Hey there are alot of threads that mention this already... in fact i think i saw your post in Volcano Wath 05... but i will respond anyway coz i feel
it is a serious issue needing addressing.
Lake Toba in Sumatra lies east of the Subduction Fault line that is the fault where the Indo-Australian Plate and the Bumese Plate meet. Now becasue
the Australian Plate subducts under the Burmese plate any volcano on the east side of the plate (I.e anywhere on sumatra) is subject to lare build ups
of Magma unless the pressure is regularly released...
Now when earthquakes, like the boxing day quake, occur, a portion of the sea floor sinks under the Burma plate... with it goes alot of water... if
there are a series of quakes in succesion, and no volacnic activity, the water (which has turned into steam due to the heat in various magma chambers)
stays under ground... now if the steam gets trapped in a magma chamber (usually occuring under volcanos) and the pressure builds for too long, there
is a high chance of a higly explosive volcanic eruption... similar to Krakatoa in 1883... i suggest you all read
Krakatoa by Simon Winchester
as it explains the process in some depth...
Krakatoa
Subduction Zone explanation with article on Sumatra