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Final straws

Big earthquakes can trigger smaller ones on the other side of the planet, scientists now say

Geoscientist Online Sunday 25 May 2008

It seems like common sense; a big earthquake should surely trigger others. Think about it – if a fault has accumulated a lot of strain, which it is on the verge of releasing, then surely the nudge provided by a big tremor somewhere else on the globe could be enough to set it off? This was always the big journalistic question in the days of nuclear tests; or when two natural earthquakes happened in quick succession, but on different parts of the globe. And yet, until 1992 when California’s magnitude-7.3 Landers earthquake set off small jolts as far away as Yellowstone National Park, experts would either refuse to be drawn, or would even flatly deny that large earthquakes could set off tremors at distant locations.  To be fair, data were lacking on those far-off days.  Now, however they are not.

Now, a definitive study shows large earthquakes routinely trigger smaller jolts worldwide, including on the opposite side of the planet and in areas not prone to quakes. Kris Pankow, a seismologist at the University of Utah Seismograph Stations and a co-author of the new study, says: “Previously it was thought seismically active regions or geothermal areas were most vulnerable to large earthquake triggers”. But Pankow and colleagues analysed 15 major earthquakes (stronger than magnitude-7.0) that had happened since 1992 - and found that at least 12 of them triggered small quakes even thousands of miles away. The findings are published online yesterday, Sunday 25 May 2008 in the journal Nature Geoscience. The authors conclude that “dynamic triggering is a ubiquitous phenomenon”. 

Pankow conducted the study with 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, California, USA. Data in the study came from more than 500 seismic recording stations and was taken five hours before and five hours after earthquakes that registered more than 7.0 on the moment magnitude. These included 15 major earthquakes from 1992 through 2006, including the 1992 Landers quake in California 800 miles southwest of Yellowstone, the 2002 magnitude-7.9 Denali fault quake in Alaska, and the magnitude-9.2 Sumatra-Andaman Islands quake (Boxing Day 2004) that generated the catastrophic Indian Ocean tsunami. 

Seismic stations that detected more than twice the normal number of small, nearby earthquakes after the passage of surface waves from major quakes centred up to thousands of miles away 1992 - 2006. Map: Aaron Velasco, University of Texas at El Paso.

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 analyse systematically all the world’s big quakes in this period and found that most of them triggered distant, smaller tremors. These are different from “aftershocks”, which occur fairly close to the main quake. The 2004 Sumatra earthquake triggered quakes in Ecuador, on the opposite side of the Earth. 

Making waves

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. The next waves to arrive 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. “We can recognise 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” says Pankow. 

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% 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% during the five hours following a 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 Kunlun, 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) and 2006 Kuril Islands, Russia (8.3). 

Only the Hector Mine, Siberia, and Kuril Islands quakes did seem to have triggered other quakes. But it is known from previous studies that the Hector Mine earthquake indeed triggered smaller quakes near California’s Salton Sea. Those were not included in the study, because they were within about 680 miles of the main shock’s epicentre. Researchers excluded triggered quakes within that distance to avoid counting aftershocks. 

So how do they do it? “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 adds. 

The study was funded by the United States Geological Survey and the National Science Foundation.