Zone generating a major tsunami on the Washington Coast during the next 50 years.

    Authority: Dr. Hal Mofjeld, NCTR, May 24, 2006, NOAA Center for Tsunami Research

  11. How does ocean bottom pressure affect sea surface height?

    To answer your question, all DART II systems consist of a sea surface buoy and a companion seafloor bottom pressure recorder (BPR) unit typically sited within 2km of one another. The BPR takes and records measurements of the overlying water column as pressure in units of absolute pounds per square inch [psia] and telemeters these measurements through the water column to the surface buoy whose sole function is communication both to and from land stations via satellite.

    All DART II BPRs measure pressure (and temperature) as frequency counts. A quartz crystal enclosed in an evacuated chamber and linked to seawater near the ocean floor by an oil filled tube oscillates as a function of the pressure exerted on it, the pressure of the overlying water column. Electronics and precise base crystal and timing clocks measure the pressure as an integration over 15-second intervals continuously for the entire time a BPR is deployed. These measurements and temperature measurements made the same way (oscillating quartz crystal integrated over 15-second intervals) are recorded in the unit as frequency counts. When a BPR detects an earthquake or tsunami, measurements are immediately sent to the surface buoy for telemetry to shore and continue to be sent on a regular but rapid reporting schedule. It is these measurements that are converted to depth in meters by algorithms controlling BPR measurements, processing, and storage.

    The precise conversion of measured pressure considers temperature, latitude, water density, and gravitational variation. Because gravitational potential and density are typically small terms, and because we are interested in rapid notification of threat based on relative sea level, the algorithms in DART II technology convert pressure to depth using a constant that assumes 'standard' seawater (Temperature = 0ÂșC, Salinity = 35 PPT (Parts per thousand)). Specifically, water depth [meters] sent to shore by the surface buoy is the result of correcting measured pressure [psia] for temperature and applying a constant 670mm/psia conversion factor.

    Here are two references in case you are interested in reading further:
    1. Saunders P.M., Fofonoff N.P. (1976): Conversion of pressure to depth in the ocean. Deep Sea Research 23:109-111.
    2. http://www.paroscientific.com/pdf/ptodepth.pdf

    Authority: Marie Eble, NOAA Center for Tsunami Research