Antonian Story

Tsunami Fact Sheet

Asia Quake, Tsunami Moved Islands, Shortened Days

The massive earthquake that triggered the Asian tsunami wobbled the earth on its axis, forced cartographers back to the drawing board and changed time by a fraction, but there's no need to adjust your clocks. Six weeks after the tsunami that may have killed 300,000 people on the shores of the Indian Ocean, scientists are discovering more about the changes wrought by the magnitude 9 quake, the fourth-largest in the last century. It caused upheaval on the sea floor near its epicenter off the northwest coast of Indonesia's Sumatra island and moved several other islands, but scientists say any movement of land mass can be measured in inches rather than tens of yards.

Chen Ji, a seismologist at the California Institute of Technology, said he found movement along the fault line of about 33 feet laterally and 13-16 feet vertically. But reports that the entire island of Sumatra -- 1,060 miles long and 250 miles wide -- 115 feet or more are wildly inaccurate, scientists say. "We know we have movements of over a meter (yard), perhaps a couple of meters," said Ken Hudnut, a California-based geophysicist with the U.S. Geological Survey. "But the idea that Sumatra has moved 100 feet is just wrong." Scientists are working on precise measurements by comparing geographic points whose locations were known before the quake with their new positions using the Global Positioning System, which reads exact locations by satellite. High-tech British and U.S. ships are investigating changes to the sea bed and local authorities are measuring depths in critical shipping channels.

SHORTER DAY

Scientists at NASA, the U.S. space agency, said the Dec. 26 quake -- the largest to rattle Earth since 1964 in Alaska -- disrupted the planet's rotation and shaved 2.68 microseconds, or millionths of a second, from the length of a day. NASA scientists B. F. Chao and Richard Gross calculated it shifted Earth's mean north pole about 1 inch and made the planet slightly less oblate, or flattened at the poles

"Physically, this is analogous to a spinning skater drawing arms closer to the body, resulting in a faster spin," they wrote in an article in Eos, a publication of the American Geophysical Union. But they said these changes are based on calculations rather than measurements. The changes are so small they are either difficult to measure or too small to detect. Many earthquakes shake the planet's axis and affect its rotation, scientists added, but their impact is too small to measure.

But environmental damage from the tsunami was vast. The killer waves gouged beaches, crushed coral reefs, smashed thousands of acres of mangrove forests and refashioned coastlines from Thailand to Somalia. A preliminary survey by Indonesia's government and the United Nations Environmental Program (UNEP) estimated the economic cost to the environment at $675 million in Indonesia alone. The survey said 60,000 acres of mangroves and 70,000 acres of coral reefs were damaged.

REEFS, MANGROVES

Some coral reefs -- undersea gardens that act as shelter and nursery to a wide range of marine species -- were crushed by the waves. Corals grow slowly, some only an inch or two a year, so their recovery could take decades. John Pernetta, a UNEP official in Bangkok, said the extent of damage to some of the coral reefs around Thailand was very high -- up to 80 percent in some places. Their recovery was uncertain. Mangroves torn out by the waves will fare better, he said, as they leave behind roots and seeds that will help them regenerate. "Long-term damage to mangroves by hurricanes or tsunamis doesn't really happen," Pernetta said. "After five to 10 years you don't even know anything has happened." Vast stretches of Sumatra's west coast were turned brown by the tsunami as rice paddies and other vegetation were swamped by salt water. It could take two or three rainy seasons to wash the salt from the saturated land, experts say The tsunami waves ate away beaches and coastal areas in Thailand, Indonesia and Sri Lanka, radically changing maps.
The waves also carried sediment ashore, said Phil Liu, a Cornell University wave researcher who led a scientific team to Sri Lanka in mid-January."There is evidence that a lot of sediment was being brought onshore," he said. "A post office on the east coast was found with sediment deposits on the roof." But it remains to be seen whether such sediment is good for the land or a bane because of its high salt content

What is a tsunami?

A tsunami (pronounced tsoo-nah-mee) is a wave train, or series of waves, generated in a body of water by an impulsive disturbance that vertically displaces the water column. Earthquakes, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies, such as meteorites, can generate tsunamis. Tsunamis can savagely attack coastlines, causing devastating property damage and loss of life.

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 earthquakes generate tsunamis?

Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the earth's crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium. When large areas of the sea floor elevate or subside, a tsunami can be created.

Large vertical movements of the earth's crust can occur at plate boundaries. Plates interact along these boundaries called faults. Around the margins of the Pacific Ocean, for example, denser oceanic plates slip under continental plates in a process known as subduction. Subduction earthquakes are particularly effective in generating tsunamis.

This simulation (2 MB) of the 1993 Hokkaido earthquake-generated tsunami, developed by Takeyuki Takahashi of the Disaster Control Research Center, Tohoku University, Japan, shows the initial water-surface profile over the source area and the subsequent wave propagation away from the source. Areas in blue represent a water surface that is lower than the mean water level, while areas in red represent an elevated water surface. The initial water-surface profile, as shown in this image, reflects a large, long uplifted area of the sea floor lying to the west (left) of Okushiri Island, with a much smaller subsided area immediately adjacent to the southwest corner of Okushiri.

This site was last updated 02/13/05