Big earthquakes trigger tremors worldwide

U.S. researchers who have studied 15 earthquakes with magnitudes greater than 7 since 1990 have found that at least 12 of them triggered smaller quakes thousands of miles away. Their study shows that large earthquakes routinely spark jolts worldwide, 'including on the opposite side of the planet and in areas not prone to quakes.' To reach this conclusion, the scientists 'analyzed data from more than 500 seismic recording stations five hours before and five hours after earthquakes.' But read more...

U.S. researchers who have studied 15 earthquakes with magnitudes greater than 7 since 1990 have found that at least 12 of them triggered smaller quakes thousands of miles away. Their study shows that large earthquakes routinely spark jolts worldwide, 'including on the opposite side of the planet and in areas not prone to quakes.' To reach this conclusion, the scientists 'analyzed data from more than 500 seismic recording stations five hours before and five hours after earthquakes.' But read more...

World map of seismic stations

As an example, you can see above a map of the world showing "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. [...] Scientists once believed big quakes could not trigger distant tremors." (Credits: University of Utah for the caption; Aaron Velasco, University of Texas at El Paso, for the map) Here is a link to a a larger version of this map.

Kristine (Kris) Pankow, assistant director of the University of Utah Seismograph Stations conducted the study with seismologist Aaron Velasco and undergraduate student Stephen Hernández, both at the University of Texas at El Paso and seismologist Tom Parsons, of U.S. Geological Survey in Menlo Park, California.

IRIS's Global Seismographic Network

The researchers obtained their data from the Incorporated Research Institutions for Seismology (IRIS), a consortium of universities. You can see above a redimensioned poster of the Global Seismographic Network maintained by IRIS. "The GSN consists of more than 125 GSN stations located around the world with near-uniform spacing -- from the South Pole to Siberia, and from the Amazon Basin to the seafloor of the Northeast Pacific Ocean. This multi-use facility provides data for scientific research, education, earthquake hazard mitigation, tsunami warning, and the international monitoring system for the Comprehensive Nuclear Test-Ban Treaty. (Credit: IRIS) Here is a link to a larger version of this poster.

Now, let's see what the University of Utah says about earthquakes. "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."

And here is what the researchers have found. "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. 'It is interesting that Rayleigh and Love waves, two very different types of surface waves, are both able to trigger these events,' says Pankow.

This research work has been accepted by Nature Geoscience and is available online as an advance online publication under the title "Global ubiquity of dynamic earthquake triggering" (May 25, 2008). Here is an excerpt from the abstract. "Here, we analyse broadband seismograms from over 500 globally distributed stations and use an automated approach to systematically identify small triggered earthquakes -- the low-amplitude signals of such earthquakes would normally be masked by high-amplitude surface waves. Our analysis reveals that out of 15 earthquakes studied of magnitude (M) greater than 7.0 that occurred after 1990, 12 are associated with significant increases in the detection of smaller earthquakes during the passage of both the Love and Rayleigh waves. We conclude that dynamic triggering is a ubiquitous phenomenon that is independent of the tectonic environment of the main earthquake or the triggered event."

Here are some other links if you want to learn more about about this study.

Still, how do the surface waves trigger small earthquakes at distant locations? Here is one possible explanation. "'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.'"

Sources: University of Utah news release, May 25, 2008; and various websites

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