Speaking at a press conference in Pasadena, California, one USGS expert said today's earthquake appeared to be the "fraternal twin" of the magnitude 9.3 temblor that rumbled in December.
Shortly after the latest shock, U.S. National Oceanic and Atmospheric Administration officials issued a warning that residents living in coastal areas within 600 miles (1,000 kilometers) of the quake's epicenter should evacuate.
For weeks seismologists have advised that another earthquake along the same fault that triggered December's deadly quake and tsunami was likely.
In a study published earlier this month in the science journal Nature, one research team predicted that an earthquake as strong as magnitude 7.5 could occur along the Sumatra Fault, but they could not say when.
John McCloskey, a seismologist at the University of Ulster in Northern Ireland, led the study. In an interview with National Geographic News two weeks ago, McCloskey said: "People believe lightning never strikes twice in the same place. Earthquakes do.
"Earthquakes cluster in space and time. When you get an earthquake, you are more likely to get another, and our calculations show the stress interaction [in South Asia] is very high."
The latest shock occurred in a zone prone to cataclysmic earthquakes, where a number of pieces of Earth's tectonic crust jostle against, over, and under one another.
Sumatra, in particular, is prone to two different types of earthquake activity.
The first occurs along the Sumatra Fault, a strike-slip fault akin to California's San Andreas Fault, where two smaller pieces of Earth's crust grate and slide against each other, shoulder to shoulder.
The second occurs along a subduction zone known as the Sunda trench. Passing within 125 miles (200 kilometers) of the west coast of Sumatra, the 3,400-mile (5,500-kilometer) fault stretches from Myanmar (Burma) toward Australia.
Along the trench, two puzzle pieces of the Earth's tectonic crustthe Indian and Australian platesare diving beneath a third tectonic plate, known as the Burma platea process called subduction.
It was there that a magnitude 9.3 earthquake trembled on December 26, 2004. The quake moved trillions of tons of rock along hundreds of miles of fault, making it the largest recorded earthquake in 40 years.
As the seafloor shifted, a huge volume of water was displaced, creating the tsunami that traveled at jet speed and lashed the coasts of 11 nations in Asia and Africa within hours.
According the U.N., more than 300,000 people were soon killed or lost. Nearly 100,000 people died in Indonesia, the agency said, and over a million people in affected nations were displaced.
Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES