Another quake in Japan today, following on from one off the east coast and another in the north in mid June - the 3rd in about 6 weeks. Quakes are not uncommon in Japan but there is a concern that large quakes can de-stabilise areas as well as relieve tension in the tectonic plates. They can be sort of pre-shocks to a more significant movement.
When I was living there, I remember being told that there are three major faults lines running through the main island and the Kanto region where Tokyo-Yokohama are and that all 3 were due/overdue to hit, that was back in the early 1990s. The Kanto area gets a big one every 70 years and the last one in 1923 caused 140,000 deaths mainly from fire. I think two of the lines were north-south and the other from the north-west to south-east. I don't think was more than a 4.5er when I was there which amounted to more or less a strong tremor where I was living. Still gave me the willies all the same.
Of course the averaging out of hits from quakes is rough work at best. I sincerely hope that the current quakes are simply relieving tension in the plates and not precursors to something major brewing in mother Earth's tummy.
Boston of all places has a risk of a magnitude 6 quake every 500 -900 years but the last was in 1755. However New England gets more moderate (magnitude 5) quakes happen every 50 to 90 years. And their problem is all the building that was done on the BackBay which is infill!