The gigantic Asian sea quake on December 26 -- the most dramatic seismic shock in more than 40 years -- made the earth wobble on its axis and permanently changed the geology of the surrounding area, scientists claim.

It was like "flicking a top," said Paul Tapponnier, head of the tectonics laboratory at the Institute de Physique du Globe (IPG), France's leading center for the earth sciences.

Tapponnier said the quake deep beneath the Pacific Ocean lasted a "colossal" 200 seconds, building up huge amounts of energy in the sea that drove towering waves onto beaches throughout south Asia.

"That earthquake has changed the map," US Geological Survey expert Ken Hudnut said in Los Angeles.

The quake, which had an epicenter magnitude of 9.0, struck 250 kilometers southeast of Sumatra Island.

One of the four biggest in the last century, it sent gigantic tsunami waves crashing around the Indian Ocean causing more than 80,000 deaths so far, but the figure is rising.

Hudnut said seismic modeling suggested the quake may have moved small islands by as much as 20 meters, and the northwestern tip of the Indonesian territory of Sumatra may also have shifted to the southwest by around 36 meters.

"That is a lot of slip," he said.

The energy released as the two sides of the geological fault line deep under the sea slipped against each other would have made the Earth wobble on its axis, Hudnut said.

Tapponnier said the quake caused a 15- to 20-meter slippage of the earth's surface along a front extending for 100 kilometers.

He said there may also have been vertical movements that possibly pushed the island of Siberut, 100 kilometers west of Sumatra, one or two meters higher, although it would be impossible to check this scientifically because of guerrilla activity in the area.

Tapponnier said it was also possible that some regions of Sumatra south of the equator have been completely swallowed up.

He said it was not rare for earthquakes to alter geological features.

"Earthquakes are the architects of landscapes," he said. "All the mountains that we see today have been modelled by earthquakes."

Tapponnier said the massive 1960 earthquake off the coast of Chile shifted the local landscape by 20 meters. A quake in Alaska in 1964 pushed islands higher and sank oyster beds 12 meters under the surface.

And a 6.3 magnitude temblor off the coast of Guadeloupe last month moved the ocean floor several tens of centimeters, he added.

Shifting lands

In Asia, "we are dealing with a quake one thousand times more powerful" than the one off Guadaloupe, a shock powerful enough to make the earth wobble on its axis, Tapponnier said.

Hudnut agreed the earth had slightly wobbled "due to the massive amount of energy exerted and the sudden shift in mass."

But minor oscillations as the earth spins like a top are well known to astronomers. The principal causes of the slight irregular motion known as nutation are the Sun and Moon as they continually change location relative to one another.

Another USGS research geophysicist agreed that the earth would have received a "little jog" from the quake and that the islands off Sumatra would have been shifted.

However, Stuart Sipkin, of the USGS National Earthquake Information Center in Golden Colorado, said it was more likely that the islands had risen higher out of the sea than they had moved laterally.

"In this case, the Indian plate dived below the Burma plate, causing uplift, so most of the motion to the islands would have been vertical, not horizontal," he said.

Richard Gross, a geophysicist with NASA's Jet Propulsion Laboratory in California, theorized that a shift of mass toward the Earth's center during the quake on Sunday caused the planet to spin 3 microseconds, or 3 millionths of a second, faster and to tilt about an inch on its axis.

When one huge tectonic plate beneath the Indian Ocean was forced below the edge of another "it had the effect of making the Earth more compact and spinning faster," Gross said.

Gross said changes predicted by his model probably are too minuscule to be detected by a global positioning satellite network that routinely measures changes in Earth's spin, but said the data may reveal a slight wobble.

The Earth's poles travel a circular path that normally varies by about 33 feet (10 meters), so an added wobble of an inch is unlikely to cause long-term effects, he said.

"That continual motion is just used to changing," Gross said. "The rotation is not actually that precise. The Earth does slow down and change its rate of rotation."

When those tiny variations accumulate, planetary scientists must add a "leap second" to the end of a year, something that has not been done in many years, Gross said.

Scientists have long theorized that changes on the Earth's surface such as tide and groundwater shifts and weather could affect its spin but they have not had precise measurements to prove it, Caltech seismologist Hiroo Kanamori said.

"Even for a very large event, the effect is very small," Kanamori said. "It's very difficult to change the rotation rate substantially."

Dramatic speed

Tidal waves like those that wreaked havoc across Asia on Sunday can travel at hundreds of kilometers per hour and even gain strength as they cross the ocean -- often causing disastrous results far from their origin.

Born of strong seismic shocks, tsunamis can reach huge heights and speeds, but despite their strength, they can be barely noticeable out at sea.

"If you are on a boat, you might not even feel a tsunami," said Wong Wing-tak, senior scientific officer at the Hong Kong Observatory.

"It becomes powerful only when it is near the shore and reaches shallow water, which then can push waves over 10 times higher than the sea water level."

The tidal waves spawned by Sunday's massive tremor off Indonesia bore down without warning on low-lying areas across the Indian Ocean at a speed of 500 kilometers per hour, a British seismologist said.

"Where there is a displacement of the ocean floor it causes a movement on the surface and it spreads out from there at a speed of about 500 kilometers an hour," David Booth of the Edinburgh Institute in Scotland said.

Despite the speed, it was "slow enough for warnings to be given, if a sophisticated warning system is set up," he stressed.

"This wave may only be a few meters high in the ocean but as it reaches shallow water the wave builds up very quickly in height and these waves can be reportedly 50 metres high," he said.

The most common cause for tsunamis is an undersea earthquake, especially in areas such as the Pacific where there is significant movement of the Earth's tectonic plates.

"Tsunamis travel outward in all directions from the epicenter of an earthquake and can savagely attack coastlines," Wong said. "It can easily roll people out to the sea, it causes flooding, devastates property damage."

Alert to the destructive capacity of tsunamis, Pacific Rim countries co-ordinate and share their observations of the ocean. A tsunami alert center in Hawaii collects information about possible tidal waves.

Booth said that vibrations from the epicenter of the quake off Sumatra were felt in London, 8,000 kilometers away, early Sunday, which means "the waves generated from the tsunami would have traveled all around the world."

The earthquake that struck Indonesia Sunday releasing devastating tsunamis across swathes of Asia was the largest in four decades and showed a rare and unpredictable natural phenomenon in action.

Superquake

The quake, which measured 8.9 on the Richter scale according to the US Geological Survey, was the most immense since a 9.2-scale quake hit Alaska in 1964 and the fifth strongest since the beginning of the 20th century.

The phenomenon, while mercifully rare, was unpredictable, "leaving authorities largely powerless to mitigate against its effects," Wong said .

"Earthquakes likes this, although rare, will occur again. This is part of nature. You cannot predict when and where it will occur again," he said.

The only quake to register a higher magnitude than the 1964 tremor in the 20th century was one that measured 9.5 in Chile in 1960.

"The extent of the damage it can cause often depends on where it happens and the structure of buildings," Wong said.

Exactly a year before Sunday's quake more than 30,000 people were killed in a 6.7-scale quake in the Iranian city of Bam, where sloppy builders were blamed for the high toll.

Enzo Boschi, the director of Italy's National Institute of Geophysics, pointed out the impact of Sunday's quake was made significantly worse by the devastating tsunami waves it triggered.

"The situation was compounded by the fact that it occurred at sea, generating a huge wave of water moving at a very high speed," he said.

South Asia bore the brunt of the deadly waves, with Sri Lanka particularly hard hit.

By contrast, the Alaskan earthquake of 1964 claimed little more than 100 fatalities, thanks mostly to the state's low density population.

The quake that hit the Japanese city of Kobe in 1995 measured a far smaller 7.2 -- meaning it registered less than one 10th of the intensity of the Indonesian quake on the exponential Richter scale -- but claimed more than 6,400 lives.

No stranger to high-magnitude quakes due to its position straddling the Pacific "Ring of Fire" marked by volcanic and seismic activity, Indonesia is one of the world's most earthquake-prone regions.

The archipelago of some 18,000 islands lies at the collision point of three tectonic plates, resulting in frequent earthquakes and volcanic eruptions as pressure between the massive segments of the Earth's crust is released.

Sumatra sits atop one of the handful of sites where several plates of the planet's crust overlap and grind. Colossal pressures build up over decades, only to release in a snap.

"These quakes are spawned by deformations of the earth's crust, which are in turn caused by a huge build-up of energy," Boschi said.

As energy accumulates and crust deformation reaches a critical level, a fracture in the crust occurs.

"The bigger the fracture, the more intense the quake," he said.

Wong described the phenomenon as a fearsome demonstration of the earth's power.

"It's pretty scary and can cause unimaginable damage and loss of life," he said.

(China Daily December 31, 2004)