Tsunami and the cause

At a magnitude of 8.6, yesterday's earthquake off the Indonesian province of Aceh was one of the largest ever recorded.
Yet the massive tremor, which was followed by an 8.2-magnitude aftershock, did not cause a severe tsunami such as the one on December 26, 2004, which devastated countries around the Indian Ocean and killed more than 200,000.

So what is the difference?
Seismologists said it was all about the horizontal and vertical movements of the sea floors and where the quake took place.
Earthquakes usually encompass three types of motions - normal faulting, reverse faulting and strike strip faulting, Wayne Peck, a senior seismologist at the Seismology Research Centre in Melbourne, said.

Yesterday's quake was a strike strip, caused by a horizontal movement of the sea bed at a fault in the Indo-Australian plate, while the 2004 tsunami, sparked by a 9.1-magnitude tremor, was generated by a vertical movement that displaced water, he said.
"If you imagine you are standing in a perfectly circular pool with a flat floor, to generate a really large earthquake the best way is to have one half of it pop up vertically by six inches while the other half remains stationary. That would generate a large wave in the pool," Mr Peck said.
"If you have a floor that tears right up in the middle and your right foot is on one side of the tear and your left foot is on the other side of the tear, and your feet move so that you end up with your right foot in front of your left foot with no vertical difference, then there's not going to be any wave generated by that motion."
The quake also took place within a plate, rather than at the boundary of two plates, senior research seismologist Gary Gibson of the University of Melbourne said.
"This time, we were lucky. It didn't happen on the main plate boundary on the Australia-India plate and the Asian plate, like the Boxing Day one did. It happened on a major crack in the Australia-India plate, well to the south-west of Sumatra," Dr Gibson told ABC Radio 702.

But its size took seismologists by surprise, he added.
"This is the largest earthquake that hasn't happened on one of the main plate boundaries ever. The previous largest was between Australia and New Zealand 15 years ago and back in 1957 in Mongolia."
While a recent spate of strong tremors - the 8.8-magnitude quake in Chile and the 7.0-magnitude quake in Haiti in 2010, and the 9.0-magnitude Japan quake and the 6.3-magnitude Christchurch quake last year - appeared to suggest that there have been an increase in earthquake activity, researchers said data showed there was no change.
A study by Peter Shearer and Philip Stark of the University of California last year found there was "little evidence that the global threat of earthquake occurrence has changed in areas far from recent activity".
But they added that the current threat of large earthquakes was "above its long-term average in regions like Sumatra, Chile, and Japan".
Mr Peck said other factors, such as the exponential growth of the human population and the improved recording of tremors, also played a part.

What Cause Tsunamis?

Tsunamis, also called seismic sea waves or, incorrectly, tidal waves, generally are caused by earthquakes, less commonly by submarine landslides, infrequently by submarine volcanic eruptions and very rarely by a large meteorite impact in the ocean. Submarine volcanic eruptions have the potential to produce truly awesome tsunami waves. The Great Krakatau Volcanic Eruption of 1883 generated giant waves reaching heights of 125 feet above sea-level, killing thousands of people and wiping out numerous coastal villages.

The 1992 Nicaragua tsunami may have been the result of a "slow" earthquake comprised of very long-period movement occurring beneath the sea floor. This earthquake generated a devastating tsunami with localized damage to coastal communities in Nicaragua.

Not all earthquakes generate tsunamis. To generate tsunamis, earthquakes must occur underneath or near the ocean, be large and create movements in the sea floor. All oceanic regions of the world can experience tsunamis, but in the Pacific Ocean there is a much more frequent occurrence of large, destructive tsunamis because of the many large earthquakes along the margins of the Pacific Ocean.

Ring of Fire

About two-thirds of the earth is covered by the waters of the four oceans. The Pacific Ocean is the world's largest, covering more than one third of the total surface area of our planet. The Pacific Ocean is surrounded by a series of mountain chains, deep ocean trenches and island arcs, sometimes called a "ring of fire." The great size of the Pacific Ocean and the large earthquakes associated with the "ring of fire" combine to produce deadly tsunamis.

In less than a day, these tsunamis can travel from one side of the Pacific to the other. However, people living near areas where large earthquakes occur may find that the tsunami waves will reach their shores within minutes of the earthquake. For these reasons, the tsunami threat to many areas (Alaska, the Philippines, Japan or the U.S. West Coast) can be immediate (for tsunamis from nearby earthquakes taking only a few minutes to reach coastal areas) or less urgent (for tsunamis from distant earthquakes taking from 3 to 22 hours to reach coastal areas).

Earth and Earthquakes

The continents and sea floor that cover the earth's surface are part of a world-wide system of plates that are in motion. These motions are very slow, only an inch or two per year. Earthquakes occur where the edges of plates run into one another. Such edges are called fault lines or faults. Sometimes the forces along faults can build-up over long periods of time so that when the rocks finally break an earthquake occurs. Examples of features produced by forces released along plate edge faults are the Andes Mountains in South America (on land) and the Aleutian Trench near Alaska (under water). When powerful, rapid faulting occurs underneath or near the ocean, a large earthquake is produced and, possibly, a tsunami.

The deep ocean trenches off the coasts of Alaska, the Kuril Islands, Russia,, and South America are well known for their violent underwater earthquakes and as the source area for destructive Pacific-wide tsunamis.

The tsunami generating process is more complicated than a sudden push against the column of ocean water. The earthquake's magnitude and depth, water depth in the region of tsunami generation, the amount of vertical motion of the sea floor, the velocity of such motion, whether there is coincident slumping of sediments and the efficiency with which energy is transferred from the earth's crust to ocean water are all part of the generation mechanism.

Tsunamis on the Move

Wave Height and Water Depth

In the open ocean a tsunami is less than a few feet high at the surface, but its wave height increases rapidly in shallow water. Tsunamis wave energy extends from the surface to the bottom in the deepest waters. As the tsunami attacks the coastline, the wave energy is compressed into a much shorter distance creating destructive, live-threatening waves.

In the deep ocean, destructive tsunamis can be small--often only a few feet or less in height--and cannot be seen nor can they be felt by ships. But, as the tsunami reaches shallower coastal waters, wave height can increase rapidly. Sometimes, coastal waters are drawn out into the ocean just before the tsunami strikes. When this occurs, more shoreline may be exposed than even at the lowest tide. This major withdrawal of the sea should be taken as a warning of the tsunami waves that will follow.

Pacific-Wide and Local Tsunamis

The last large tsunami that caused widespread death and destruction throughout the Pacific was generated by an earthquake located off the coast of Chile in 1960. It caused loss of life and property damage not only along the Chile coast but in Hawaii and as far away as Japan. The Great Alaskan Earthquake of 1964 produced deadly tsunami waves in Alaska, Oregon and California.


In July 1993, a tsunami generated in the East Sea killed over 120 peoples in Japan. Damage also occurred in Korea and Russia but not in other countries since the tsunami wave energy was confined within the Sea of Japan. The 1993 Sea of Japan tsunami is known as a "local event" since its impact was confined to the nearby regional area in the proximity of the earthquake that generated the tsunami. For people living along the northwestern coast of Japan, the tsunami waves followed the earthquake within a few minutes. Local tsunamis also occurred in Nicaragua (1992), Indonesia (1992, 1994) and the Philippines (1994) killing thousands of people. Scientific studies indicate that local tsunamis generated off the northern California, Oregon and Washington coast can arrive within five to 30 minutes after the earthquake is felt.

How Fast?

Where the ocean is over 20,000 feet deep, unnoticed tsunami waves can travel at the speed of a commercial jet plane, nearly 600 miles 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 since the speed of the waves varies with the square root of the water depth. Tsunamis travel much slower in shallower coastal waters where their wave heights begin to increase dramatically.

How Big?

Offshore and coastal features can determine the size and impact of tsunami waves. Reefs, bays, entrances to rivers, undersea features and the slop of the beach all help to modify the tsunami as it attacks the coastline. When the tsunami reaches the coast and moves inland, the water level can rise many feet. In extreme cases, water level has risen to more than 50 feet for tsunamis of distant origin and over 100 feet for tsunami waves generated near the earthquake's epicenter. The first wave may not be the largest in the series of waves. One coastal community may see no damaging wave activity while in another community destructive waves can be large and violent. The flooding can extend inland by 1000 feet or more, covering large expanses of land with water and debris.

How Frequent?

Since scientists cannot predict when earthquakes will occur, they cannot determine exactly when a tsunami will be generated. However, by looking at past historical tsunamis, scientists know where tsunamis are most likely to be generated. Past tsunami height measurements are useful in predicting future tsunami impact and flooding limits at specific coastal locations and communities. Historical tsunami research may prove helpful in analyzing the frequency of occurrence of tsunamis and their relationship to large earthquakes.