Why Earthquake Off Sumatra Did Not Trigger A Devastating Tsunami -- The Guardian
Main shock was magnitude 8.6 but horizontal seafloor movement meant warning buoys only measured small waves.
The earthquake off the coast of northern Sumatra in Indonesia began with a sudden shift in a strike-slip fault, a line of weakness in the sea floor where two huge bodies of rock can slide past one another.
Unlike the earthquake that triggered the devastating 2004 tsunami in the region, the sea floor moved sideways instead of vertically, meaning it displaced less water and did not send giant waves around the Indian Ocean.
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My Comment: There region was lucky this time.