So called megathrust earthquakes such as the one that struck off the coast of Miyagi Prefecture on March 11, 2011, tend to occur in pairs, with a relatively short gap between them.
Such was the case in the Ansei Era (November 1854 through March 1860), during which Japan was successively shaken by several major earthquakes.
Since January, Shukan Gendai and several other weekly magazines, as well as sports tabloids, have produced a stream of copy about the inevitability of the next big one.