Scientific thought: The forces driving Plate tectonics can happen fast when conditions are right.
Non-scientific thought: Thor done dropped his hammer hard.
Random thought: There are plenty of faults in North America that could pop with an equivalent amount of force (Mississippi River, Hudson River faults underlying NYC, San Andreas in CA... to name a few).
All things being equal, the Japanese have built their infrastructure in anticipation of events like the recent quake/tsunami (although no amount of anticipation can account for total safety when the event is of such a horrific magnitude).
If a M8 quake hit NYC, there'd be nothing but a pile of ruins after everything was done.