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Big quakes spark jolts worldwide, study finds
May 27, 2008
Courtesy University of Utah
and World Science staff
Large earthquakes routinely trigger smaller jolts worldwide, including on the opposite side of the globe and in places not prone to quakes, a study has found. This world map 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 centered hundreds to thousands of miles away, from 1992 through 2006. A study
has found that at least 12 of the 15 major earthquakes in this
period triggered small quakes in distant parts of the world.
(Credit: Aaron Velasco, U. of Texas at El Paso) Send us a comment
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Scientists didn’t think such distant tremors occurred before California’s magnitude-7.3 Landers earthquake in 1992 set off small jolts as far as 800 miles (1,300 km) away.
In the study, researchers analyzed 15 major quakes stronger than magnitude-7 since and including the Landers event. They found that at least 12 triggered small quakes hundreds and even thousands of miles away.
This process, called dynamic triggering, is “a ubiquitous phenomenon,” they wrote, in findings published online May 25 in the research journal
Nature Geoscience.
They reviewed measurements from more than 500 seismic recording stations five hours before and after the quakes. The data came from the Incorporated Research Institutions for Seismology, a consortium of universities.
The distant tremors differ from 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, researchers said.
An earthquake first releases energy in the form of shock waves that move through the ground. The first waves are called pressure waves, which
travel quickly with an up-and-down motion. Next come shear waves,
which move from side to side, causing much damage from an earthquake. The next waves are two types
that move along the surface: Love waves in a side-to-side fashion, followed by Rayleigh waves, which have a rolling motion.
The researchers found that magnitude-4 or smaller events often are triggered when either Love or Rayleigh waves from a major quake pass a given point. “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,” said co-author Kris Pankow, a seismologist at the University of Utah.
About 600 small seismic events occur worldwide 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,
the investigators found.
“Rayleigh and Love waves, two very different types of surface waves, are both able to trigger these
events,” said Pankow. The researchers measured quake strength using
the so-called Moment Magnitude scale, which estimates the energy
released and is a newer system than the oft-cited Richter scale.
Both, however, give similar numerical results in most cases.
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Large earthquakes routinely trigger smaller jolts worldwide, including on the opposite side of the globe and in places not prone to quakes, a study has found. Scientists didn't think such distant tremors occurred before California's magnitude-7.3 Landers earthquake in 1992 set off small jolts as far as 800 miles (1,300 km) away. In the study, researchers analyzed 15 major quakes stronger than magnitude-7 since and including the Landers event. They found that at least 12 triggered small quakes hundreds and even thousands of miles away. This process, called dynamic triggering, is “a ubiquitous phenomenon,“ they wrote, in findings published online May 25 in the research journal Nature Geoscience. They reviewed measurements from more than 500 seismic recording stations five hours before and after the quakes. The data came from the Incorporated Research Institutions for Seismology, a consortium of universities. The distant tremors differ from 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. An earthquake releases energy 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. The researchers found that magnitude-4 or smaller events often are triggered when either Love or Rayleigh waves from a major quake pass a given point. “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,“ said co-author Kris Pankow, a seismologist at the University of Utah. About 600 small seismic events occur worldwide 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. “Rayleigh and Love waves, two very different types of surface waves, are both able to trigger these events,“ said Pankow. The magnitude scale used by the researchers was the so-called Moment Magnitude scale, a newer measuring system than the older Richter scale, which however gives similar numerical results in most cases.