Size of Japan’s quake surprises seismologists

The magnitude 8.9 earthquake that struck Friday off the coast of Japan “is going to be among the top 10 earthquakes recorded since we have had seismographs,” said seismologist Susan Hough of the U.S. Geological Survey in Pasadena. “It’s bigger than any known historic earthquake in Japan, and bigger than expectations for that area.”

Geologists had expected the portion of the Pacific “Ring of Fire” that produced this quake to yield a temblor on the order of magnitude 8 or perhaps 8.5, she said. “Something as big as an 8.9 is a bit of a surprise,” she said.

A quake that big usually requires a long, relatively straight fault line that can rupture, such as those found in Peru and along the eastern coast of South America. Friday’s quake occurred in the Japan Trench, where the Pacific tectonic plate slides under the Japan plate.

Photos: Scenes from the earthquake

Scientists did not expect such a big quake in the area because the plate boundary is not straight, but is fairly irregular. According to Lucile Jones of the U.S. Geological Survey, a quake of that size would require rupturing a zone at least 300 miles long.

The region had a magnitude 7.2 temblor Wednesday in almost exactly the same area. Typically, with any large earthquake, there is about a 5% chance that such a quake is a precursor of a larger quake. This appears to have been that 1-in-20 chance.

There have been at least two aftershocks of magnitude 7 or greater, and researchers expect more.

The quake was a “perfect storm for tsunami generation,” Hough said — it was large in magnitude and very shallow. The quake was so close to land, about 80 miles offshore, that people on the shore really had no warning that a 15-foot wave was imminent.

The Indonesian earthquake that produced the Indian Ocean tsunami on Dec. 26, 2004, was a magnitude 9.1. The largest quake on record was the 9.5 temblor that struck Valdivia, Chile, in 1960.

Photos: Scenes from the earthquake

thomas.maugh@latimes.com