From an amateurs point of view (2 years studying geology in college), since we are all seeing the effects of it, I started looking in to what caused the shaker in the IO off Sumatra.

This is from Wikipedia.org about the Indian Ocean and Sumatra Tsunami, the best general information WHY it happened. The article has several links in it that break down just how much energy was contained in the 9.0 quake that started it ("enough to boil 1,300 US gallons of water for every person on Earth."), and about other quakes that occurred before and after (after shocks). Interesting reading if you think of the geologists point of view.

This USGS chart shows were the epi-center was, (about 27E -027S) some after shock magnitudes and why it happened where it did - an understanding of plate tectonics is handy here. Basically a 'rift' is where 2 plates are spreading, a 'subduction' is where one plate is sliding under another and a transform is where they are sliding past each other (San Andreas fault line is a transform). The plate being subducted (going down) is under tremendous heat and pressure, melting the rock and usually creating volcanoes (Hawaii, Japan, & Guam with it's 7 mile deep Marianias Trench, are probably the best known volcanic island chains created by subduction). If you look at the graphic, the rifts and subductions, it appears the India plate is riding over the Berma plate (subduction) allowing the rift to happen between the Berma and Sandia plates.

If you look just South West of the epicenter you can see 2 converging plates (collision) and a very long subduction (the Sundia Trench at about 4.6 miles deep). It looks like the India plate is rotating counter-clockwise eating the Berma plate, pushing the Australian plate and that is affecting the Sandia and Eurasia plates. We haven't seen the end to the big shakers yet in my geological guestimate.

Mudrat