If memory serves, there were numerous major aftershocks and quakes after the Indonesia tsunami, so this may not be unusual.
Yes, there were. In the 1994 quake in Northridge, we had aftershocks (emanating from the same fault line) almost 5 years later.
From what I've read and heard, the undersea land was displaced 150 miles long x 50 miles wide in some areas. That's a HUGE area to be considered "epicenter". We can expect lots and lots of aftershocks, most in the 6-7 range, for the first week or so.
Over the next few weeks, they will both lessen in frequency, and be lower magnitude.
Frankly, historically, this quake has had far fewer aftershocks than I would have expected. There are only 180 on the map (which changes on a 7-day basis) at this time; it's been my experience that there were thousands of aftershocks to some quakes, and multiple hundreds (not 180ish...I mean, 4-500) of aftershocks in others.
If I am correct in my memory, this tells me that we may not have seen the last of the shaking (huge number kind of shaking), and that if the little ones (little in contextuality, not reality) don't pick up in numbers, the risk is greater for a "lock" or "break/slip" to happen.
A "lock" is when one plate exerts tremendous pressure on another plate, but there is no release; over time, the lock literally will break down, and generally a huge quake is the result. The San Andreas Fault, near me, is "locked". You can see the "elbow" north of Los Angeles; it literally bends. A locked quake ends up as a thrust quake, which has significant up/down displacement.
A break/slip is, simply, an earthquake. It's a type of earthquake, though, that isn't quite as violent as what I suspect the Japan quake was, and what I know the 1994 quake was. The break/slip is a type of quake which slides past each other, but doesn't move earth up/down as much.
I would rather a break/slip to a lock/thrust any day of the week. They tend to be smaller, more gentle (lots of side to side) motion, and are more easily survived. I've been in both, several times each, and while I know that it's sorta splitting hairs, I sure would rather the side to side than the up and down...
BTW, USGS states that the 6.8 is a new - NEW - quake. Confirmed by Fox.
Best-
Herding Cats
