http://e.nikkei.com/e/fr/tnks/Nni20110311D11JFA11.htm
"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.
"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.
...
"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."
http://www.heraldtribune.com/article/20110311/WIRE/110319867/-1/features?p=2&tc=pg
"The quake was caused when one giant tectonic plate was shoved under another, the type of movement that produces the biggest earthquakes. It's the same kind of quake that caused the devastating 2004 Indonesian tsunami.
...
"The quake happened at the intersection of the North American and Pacific plates in the northwestern chunk of the "Ring of Fire," in an area that "has been incredibly quiet," Applegate said."
