[Widdekind]

According to the History Channel documentary How the Earth was Made -- Tsunamis (TV), as the Indian Tectonic Plate "subducts" under the Sunda Plate, the lip of the latter is pulled down, being held, by friction, to same descending spot of the former. But, because Crustal rock is actually Elastic, this builds up stress in the Sunda Plate, like a giant sheet of plywood. Eventually, the force of friction fails, and the Sunda Plate slips free and springs back up, in a series of steps, gradually reducing the pent up tensions. Thus, every 200-250 years, there is a series of several such Earthquakes. The 2004 AD Indonesian Tsunami was caused by one such spring-back of the Sunda Plate. And, another such spring-back is expected sometime soon, according to Wikipedia.


ALTERNATIVE THEORY:

Crustal rock is weak. Crustal Slabs are a hundred times wider than they are thick. Thus, they resemble "tissue paper"*.

* R.H. Dott, Jr. & D.R. Prothero. Evolution of the Earth (5th ed.), pp. 138,152.

Therefore, it seems impossible that powerful Tension-forces can build up in deformed Crustal Slabs, being pulled downward, through Friction, by Subduction.

Rather, it seems intuitively more plausible, that as the Sunda Plate is driven downward, into the Mantle, powerful Pressures build, by the Compression of that Mantle material. Thus, it could be Compression of the underlying Mantle, not Tension in the Crustal Slab, that drives such Undersea Earthquakes and associated Tsunamis.

[Geosciences-Forum.com]

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