The 8.6-magnitude quake that struck Indonesia yesterday will go in the books as the 11th largest since 1900, and is presumed to be the largest of its type ever.
Scientists believe the event was what is known as a strike-slip, which is when rocks slide together horizontally, although they aren't known for creating earthquakes this large. A famous example of a strike-slip fault is in California--the San Andreas.
Geologists say the horizontal shift was a good thing, as it kept ground-level damage to a minimum and played a part in keeping tsunami-sized waves at bay. Officials in northern Indonesia say the devastating quake-triggered tsunami in 2004 was the jumping-off point for an upgrade in early warning systems, and thousands of people were evacuated or moved to higher ground yesterday in anticipation of dangerous waves. Luckily, none ever came.
Keven Furlong, a geosciences professor at Penn State, said such a big event is rare enough to make history. "A week ago, we wouldn't have thought we could have a strike-slip earthquake of this size," he said.
Early data indicates that a major factor in the quake's size is the odd unbalance that occurred; one side of the fault was pushed 70 feet past the other side, an enormous difference when compared to other, similar events--such as a 1906 quake along the San Andreas fault in which the ground only shifted 15 feet.