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Tsunami Information
Tsunamis are formed by a displacement of water - a landslide, volcanic eruption, or, as in this case, slippage of the boundary between two of the earth's tectonic plates -slabs of rock 50 to 650 feet (15 to 200 m) thick that carry the Earth's continents and seas on an underground ocean of much hotter, semi-solid material.

The December 26 2004 mega tsunami was caused by slippage of about 600 miles (1,000 km) of the boundary between the India and Burma plates off the west coast of northern Sumatra. The convergence of other plates strains the area, and at the quake's epicenter, the India plate is moving to the northeast at 2 inches (5 cm) per year relative to the Burma plate. The aftershocks were distributed along the plate boundary from the epicenter to near Andaman Island.

Tsunamis can travel up to 600 mph (965 km per hour), 521 knots) at the deepest point of the water, but slow as they near the shore, eventually hitting the shore at 30 to 40 mph (48 to 64 kph or 26 to 35 knots). The energy of the wave's speed is transferred to height and sheer force as it nears shore.

The 7.3 magnitude aftershock might have been powerful enough to create further tsunamis, but did not.

Historical information
The Chilean quake and tsunamis were responsible for 5,700 deaths, and the Prince William Sound quake and tsunamis killed 125; the Andreanof Island and Kamchatka events killed no one

The deadliest earthquake recorded since 1900 occurred on July 27, 1976, in Tangshan, China, when the official death count reach 255,000 for a 7.5 magnitude quake. Estimated death counts, however, reached as high as 655,000.

The highest toll for an earthquake-tsunami combination since 1900 took place on December 28, 1908, when a 7.2 magnitude quake struck Messina, Italy, killing an estimated 70,000 to 100,000 people.

The deadliest earthquake ever recorded is believed to have occurred on January 23, 1556, in Shansi, China, killing 830,000 people.

The worst tsunami in recent history followed the August 27, 1883, the eruption of the volcano Krakatau. The resulting wave swept over the Indonesian islands of Java and Sumatra, ultimately killing 36,000 people.

A tidal wave - caused not by water displacement on the sea floor but by the floods and high tides accompanying Cyclone Marian - swept Bangladesh in 1991, killing nearly 140,000 people.

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