26 May 2008
Scientists Mull Earthquake Love Waves
by Kate Melville
A new study in the journal Nature Geoscience shows that large earthquakes routinely trigger smaller jolts worldwide, sometimes as far away as on the opposite side of the planet and in areas not normally prone to quakes. "Previously it was thought seismically active regions or geothermal areas were most vulnerable to large earthquake triggers," says Kris Pankow, a seismologist and a co-author of the new study. But Pankow and colleagues 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. The researchers contend that this "dynamic triggering" is a ubiquitous phenomenon.
Pankow, with seismologists Aaron Velasco and Tom Parsons, 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. The data included 15 major earthquakes from 1992 through 2006, including the 1992 Landers quake in California 800 miles southwest of Yellowstone, 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.
Pankow says that most of these major quakes triggered distant, smaller tremors. These are different than aftershocks, she said, which occur fairly close to the main quake. She cited the devastating 2004 Sumatra earthquake which triggered quakes in Ecuador, on the opposite side of the Earth.
To understand how earthquakes can trigger other quakes at such great distances, the researchers have been analyzing the different types of movement that constitute an earthquake. 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 of the 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.
Pankow 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. "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," she explained.
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.
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 added.
Source: University of Utah