Devastating earthquakes have rocked the world recently, from Haiti to New Zealand to Japan, but while scientists now find evidence that these major quakes can increase the risk of more temblors nearby, they aren't likely to trigger massive quakes around the globe.
An analysis of large earthquakes over the past 30 years finds that that the largest follow-up quakes tended to occur within 620 miles (1,000 kilometers) of the original. The reason for this is not clear.
"We need to understand the physics of why small events can be linked to mainshocks at global distances, but not larger ones," researcher Tom Parsons, a U.S. Geological Survey geophysicist at Menlo Park, Calif., told OurAmazingPlanet.
The planet has seen a spate of powerful earthquakes in the last year or so — the magnitude 9.0 quake in Japan this month, the magnitude 6.8 shock that hit Myanmar on March 24, and the magnitude 6.3 that struck New Zealand in February, as well as the magnitude 8.8 temblor in Chile and magnitude 7.0 earthquake in Haiti that both struck in 2010.
Scientists in recent years have discovered that large earthquakes routinely trigger small ones elsewhere in the world. The obvious question then was whether or not big quakes also raised the risk of large, damaging earthquakes globally, Parsons said.
More quakes nearby
Parsons and his colleague Aaron Velasco analyzed a 30-year catalog of all earthquakes from the Advanced National Seismic System and Global Seismograph Network. They focused on those larger than magnitude 5 that could have been triggered by a shock of magnitude 7 or greater within the previous 100 days.
The researchers found that powerful earthquakes did increase the risk of more large quakes around them. However, after a certain distance — from the radius around the main quake's epicenter to about 620 miles, or two to three times the length of the rupture to blame — only small follow-up earthquakes seem to be triggered.
There are fears the March 11 earthquake in Japan might boost the risk of quakes worldwid e, but Parsons noted that while "the regional rate of large triggered aftershocks is very high in Japan now, again this seems restricted to within about 1,000 kilometers of the mainshock."
This connection, or lack thereof, leaves geophysicists with a puzzle to solve.
"It suggests that there might be a delay in their response such that we cannot detect them, or that there is a different process by which large earthquakes begin," Parsons said.
A current example
Looking for connections between quakes that happen close together in time or space can be tempting, but the findings aren't always clear, Parsons said.
The magnitude 6.8 shock that hit Myanmar March 24 does lie outside the 620-mile radius of the earthquake in Japan. Still, "it is also not so far off the end of the 2004 Sumatra rupture," Parsons said. "It's thus feasibly related to either quake or neither. It's always tempting to draw lines between distant events, but when we look at enough events to make a statistical test, it's hard to make that case."
Parsons and Velasco detailed their findings online March 27 in the journal Nature Geoscience.