myBlessedHope

TOKYO (Nikkei)–The major earthquake that struck northeastern Japan on Friday had the hallmarks of an interplate quake, which occurs at the boundary between two tectonic plates.

At the area near the epicenter, the North American plate, on which part of the Japanese archipelago lies, slips under the Pacific plate. The temblor was likely triggered when the North American plate snapped upward, releasing the accumulated strain.

A whirlpool caused by the quake-triggered tsunami at Tomakomai port in Hokkaido.

The earthquake occurred near the site of another tremor that shook Miyagi Prefecture on Wednesday, so the earlier quake may have been a foreshock, according to the Japan Meteorological Agency.

“There aren’t many instances in which such a large earthquake occurs right after a magnitude 7-class quake,” said an official at the agency. “This is a highly unusual case.”

The agency warns that aftershocks registering around magnitude 7 could take place over the next month.

Major quakes have occurred off the coast of Miyagi Prefecture about every 40 years, the previous being a magnitude 7.4 temblor in 1978. Government experts had expected that one registering between 7.5 and 8 would hit, but the latest packed a wallop of magnitude 8.8.

The plate may have shifted over a stretch of several hundred kilometers along the fault on Friday, according to the agency, with some experts putting the figure at upwards of 400-500km.

“In last year’s magnitude 8.8 earthquake in Chile, the fault is said to have moved over a span of about 800km, so the latest quake may be similar,” said Kyoto University Professor Manabu Hashimoto.

Friday’s interplate earthquake is different from the Great Hanshin Earthquake that rocked the Kobe area in 1995, which was an intraplate quake.

(The Nikkei March 12 morning edition)