When quakes swarm: Are quakes contagious? Unusual number of earthquakes has some wondering what it means

Something seismically remarkable has happened in the South Pacific: In one 24-hour period this week no less than seven major earthquakes have rocked a small area near Vanuatu and the Santa Cruz Islands.

The unusual swarm of four magnitude 6-plus and three magnitude 7-plus events, and many smaller aftershocks, has a lot of people wondering what it means and whether the quakes have any connection to the very recent large quake in Samoa or even the 9.2 Sumatra-Andaman monster earthquake of five years ago.

"It's not unprecedented, but getting three events above 7 is unusual," said seismic researcher Susan Hough of the U.S. Geological Survey. Hough has investigated similar, even more powerful quake swarms — most notably the five large 7 and 8-magnitude quakes that struck the New Madrid area of Missouri in the winter of 1811-1812.

What New Madrid and other clusters of large quakes have taught seismologists so far is that it's largely a game of chance, Hough told Discovery News. Since we don't have enough data on the exact happenings inside the Earth's crust to be more precise, we must resort to working out the odds of one shock being a foreshock of a larger quake or not, based on what's happened before.

For instance, anytime an earthquake happens there's a one-in-20 chance something bigger will follow, Hough explained. The odds of there being two such large quakes crowded close to a larger shock is about one-in-100, she said. Think of that in horse racing terms, for instance, and the rarity of the new South Pacific swarm becomes clearer.

In the case of the Vanuatu swarm there was first a 4.9 foreshock, then hours later a mighty 7.6 shock. That second quake might have been mistaken for a main event had it not been followed 15 minutes later by 7.8-magnitude show stopper. An hour later there was a vigorous 7.3-magnitude aftershock, then a couple of dozen more aftershocks ranging from magnitude 4.9 to 6.8.

The good news from all this, however, is that despite the rarity of the swarm, these quakes, individually, are not the worst sorts of events produced in the collision zone of tectonic plates in that part of the world, Hough said. A genuinely big quake for a subduction zone is more in the range of a magnitude 9, she said.

Quakes Triggering Quakes
Then there is the matter of whether the South Pacific cluster is in any way "teleconnected," as seismologists say, to the 8.0 magnitude Samoan event of Sept. 29 or the 9.2 magnitude Sumatra-Andaman event of 2004.

"It's well within the realm of possibility," Hough said. "The [seismic] waves come tumbling through and disturb the fault zone and it causes little earthquakes...The trouble is, how do you prove it?"

Other researchers working on quake-triggered-quakes tend to agree."It is hard to guess whether there are teleconnections between the Samoa earthquake, the Sumatra earthquake and this earthquake swarm," said seismologist Fenglin Niu of Rice University. "Considering the fact that the first two earthquakes are almost 9,000 kilometers away from each other and the Samoa earthquake is much smaller than the 2004 Sumatra earthquake, probably there is little connection between the Samoa and the Sumatra events."

On the other hand, the Vanuatu swarm is less than 3,000 kilometers away from the Samoa event, so they could be related, Niu said.