(Associated Press) The massive earthquake off Indonesia surprised scientists: Usually this type of jolt isn’t this powerful.

The biggest earthquakes tend to occur in subduction zones where one plate of the Earth’s crust dives under another. This grind produced the 2004 magnitude-9.1 Indian Ocean disaster and the magnitude-9 Japan quake last year.

Wednesday’s magnitude-8.6 quake occurred along a strike-slip fault line similar to California’s San Andreas Fault.

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