Subduction zones generate the world's largest and most destructive earthquakes and most of the world' s destructive tsunamis, as has been recently shown by the devastating Andaman-Sumatra event on 26 December 2004. Understanding the factors leading to Earth's largest and most destructive earthquakes is not only an obviously important goal, as stated in the U.S. National Science Foundation's Margins Science Report 2004, but it is also an utmost important goal for the whole geoscience community.Interrelated with this topic are still unsolved questions in seismologyc Why do subduction zones occasionally generate the largest known (Mw > 9) earthquakes? And why are only a few subduction zones capable of generating Mw ≈ 9 earthquakes while the rest only produce upto Mw ≈ 7.5?