Washington, May 26 : A new study has shown that large earthquakes routinely trigger smaller jolts worldwide, including on the opposite side of the planet and in areas not prone to quakes.
The study was carried out by Kris Pankow, a seismologist at the University of Utah Seismograph Stations, and colleagues.
"Previously it was thought seismically active regions or geothermal areas were most vulnerable to large earthquake triggers," said Pankow.
But, the team analyzed 15 major earthquakes stronger than magnitude-7.0 since 1992, and found that at least 12 of them triggered small quakes hundreds and even thousands of miles away.
"We conclude that dynamic triggering is a ubiquitous phenomenon," said the researchers.
Pankow's team 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 data included 15 major earthquakes from 1992 through 2006, including the 1992 Landers quake in California, 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.
Scientists previously noted that those three major quakes triggered not only nearby aftershocks, but small quakes at great distances.
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.
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, and the second waves are called S or shear waves.
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.
Pankow and colleagues 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.
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," said Pankow.