FACTBOX: Sumatra quake tops magnitude 8 -- what does this mean?

(Reuters) - A magnitude 8.4 earthquake that hit South Sumatra on Wednesday is the largest of four of magnitude 8 or above that have hit worldwide in 2007, and the most powerful to hit Indonesia since the magnitude 8.6 "Nias" earthquake in March 2005, according to the U.S. Geological Survey (USGS). Here are five facts about magnitude 8+ quakes.

WHERE HAVE THIS YEAR'S MAGNITUDE 8+ QUAKES BEEN?:

* January 13: East of the Kuril Islands, which stretch between Japan and Russia, magnitude 8.1

* April 1: Solomon Islands, east of Melanesia, magnitude 8.1

* August 15: Near the coast of central Peru, magnitude 8.0

* September 12: South Sumatra, Indonesia, magnitude 8.4

-- All four locations are on the Pacific "Ring of Fire", a horse-shoe shaped 25,000 mile (40,000 kilometer)-long arc of volcanoes and fault lines that generates about 90 percent of the world's earthquakes.

WHAT TRIGGERED THE SOUTH SUMATRAN QUAKE?:

-- The quake hit 130 km (80 miles) southwest of Bengkulu, on Sumatra island, as a result of thrust faulting at the boundary of two tectonic plates -- the Australian plate and the Sunda plate. At this location, the Australian plate moves northeast from the Sunda plate at a velocity of about 69 mm/year, according to the USGS.

HOW MANY MAGNITUDE 8 EARTHQUAKES ARE THERE PER YEAR?:

-- On average, about one. And there are about 15 to 20 magnitude 7 earthquakes per year, the U.S. Geological Survey says.

HOW MANY PEOPLE HAVE BEEN KILLED BY MAGNITUDE 8+ QUAKES?:

-- The 17 magnitude 8+ quakes since 1990 have killed about 285,491 people.

WHICH WAS THE MOST DEADLY?

-- The biggest quake -- December 2004's 9.1 magnitude quake off the west coast of north Sumatra, Indonesia -- was by far the most deadly: accounting for 283,106 deaths from the total.

-- Indonesia's March 2005 northern Sumatra quake was the second-most deadly in this period -- killing about 1,313 people.

Sources: Reuters, U.S. Geological Survey (www.usgs.gov, here)