Tags: Quake's | Movement | Behind | Lack | Tsunami

Quake's Movement Behind Lack of Tsunami

Wednesday, 30 March 2005 12:00 AM

Keiji Doi from Tokyo University's Earthquake Research Institute said the size of a tsunami essentially depends on two factors: the volume of sea water above where the quake occurs and the extent of the movement.

"This quake occurred in much shallower waters because the area is a continuation of the Sumatra land mass, so less water was displaced," Doi said.

The epicenter of Monday's quake was about 56 miles south of Simeulue Island, off Sumatra's western coast.

Doi said waters there are probably 330 to 660 feet deep, compared to the oceans above the Dec. 26 temblor off Sumatra's Aceh province, which were about 10 times that depth.

He also said the ocean floor heaved across a much larger area in December - about 92,700 square miles compared to 11,600 square miles on Monday.

"The amount of water impacted was on a completely different scale," Doi said.

The magnitude-9.0 quake in December spawned killer waves that hit 11 countries, leaving more than 280,000 dead or missing. Waves more than 100 feet high were observed along Sumatra's west coast, according to the U.S. Geological Survey.

This week's quake barely caused a ripple. The highest wave recorded by Japan's Meteorological Agency was off Salalah, Oman, and measured a mere 12 inches.

Hiroshi Ueno, from the Earthquake and Tsunami Observations Division of Japan's Meteorological Agency, said the depth at which displacement in the earth's crust occurs during a quake can also affect the size of a tsunami.

According to the U.S. Geological Survey, both December's and Monday's quakes had epicenters about 19 miles under the seabed.

But displacement along a fault line during an earthquake can be uneven.

The point at which the greatest shift occurred Monday was relatively deep underground, whereas in December, a large portion of the seabed jolted almost uniformly along the fault line, Ueno said.

"If the vertical movement is large and across a wide area, it will displace more water, more quickly," Ueno explained.

Both Doi and Ueno said their explanations were educated guesses based on current data and further studies are planned.

"It just shows how difficult it is to predict tsunamis," Doi said. "It's not an exact science, especially right after the fact, when you don't have this kind of data."

Tokyo University's Earthquake Research Institute said other quakes of magnitude 8 or higher had occurred previously in the region in 1833, 1861, 1907 and 1941.

Along such an active fault line, Doi said, there is always a likelihood of another big quake - and a tsunami.

"But you can't predict these things," he said.

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Keiji Doi from Tokyo University's Earthquake Research Institute said the size of a tsunami essentially depends on two factors: the volume of sea water above where the quake occurs and the extent of the movement. "This quake occurred in much shallower waters because the...
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2005-00-30
Wednesday, 30 March 2005 12:00 AM
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