As far as I know, it's not uncommon for earthquakes to happen in clusters. The pressure builds up until the lock breaks and the fault moves, until it hits another lock that stops it. The moving plates can load several weak locks on the fault to failure over a period of months, breaking them and causing earthquakes, until they eventually hit a lock that holds them for a decent period of time.
Of course, all this is irrelevant, because we all know the Japanese earthquake was God's punishment for not following the Ten Commandments:
http://nymag.com/daily/intel/2011/03...s_earthqu.html
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