FAQ: What’s the Science Behind Japan’s Quake and Tsunami?
The epicenter of the March 11 earthquake occurred near the east coast of Honshu, Japan.
Credit: USGS

On Friday afternoon in Japan, disaster struck as a massive 8.9 magnitude earthquake generated a wall of water that surged over the east coast of the island nation, sweeping many to their deaths.

Here's what you need to know about the quake and its aftermath:

Where did the earthquake come from?

Japan is situated along the world's most active earthquake belt, the Pacific Ring of Fire, where rigid plates in the Earth's crust collide along the rim of the Pacific Ocean. This earthquake originated 231 miles (373 kilometers) northeast of Tokyo and 80 miles (130 km) east of Sendai, Honshu in the Pacific Ocean. The earthquake occurred at a depth of about 15.2 miles (24.4 kilometers).

In this area, the Pacific Plate, the plate beneath the Pacific Ocean, is moving almost due west and being pushed down into the Earth's interior along a trench off Japan's east coast. On average, the Pacific Plate is moving at 3.5 inches (8.9 centimeters) per year, but this process is not continuous, according to Keith Sverdrup, a professor of geophysics at the University of Wisconsin-Madison. [The Science behind Japan's Deadly Earthquake]

Movement may stop as the plates stick together for a period and energy will build up, so when the movement does finally occur, it is much more dramatic, Sverdrup said. 

The process of one plate being pushed beneath another is called subduction, and it occurs all along the Ring of Fire, producing other earthquakes, including the 7.7 magnitude quake that struck off the coast of Indonesia in October. 

How does this earthquake rank in global records?

While the magnitude estimate of this latest earthquake may later be revised, 8.9 is the largest Japanese quake on record and the fifth-largest quake worldwide since 1900, according to the U.S. Geological Survey (USGS).

How did the earthquake generate a tsunami?

The earthquake caused the seafloor at the fault to suddenly shift vertically and this motion, in turn, shifted the water column above it. This motion then spawned a series of massive waves known as a tsunami. [Tsunami Hits Japan after Massive 8.9 Earthquake]

"It is not unlike dropping a pebble into a pond and seeing the ripple move," Sverdrup told LiveScience.com. The waves simultaneously spread toward the east coast of Japan and toward the west, on a much longer path to the west coast of North America.

"Last night, Japan's tsunami was at a location at which we have had giant tsunamis in the past," said Harry Yeh, a professor at Oregon State University who was born in Japan. In 1896 and 1933, tsunamis hit in the same area.

Yeh said his family in Tokyo is fine. "My father is 91 years old and he and my 92-year-old mother were taking a walk, and walking around they felt some kind of ground shaking," he said. "He thought he had a stroke."

Yeh is an expert on ocean and coastal wave dynamics, including tsunamis.

"A tsunami is really an intriguing phenomenon because it rarely happens," he said. The wave itself is unique, resembling neither a storm wave nor a long, tidal wave, and it alters both the ecology and the terrain of the land it touches. Watching the footage on TV, he said he was struck by how the speed of the water surging over fields and homes resembled that of a flooded river.

The highest of the waves are believed to have reached 30 feet (9.1 meters).

How well can we predict events like this?

"Earthquake prediction is something we haven't really been able to master yet," Sverdrup said. "With earthquakes, the best we can say is these are areas where earthquakes are likely to occur and this is our best estimate of the statistical probability of an earthquake of a certain size happening within a certain period of time."

Once an earthquake like this one has occurred, it's possible to assess whether or not it will generate a tsunami by determining whether a vertical shift occurred at the fault and by looking at measurements of water height recorded around the Pacific Ocean basin by the Tsunami Warning Network.

The warning network alerts people living in the area that will possibly be hit by a tsunami."Unfortunately, the closer you get to the earthquake, the less time you have. Clearly in the case of the country of Japan, they would not have had much warning at all," Sverdrup said.

With the westbound wave traveling at about 500 mph (804.7 kilometres per hour), roughly the speed of a commercial flight, Hawaii and the west coast of North America were better off.

In Japan, the tsunami warning went out about five minutes after the earthquake and included an estimated height for the waves, Yeh said. Reports of the time between the warning and the arrival of the giant waves varied, and as of Friday (March 11) afternoon, the death toll was still uncertain, with hundreds reported missing.

Yeh said the island nation was as well-prepared as it could have been for the disaster.

"It's a real problem in Japan, so they are always worried about tsunamis, they have tsunami drills and they have tsunami warnings," he said.

Why do some earthquakes cause tsunamis while others don't?

A few factors come into play: the strength of the quake, the direction of the temblor's motion and the topography of the seafloor.

First, the magnitude of the quake — which is a measure of the amplitude of the largest seismic wave recorded for the earthquake — must exceed a certain threshold. The 8.9-magnitude of Japan's earthquake was enough to trigger a tsunami, but the magnitude-7.7 earthquake that struck Indonesia in October 2010 just surpassed the threshold for causing a tsunami.

"Earthquakes below 7.5 or 7.0 usually do not trigger tsunamis," said geophysicist Don Blakeman of the U.S. Geological Survey's National Earthquake Information Center.

Earthquakes trigger tsunamis when the seismic activity causes the land along fault lines to move up or down. When parts of the seafloor shift vertically, either becoming raised or lowered, entire water columns become displaced. This creates a "wave" of energy, which propels the water, Blakeman explained.

Earthquakes that push land mainly in the horizontal direction are less likely to cause the devastating waves, according to USGS geophysicist John Bellini. When energy pushes the plates horizontally, the land does not raise or lower the water above it enough to cause a tsunami, Bellini said.

The height of a tsunami wave is influenced by the ground's vertical movement, so changes in the seafloor's topography can either amplify or dampen a wave as it travels along.

Why is there a danger of radiation leaks at two Japanese nuclear reactors?

The cooling systems have failed at the two facilities as a result of power loss in the region. Even though the reactors have been shut down, a small, base level of nuclear decay is still happening, and that means a small amount of residual heat is still coming off the nuclear fuel rods. Under normal conditions, the cooling systems use water to surround the nuclear fuel rods and dissipate that heat, but power loss means the plants aren't able to pump water around the rods. If the fuel rods get exposed, they could melt, and in that case their containers may not be able to contain the radiation emitted from them. Dangerous leaks could occur.

Why did a whirlpool form in a Japanese harbor after the tsunami hit?

As a tsunami hits the coastline, the force of the water interacts with the shape of the coast and seafloor. The resulting chaos can cause all sorts of underwater vortexes that often scar the ocean bottom, according to "Tsunami: The Underrated Hazard" (Cambridge University Press, 2001) by Edward Bryant of Wollongong University in Australia. Whirlpools don't leave much of a geological record, but eyewitness accounts suggest they're relatively common after large tsunamis.

Stephanie Pappas, Remy Melina and Natalie Wolchover contributed to this article.

You can follow LiveScience writer Wynne Parry on Twitter @Wynne_Parry.