About How Long Does the Water Associated With the First Tsunami Wave Continue to Come in

Frequently Asked Questions

On this page we provide answers to frequently ased questions about the Indian Ocean Tsunami. These are based on real questions that we have received during the days after the disaster.

Q: "Can you tell whether there will be a similar tidal wave coming in the future from today, when and what will be the magnitude"

A: Unfortunately, scientists cannot predict when the next large earthquake will occur, what its size will be, and whether a tsunami will be generated. From history, however, we do observe that large or great earthquake don't occur often. And, since not all earthquakes generate tsunamis, it is a very infrequent occurrence. While there have been many aftershock earthquakes in Indonesia, Nicobar and Andaman Islands in the last day (27 so far of magnitude greater than 5.4), none of these have generated tsunamis. Of some concern is that sometimes, smaller earthquake might trigger underwater landslides which could generate tsunamis, but these tsunami generally only affect the area very near to the source., e.g., near Indonesia only.

Q: Has a tsunami hit the Indian Ocean before?

A: The 1883 Krakatau volcanic eruption has generated a destructive tsunami higher than 40 m on the Indonesian coast where more than 36 000 lives were lost. [read more here]

Q: Can an aftershock generate a tsunami?

A: Yes, but only if the aftershock is very large. Such a large aftershock is now very unlikely. No aftershocks of the magnitude 9.4 Alaska earthquake of 1964, nor of the magnitude 9.0 Central Aleutian earthquake of 1957 generated tsunamis large enough to be damaging. The great Chile earthquake of 1960 (magnitude 9.6) had a foreshock that generated a tsunami, but that foreshock was exceptionally large, magnitude 7.9.

Q: WHY DOES THE OCEAN OFTEN RECEDES AWAY FROM SHORE JUST BEFORE A TSUNAMI WAVE HITS LAND?

A: Although tsunamis are commonly depicted as a giant breaking wave with a crest towering over the land, this image is hardly if ever the case. Instead tsunamis can more accurately be described as a rapidly-rising tide without a developed wave face, which quickly and forcefully floods low-lying coastal areas.

Ironically, in deep, open-ocean water, tsunami waves are often less than a meter high and can travel at speeds up to 1,000 kilometers per hour. However, as a tsunami wave approaches shallower waters along the coast, the leading edge of the wave begins to slow down while the rest of the wave begins to "pile up" behind it — causing it to grow in height while maintaining its strength. The crest of this wave can be several meters high by the time it reaches the shoreline. Sometimes, however, the crest of the wave isn't the first to arrive — the trough is (this is often the case when the tsunami originates from an oceanic earthquake associated with land subsidence (sinking), which causing the water column to drop down at the earthquake site). In this case, instead of extremely high water levels, the first sign of a tsunami is what appears to be an unusually low low-tide. Although onlookers might be intrigued by this unusual site, this major withdrawal of the sea should be taken as a warning that a tsunami wave will soon follow.

As the coastal ocean waters recede from the shore, it often leaves large portions of the sea floor exposed. Individuals who do not recognize this as a common precursor to tsunami waves often find themselves gravitating toward the exposed shore. Unfortunately, they often perished as they rush to gather fish left high and dry on the exposed beach or to view never before seen rock and reef formations — only to be hit moments later by the incoming wall of water. Experts believe that a receding ocean may give individuals more familiar with "nature's tsunami warning signal" as much as a five minute warning to evacuate the area. This cycle may be repeated several times as successive wave crests arrive five minutes to an hour apart. Seek higher ground and stay out of danger areas until an "all-clear" is issued by a competent authority.

There are several terms that are used to describe this phenomenon. Which one to use depends on the circumstances (e.g., whether the tsunami occurs along the ocean or is in a bay) and on personal preference. The terms "drawdown," "negative wave" and "withdrawal" are most often used to describe this phenomenon. Less formal terms include "waterline receding" and "bay emptying."

So remember:
• An approaching tsunamis is sometimes preceded by a noticeable rise or fall of coastal water. This is a natural warning; people should move inland away from the shoreline.

• When the sea begins to drain away, do not go to investigate, but quickly go inland away from the shoreline.

• Never go down to the beach to watch for a tsunami. When you can see the wave you are too close to escape. Tsunami can move faster than a person can run!

• Stay tuned to your local radio, marine radio, (USA: NOAA Weather Radio), or television stations during a tsunami emergency - bulletins issued through your local emergency management office and the (USA:NOAA) National Weather Service offices can save your life.

Q: Historically, how often and where do tsunamis occur?

A: Tsunamis are disasters that can be generated in all of the world's oceans, inland seas, and in any large body of water. Each region of the world appears to have its own cycle of frequency and pattern in generating tsunamis that range in size from small to the large and highly destructive events. Most tsunamis occur in the Pacific Ocean (85%) and its marginal seas. The reason is that the Pacific covers more than one-third of the earth's surface and is surrounded by a series of mountain chains, deep-ocean trenches and island arcs called the "ring of fire" - where most earthquakes occur (off the coasts of Kamchatka, Japan, the Kuril Islands, Alaska and South America). It is in the ring of fire where the main tectonic plates forming the floor of the Pacific collide against themselves or against the continental plates that surround the ocean basin. Many tsunamis have also been generated in the seas which border the Pacific Ocean. Tsunamis are generated, by shallow earthquakes all around the Pacific, but those from earthquakes in the tropical Pacific tend to be modest in size. While such tsunamis in these areas may be devastating locally, their energy decays rapidly with distance. Usually, they are not destructive a few hundred kilometers away from their sources. That is not the case with tsunamis generated by great earthquakes in the North Pacific or along the Pacific coast of South America. On the average of about 6 times per century, a tsunami from one of these regions sweeps across the entire Pacific, is reflected from distant shores, and sets the entire ocean in motion for days. Although not as frequent, destructive tsunamis have been also been generated in the Atlantic and the Indian Oceans, the Mediterranean Sea and even within smaller bodies of water, like the Sea of Marmara, in Turkey.

The U.S. is vulnerable to tsunamis generated by seismic events anywhere along the Pacific Basin's Ring of Fire. The States of Hawaii, Alaska, Washington, Oregon, and California are vulnerable to tsunamis triggered by local seismic events as well as teletsunamis generated by distant seismic events along the Ring of Fire. The PTWC was established in 1946 as a result of a tsunami generated by a seismic event in Alaska's Aleutian Island Chain that led to a teletsunami affecting the West Coast of the U.S., Hawaii (severe damage and 165 fatalities) as well as Japan. Similarly, seismic events in South America have triggered both local and teletsunamic events.

During the 101-year period from 1900 to 2001, 796 tsunamis were observed or recorded in the Pacific Ocean according to the Tsunami Laboratory in Novosibirsk. 117 caused casualties and damage most near the source only; at least nine caused widespread destruction throughout the Pacific. The greatest number of tsunamis during any 1 year was 19 in 1938, but all were minor and caused no damage. There was no single year of the period that was free of tsunamis. 17% of the total tsunamis were generated in or near Japan. The distribution of tsunami generation in other areas is as follows: South America, 15%: New Guinea Solomon Islands, 13%; Indonesia, 11%; Kuril Islands and Kamchatka, 10%; Mexico and Central America, 10%; Philippines, 9%; New Zealand and Tonga, 7%; Alaska and West Coasts of Canada and the United States, 7%; and Hawaii, 3%.

Note: The Indian Ocean tsunami is now reported to be one of the strongest in the world over the past 40 years. More than 75,000 (12/30/04) lives have been lost and material damage is in the Billions of Dollars.

General questions about tsunamis (links to the web site of the International Tsunami Information Center, Honolulu):

What is a tsunami?
How do earthquakes generate tsunamis?
How do volcanic eruptions generate tsunamis?
How do submarine landslides, rock falls and underwater slumps generate tsunamis?
Can asteroids, meteorites or man-made explosions cause tsunamis?
Where and how frequently are tsunamis generated?
How does tsunami energy travel across the ocean and how far can tsunamis waves reach?
Why aren't tsunamis seen at sea or from the air?
What are the factors of destruction from tsunamis?
What determines how destructive a tsunami will be near the origin and at a distant shore?
Why are locally generated tsunamis so dangerous?
What is a mega-tsunami and can it happen today?