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Big Quakes Trigger Small Quakes

Posted by alec8c on November 1, 2010


Seismologists Find Large Earthquakes Can Trigger Smaller Ones In Unlikely Locations

December 1, 2008 ā€” Seismologists found that L-waves, slow-moving seismic surface waves, from larger earthquakes can trigger smaller earthquakes as they travel through the ground. Different from aftershocks because they can happen farther away and hours or days later, the triggered tremors rate much lower on the magnitude scale–often undetectable without a seismograph.

An earthquake in Alaska could trigger one near you, even if you’re not in an earthquake-prone area, new research shows. Seismologists are now finding earthquakes in some unexpected places.

City Hall in Park City, Utah, is undergoing a $10 million seismic update. Park City is near the Wasatch Fault, an area overdue for an earthquake, so leaders have been concerned about earthquake-proofing the building for years. “It’s something we absolutely expect,” said Ron Ivie, a building official in Park City. “The question is, ‘What day?'”

While earthquakes along fault lines are expected, seismologist Kris Pankow and her research team recently found slow-moving seismic surface waves, or L waves, from large earthquakes travel along the ground and trigger smaller earthquakes as they go.

“It’s sort of like if a tree falls in the forest, does anyone hear it?” said Pankow, assistant director of the University of Utah seismic stations in Salt Lake City. “The same question was here: If the seismic waves go by everywhere, do they generate earthquakes everywhere?”

Unlike the earthquake risk in Park City, Pankow says the risk of these smaller earthquakes is minimal. The team tracked 15 large earthquakes and found 12 of them actually triggered smaller jolts. These are different than aftershocks because they happen miles away and sometimes hours or days later.

The team also found earthquakes in unlikely places like Canada, Australia and western Africa.

Pankow doesn’t want to alarm anyone. She says the triggered earthquakes that have been observed have been small. Without a seismograph, you may not even notice them.

Seismologists still don’t completely understand why earthquakes happen. Pankow and her team hope their work with these dynamically triggered earthquakes will help lead to an answer.

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WHAT CAUSES EARTHQUAKES? An earthquake is the result of a sudden release of stored energy in the Earth’s crust triggered by shifting tectonic plates. The Earth’s lithosphere is an elaborate network of interconnected plates that move constantly — far too slow for us to be aware of them, but moving, nonetheless. Occasionally they lock up at the boundaries, and this creates frictional stress. When that strain becomes too large, the rocks give way and break and slide along fault lines. This can give rise to a violent displacement of the Earth’s crust, which we feel as vibrations or tremors as the pent-up energy is released. However, only 10% or so of the total energy is released in the seismic waves. However, the rest is converted into heat, used to crush and deform rock, or released as friction.

HOW DO SCIENTISTS RATE EARTHQUAKES? An earthquake’s magnitude describes how much the ground moves. The scale is logarithmic, which means that when the magnitude increases by one (say from 3 to 4, or from 4 to 5) the amount of ground motion increases by ten times. That is, a magnitude 3 quake leads to ten times as much ground motion as a magnitude 2 quake, and a magnitude 2 leads to ten times as much motion as a magnitude 1. This means that a magnitude 3 is a hundred times as violent as a magnitude 1, and a hundred times less violent than a magnitude 5.

The magnitude scale also tells us just how much energy an earthquake released. For example, a magnitude 1 earthquake releases the same amount of energy as 30 pounds of TNT exploding. Although a magnitude 2 earthquake makes the ground move ten times as much as a magnitude 1, it releases 32 times as much energy — or roughly as much as a ton of TNT. A magnitude 5 earthquake packs the punch of a moderate nuclear weapon, and a magnitude 12 quake would be enough to put a crack all the way through the center of the Earth.

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