EARTHQUAKE STRIKES: An 8.6-magnitude earthquake and an 8.2-aftershock hit off Indonesia's western coast Wednesday but did not cause a major tsunami.

WHY NOT? These two quakes were strike-slip quakes, which cause a horizontal tearing movement that does not displace the seabed, causing vibrations in the sea instead.

MEGA-THRUST: The tsunami that devastated Japan last year and the 2004 tsunami off Indonesia were mega-thrust earthquakes. They cause the seabed to heave vertically, dispersing large volumes of water.

MAGNITUDE: The magnitudes of Wednesday's quakes were smaller. Japan's temblor was 9.0-magnitude, about four times more powerful than Wednesday's first quake. The 9.1-magnitude quake in 2004, which triggered an Indian Ocean tsunami that killed 230,000 people, was five times more powerful.