The plate boundary off the coast of Sendai had not had a “mega-quake” in the modern era. It may not have suffered a major rupture like this for more than 1,000 years. The closest analog may be a tremor recorded by monks in the year 869, according to Dave Applegate, a senior earthquake specialist at the U.S. Geological Survey...

“This is a continuation in a sense of the cold shower that we got in Sumatra — these mega-earthquakes take place in places we do not expect them,” said Emile Okal, a Northwestern University geophysicist reached in Tahiti, where he was preparing to evacuate in advance of the tsunami generated by the quake. The huge Sumatra earthquake six years ago that generated the devastating tsunami along the rim of the Indian Ocean happened on what had been presumed to be a relatively quiescent stretch of a subduction zone.

“That means that on a global scale we should consider that all subduction zones are potential locations for such events,” Okal said.