Big Quakes Cause Shaking Halfway Around the World

This map of the world shows seismic stations that detected more than twice the normal number of small, nearby earthquakes after the passage of what are known as "surface waves" from major quakes that were centered hundreds to thousands of miles away and occurred from 1992 through 2006. A new study co-authored by University of Utah seismologist Kris Pankow found that at least 12 of the 15 major earthquakes (greater than magnitude-7) during 1992-2006 triggered small quakes in distant parts of the world. Scientists once believed big quakes could not trigger distant tremors.

Photo: Aaron Velasco / University of Texas at El Paso

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Big earthquakes like the deadly 7.8-magnitude quake that recently hit China not only spawn aftershocks that can be felt hundreds of miles away – China's main quake was felt as far away as Thailand and Vietnam. New science shows how those major quakes can trigger tremors halfway around the world.

Analyzing 15 major earthquakes stronger than magnitude-7.0 since 1992, researchers found that at least 12 of them triggered small quakes hundreds and even thousands of miles away. The results were published in Nature Geoscience.

“Previously it was thought seismically active regions or geothermal areas were most vulnerable to large earthquake triggers,” says Kris Pankow, a seismologist at the University of Utah Seismograph Stations and a co-author of the new study. His co-authors were from the University of Texas at El Paso and the U.S. Geological Survey.

Among the earthquakes analyzed were the 1992 Landers quake in California which triggered tremors 800 miles away in Yellowstone National Park, the magnitude-7.9 Denali fault quake in Alaska in 2002, and the magnitude-9.2 Sumatra-Andaman Islands quake near Indonesia in 2004 that generated a catastrophic tsunami blamed for more than 200,000 deaths.

China's May 12 earthquake, which spawned another destructive aftershock just this weekend (there have been 182 recorded in two weeks), was not part of the study. The earthquake has killed more than 65,000, according to state-run media. More than 360,000 were injured, 23,150 remain missing and 14.4 million people have been evacuated from Sichuan Province.

Here's how the researchers described the science behind their findings:

When an earthquake begins, energy is released in the form of shock waves that move through the ground. The first waves are called P or pressure waves, which move at high speed with an up-and-down motion. The next waves are S or shear waves. These move from side to side, causing much damage from an earthquake. The next waves are two types of surface waves: Love waves move in a shearing fashion, followed by Rayleigh waves, which have a rolling motion.

Researchers showed that magnitude-4 or smaller seismic events often are triggered when either Love or Rayleigh waves from a major quake pass a given point.

There are about 600 small seismic events around the Earth every five minutes. For five hours after the arrival of Love waves from a major quake, the researchers saw a 37 percent increase in the number of small quakes worldwide. And after Rayleigh waves from the same large quake followed the Love waves, the number of small quakes worldwide shot up by 60 percent during the five hours after the major quake.

How do the surface waves trigger small earthquakes at distant locations?

“The physical mechanism is not known,” says Pankow. “It has been proposed that the passage of the waves may change the water flow in a fault, possibly increasing the number of conduits that water can flow through which could cause the fault to slip.”

Other theories are that surface waves may increase the strain on a fault, or loosen a fault so that it prematurely breaks or slides, she adds.

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