I-MAG STS    Corporation
The magnitude rating of very large earthquakes is undergoing revision
There are three types of movements along a fault: dip-slip, otherwise known as thrust, moves one half of the fault
vertically (higher or lower) relative to the other half; strike-slip moves one half of the fault horizontally (to the right or
left) relative to the other half; oblique slip does both. Thrust earthquakes are far more likely to generate tsunamis.

Magnitudes below 6.5 - earthquakes of this magnitude are very unlikely to trigger a tsunami.

Magnitudes between 6.5 and 7.5  - earthquakes of this size do not usually produce destructive tsunamis.  
There may be secondary effects such as landslides or submarine slumps.   

Magnitudes between 7.6 and 7.8 - earthquakes of this size may produce destructive tsunamis near the epicenter

Magnitude 7.9 and greater - destructive tsunamis capable of crossing oceans are possible
Generally, one can expect a maximum aftershock with magnitude 1.5 less than the original earthquake.