On Friday, an earthquake, the largest in at least 100 years (1000 years?), struck Japan. It triggered a local tsunami with waves of 30 feet. The combined calamity killed hundreds. In addition, the 8.9-magnitude earthquake triggered tsunami alerts across the Pacific from Indonesia and New Zealand to Russia to the coasts of Alaska, Oregon and California.
According to the U.S. Geological Survey, the earthquake was 130 kilometres east of Sendai on the main island of Honshu. For the latest news, see live blogging from TMV.
What Causes An Earthquake?
The earth's continents and sea floor are composed of tetonic plates that cover the magma mantle and that are in incremental (an inch or two a year) motion. Earthquakes happen where these plates abut one another; these edges are called fault lines. Sometimes the plates get stuck. Eventually, when the resulting friction reaches a breaking point, the stuck plates move and release vast amounts of energy that we know as an earthquake. The point of origin, beneath the earth's surface, is called the hypocenter; the epicenter is the point on the surface directly above the hypocenter. The released energy radiates outwards and generates side-to-side shaking. From BBC: How Earthquakes Happen (pdf) and NOAA: Causes of tsunamis
Watch an earthquake animation from the BBC.
What Causes A Tsunami?
Tsunami means "harbor wave" in Japanese but it is more accurately a series of waves. Tsunamis can result from earthquakes that happen in a subduction zone; this is where one oceanic plate moves beneath another one, down into the mantle. Earthquakes cause tsunamis by causing a vertical displacement of the seafloor. The size of a tsunami is correlated with the size of the earthquake, where size is a measure of released energy. As the tsunami travels across open sea, at speeds up to 1,000 km/hour, it may not be visible to the human eye.
However, as the illustration above shows, the tsunami changes as it moves into the more shallow waters near the coast: the waves slow down and they begin to compress, becoming taller and closer together. From Geology.com: What causes a tsunami? and Tulane University: Tsunami
Watch a tsunami in action (from the British Columbia government).
What Is The Pacific Rim of Fire?
The "Pacific Ring of Fire" is an area of active seismic activity that stretches, clockwise, from New Zealand up along the eastern edge of Asia, north across Alaska's Aleutian Islands and then south along the coast of North and South America. Learn more from Geography at About.com.
See a map of the Pacific Rim of fire and plate tetonics from the US Geologic Service