Big earthquakes don't just cause repeat aftershocks near their epicenter as they are in China, they can ripple worldwide and trigger tremors on the opposite of the planet even in places not prone to quakes.
A new study co-authored by a University of Utah seismologist and published last week in the journal Nature Geoscience reports that at least 12 of the 15 major earthquakes stronger than magnitude 7.0 since 1992 triggered small quakes thousands of miles away.
The findings contradict both science and the rule of thumb among seismologists that quakes do not spark tremors in distant locations.
"If you were to ask any seismologist prior to 1992 if nonseismically active areas are vulnerable to large earthquake triggers they would have said absolutely not," Kris Pankow said. "What's new in this study is we conclude that dynamic triggering is a ubiquitous phenomenon."
The new study is the first to systematically analyze all the world's big quakes during 1992-2006 and find that most of them triggered distant, smaller tremors. These are different than aftershocks, which occur fairly close to the main quake. After the devastating 2004 Sumatra earthquake, triggered quakes even occurred in Ecuador, on the opposite side of the Earth.
The data were gathered from a consortium of universities and included the '92 Landers quake in California 800 miles southwest of Yellowstone, the magnitude-7.9 Denali fault quake in Alaska in '02, and the magnitude-9.2 Sumatra-Andaman Islands quake near Indonesia in '04 that generated a catastrophic tsunami blamed for most of the quake's 227,898 deaths in South Asia and East Africa.
Pankow and colleagues seismologist Aaron Velasco and undergraduate student Stephen Hernandez, both at the University of Texas at El Paso, and seismologist Tom Parsons, of U.S. Geological Survey in Menlo Park, Calif., analyzed data from more than 500 seismic recording stations five hours before and five hours after earthquakes that registered more than 7.0 on the "moment magnitude" scale, which scientists say is the most accurate scale for large earthquakes. (The Richter scale measures only relatively small, nearby quakes.)
Pankow and her colleagues showed that magnitude 4 or smaller seismic events often are triggered when either Love waves, energy that moves in a shearing fashion, or Rayleigh waves, which roll, move out from a major quake, according to a U. press release.
"We can recognize the different kinds of waves as they pass and can filter out everything except the small seismic events, which are presumed to be local, small earthquakes," Pankow said.
The large quakes, like the one in Sichuan province in China on May 12 which is still rumbling, are obviously the most destructive, she said. But the interior of the earth is a very active place, with 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.
"It is interesting that Rayleigh and Love waves, two very different types of surface waves, are both able to trigger these events," says Pankow.In addition to the 1992 Landers, 2002 Denali and 2004 Sumatra-Andaman Islands quakes, the other 12 major quakes in the study (and their moment magnitudes) were:
• 1998 Balleny Island near Antarctica (8.1)
• 1999 Izmit, Turkey (7.6)
• 1999 Hector Mine, Calif. (7.1)
• 2000 New Ireland, Papua New Guinea (8.0)
• 2001 Peru (8.4)
• 2001 Kinlun, China (7.8)
• 2003 Hokkaido, Japan (8.3)
• 2003 Siberia, Russia (7.3)
• 2004 Macquarie Ridge, near New Zealand (8.1)
• 2005 Sumatra, Indonesia (8.7)
• 2006 Java, Indonesia (7.7)
Exactly how surface waves trigger small earthquakes at distant locations isn't understood, Pankow said.
"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."
She said surface waves may increase the strain on a fault, or loosen a fault so that it prematurely breaks or slides, she noted.The study was funded by the United States Geological Survey and the National Science Foundation, Pankow says. A map of seismic stations that recorded triggered quakes may be downloaded from www.unews.utah.edu/p/?r=051908-1