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.