Earthquake strikes: An 8.6-magnitude earthquake and an 8.2-aftershock have hit off Indonesia's western coast, but a major tsunami is unlikely.
Why no tsunami? These two quakes were strike-slip quakes, which cause a horizontal tearing movement that does not displace the seabed, meaning the chances of a tsunami are low.
Japan: The earthquake that triggered a tsunami that devastated northeastern Japan last year was a thrust earthquake. This kind of quake causes the sea bed to flip up, dispersing large volumes of water.
Magnitude: The magnitude of the Japan quake was also greater at 9.0, it was about four times more powerful than Wednesday's quake. The 9.1-magnitude quake off Indonesia in 2004, which triggered an Indian Ocean tsunami that killed 230,000 people, was five times more powerful.