Planet on a Roll: Two earthquakes related ?

But that's not the case: "This is not out of the ordinary as far as the year goes," said John Bellini, a geophysicist with the U.S. Geological Survey.

While it's not unusual to have several large quakes in any given year, these rumblings of the Earth can have different profiles, occurring on different types of faults and triggering varying series of events, including tsunamis possibly even other earthquakes.

Scientists said it's possible the two quakes this week are related, but they don't know yet. Meanwhile, researchers have some preliminary explanations as to why the Samoan temblor generated a deadly tsunami but the Indonesian quake did not.

A year in earthquakes

More than a million earthquakes are estimated to shake the Earth's surface every year, though most of these are minor tremors, barely perceptible to humans if at all.

As you go up in earthquake magnitude (a measure of a quake's strength), these events become rarer: About 17 magnitude 7.0 to 7.9 quakes occur around the globe each year, while on average only one magnitude 8.0 or higher strikes over the period of a year. (The logarithmic nature of the magnitude scale means that an 8.0-magnitude quake is 10 times stronger than a 7.0-magnitude.)

The underwater quake that struck of the coast of Samoa and American Samoa yesterday measured about 8.0, according to the USGS. The quake off the coast of Sumatra, Indonesia came in at an estimated 7.6 magnitude. (The L'Aquila quake, which struck in April, was only a 6.3-magnitude quake.)

The occurrence of all these earthquakes falls entirely within the average for a given year. "We're just having a busy week," Bellini said.

Different effects

While the Samoan earthquake generated a large tsunami that rushed ashore on the South Pacific islands, leveling towns with waves estimated to be 10 foot (3 meters) or higher and killing scores, the Indonesian quake sprouted only a small local tsunami. The shaking did most of the damage in the Indonesian event.

The damage done by a quake - and how it is done - isn't just a matter of how strong the quake is, it's "dependent on how the fault breaks," Bellini told LiveScience.