Great off-trench earthquakes are rare in the instrumental era; only four are known with confidence since 1900 (1933 Japan, 1977 Sunda, 2007 Kuriles, and 2009 Tonga. The 29 Sept 2009 Tonga earthquake with its destructive and fatal tsunami waves in Samoa is the most recent example. This lecture explores common features in the tectonic settings of such earthquakes and common flexural mechanics. There are other possible subduction segments that also share these feature and hence may be prone to such earthquakes in the future.