That observation inspired a University of Utah seismologist and colleagues to marshal and crunch old data from around the world to see whether the number of earthquakes rises in other places following major seismic events. Not only does that happen, but it happens routinely, even in places not normally associated with seismic activity, according to a study published Monday in Nature Geoscience.
"Most large events are triggering earthquakes and they are triggering them worldwide," said co-author Kris Pankow, an assistant research professor at the U.'s seismograph stations. "The only place where we could prove triggering in the past was in seismically active places, like Utah or Yellowstone. This study shows that even in areas that aren't active, like Australia, we are seeing small earthquakes being triggered."
Following the magnitude 7.9 Denali event on Nov. 3, 2002, Pankow noticed Utah stations detecting higher-than-usual earth movements.
"Yellowstone went crazy. The number of quakes was just phenomenal. We were just watching the coils go off," she said. Pankow documented 65 earthquakes in Utah in the first 24 hours following Denali. It took 25 days before Utah's earthquake rate subsided to background levels, according to a 2004 study U. seismologists published in the Bulletin of the Seismological Society of America. The new study builds on this Utah work by taking a global look at the effects of massive quakes, the magnitude 7 and greater events that occur only once or twice a year.
Researchers examined all 15 of the world1s major earthquakes between 1992 and 2006 and discovered that all but three yielded statistically significant correlations with subsequent upsurges of small quakes.
They gathered data from 500 seismic monitoring stations around the world, looking at seismic activity five hours before and after each of the 15 big events. Researchers chose the five-hour windows because that's about as long as it takes for a major earthquake's shock waves to travel around the globe once or twice. The study did not count jolts within 1,110 kilometers (10 degrees of latitude) of the epicenter of major quakes to exclude aftershocks.
Researchers looked for increases in the numbers of small earthquakes, less than magnitude 4. The earth experiences 600 of these every five minutes, on average. Many of the stations reported more than double the incidence of earthquake activity, but the timing of these upsurges gives tantalizing clues to how shock waves trigger distant earthquakes, said lead author Aaron Velasco of the University of Texas-El Paso.
"We just did the tip of the iceberg. We1re going to look at these events closely to study the exact stresses that answer the question of the mechanics of triggering," he said.
Earthquakes release energy in the form of ground waves, followed by surface pulses known as Rayleigh waves, which move in an elliptical fashion, and Love waves, which shear. Rayleigh waves move more slowly than Love waves, around 10 times the speed of sound. The research demonstrated that the incidence of small quakes shot up when big quakes' surface waves passed a given point. For example, the rates of earthquakes increased 37 percent above background levels when the Love waves passed, then spiked 60 percent as the Rayleigh waves passed.
"The physical [triggering] mechanism is not known," Pankow said. "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."
Her findings are expected to open important new lines of inquiry into how earthquakes are caused.
"The surface waves are carrying most of the energy. They are having greater effect than the body waves traveling through the interior of the earth," said Walter Arabasz, director of the U.'s seismographic stations. "Our general assumption has been that earthquakes occur randomly. If you are challenged to find a special pattern you have to have the tools to make the observations and sort out what1s non-random and what1s related to the passage of waves from distant earthquakes. "This study is prodding people to look harder at the kinds of mechanisms related to earthquake triggering."
Seismologists believe major quakes routinely trigger smaller earthquakes all over the world.
A new study co-authored by University of Utah seismologist Kris Pankow reviews seismic data generated around the world during the five hours before and after each of the world's 15 major quakes between 1992 and 2006. In most cases, the incidence of small earthquakes spiked following these big events. Even more interesting, spikes occurred at seismic stations with the arrival of the major quakes' surface waves.
The earthquakes studied were: 1992 Landers, Calif. (7.3); 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); 2002 Denali (7.9); 2003 Hokkaido, Japan (8.3); 2003 Siberia, Russia (7.3); 2004 Macquarie Ridge, near New Zealand (8.1); 2004 Sumatra-Andaman Islands (9.2); 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 not show triggering events.