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It is generally accepted that the earthquake increased the stress on the adjoining faults.  For an earthquake of this size, the aftershock series will continue for many years, but at a decreasing rate with time.

There is nothing thus far discovered that distinguishes aftershocks from new earthquakes except for the relationship in time and space with the major event.  Thus, they will be called aftershocks if they are close to the main event until the seismicity rate drops to what is considered "normal".

Scientists have not yet found any clear relationship between the timing of earthquakes on opposing sides of the Pacific plate.  However, this is a subject of ongoing research.

Depending on the design of the absorbing components, it is possible that they have been damaged to the point that they have lost their ability to protect the structure.  Earthquake engineers will have to inspect the components to determine what actions need to be taken.

Modeling of the stress changes due to the Tohoku earthquake show stress increases to the north and south of the main rupture surface, including the Tokyo area.  However, we are not able to translate this information into a better prediction of the timing of future events.  This stress increase does not necessarily translate into a stronger earthquake because that depends on an extensive history of stress build-up that extends for hundreds to thousands of years in the past.  While an aftershock could possibly trigger an earthquake nearby, it is difficult to predict this in advance.

For an earthquake of magnitude 9.0, the aftershock series could easily last for a decade with decreasing seismic frequency with time.  The aftershock series is judged over when it reaches the "normal" earthquake frequency level, which is quite high in Japan.

Right now, scientists can't find any physical or statistical reason to use the Chile, New Zealand, and Japan earthquakes to predict the timing of a future event in the Pacific Northwest.  However, we know that a great earthquake occured there in 1700, and that we must do everything we can to prepare for a recurrence of that event.

As I said before, an aftershock and a "normal" earthquake cannot be distinguished from one another.  An aftershock is within the same area, approximately within a fault length, and some time after the main event.  It is no longer an aftershock when the seismicity has dropped to a normal level for that region.

The Tohoku earthquake is a wakeup call for everyone in seismically active areas, or on vulnerable coastlines, to evaluate earthquake and tsunami hazards to nuclear plants, the built environment in general, and evacuation plans.

Earthquakes can happen around the Pacific rim, the so-called "ring of fire", at any time.  However, we haven't been able to make a clear connection between the timing of events that are distant from one another.  That is, we haven't been able to confirm a domino effect around the Pacific rim.

That is essentially correct.  The duration of the shaking is directly a function of how extensive is the fault rupture surface.  It can be considered to be multiple earthquakes on the fault surface and the further you are from each rupture, the longer it takes for seismic waves to arrive.  This is a general characteristic of great earthquakes which by definition have extensive fault surfaces.  It is hard to say what it tells you about the time of the next earthquake further down the fault.