Greatest Mysteries: What Happens Inside an Earthquake?
| Aerial view of the San Andreas fault slicing through the Carrizo Plain in the Temblor Range east of the city of San Luis Obispo. Credit: Robert E. Wallace, USGS |
Editor's Note: We asked several scientists from various fields what they thought were the greatest mysteries today, and then we added a few that were on our minds, too. This article is one of 15 in LiveScience's "Greatest Mysteries" series running each weekday.
When a sizable earthquake strikes, experts can explain exactly where it started and what type of fault is involved and maybe even predict how long aftershocks will last. But the strange truth is that seismologists and geophysicists are quite unsure of what happens inside the planet during a quake.
Earthquake physics has undergone a revolution during the past decade, thanks to new insights from lab experiments, field studies of exhumed faults and better theories.
But the nature and behavior of the forces that keep faults from moving and then suddenly fail are still unknown.
And when faults do move, something is missing—there is little to no evidence of the extremely high levels of friction and melting that would be expected to follow above ground when two giant rocks slid against each other.
"There are many reasons to believe that something exotic is happening," said Caltech geophysicist Tom Heaton.
"The problem of frictional sliding in earthquakes is one of the most fundamental problems in all of Earth science," Heaton said. "It has been a 30-year mystery story of figuring out the basic physics of the earthquake problem."
Gentle earthquakes
Most earthquakes happen where tectonic plates meet and glide against each other. Quakes occur when the frictional stress of the movement exceeds the strength of the rocks, causing a failure at a fault line. Violent displacement of the Earth's crust follows, leading to a release of elastic strain energy. This energy takes the form of shock waves that radiate and constitute an earthquake.
One of the strangest things about earthquakes is how gentle they are, Heaton said.
For instance, some scientists thought they had figured out how to simulate mini-earthquakes in the lab. But when they scale up the energies observed in the lab to the size of real faults, the model would predict extensive melting on faults. And such models predictdevastation far beyond what killed more than 500 people this week in Peru, more than 80,000 people in the 2005 Pakistan quake or more than a quarter of a million people in the 2004 earthquake off the coast of Sumatra.
"Earthquakes would be so violent that no living thing could survive the shaking," Heaton said.
Therefore, no one has actually simulated anything close to a real earthquake yet.
A machine design problem
The simulation problem lies partly in the fact that it's very difficult to make lab machines generate all the environmental conditions that occur miles below ground during an earthquake—including high stress, high pressure, elevated temperatures and a slip rate of about a yard per second (about the pace at which we walk).
David Goldsby and his rock mechanics colleagues at Brown University have designed machines that can apply the high stresses of temblors to rock specimens so the geophysicists can study friction at depth.
"We can apply normal stresses as high as occur throughout the entire seismogenic zone of the Earth's crust, about 10 kilometers [6 miles] in depth," he said.
That's incredibly impressive and important for earthquake science, but it still leaves a lot of questions unanswered, because what happens inside the Earth is so strange in magnitude and physics.
"No apparatus in the world is yet capable of meeting all of these criteria," Goldsby said.
Normal friction
Above ground, friction is a steady, stubborn force that opposes motion. Friction generates heat, as people with cold hands know, and increases with the stress you put on objects. So the heat on faults during sliding should increase with depth in the Earth. The rocks should definitely melt where they meet.
But underground, during earthquakes, two huge, hard, weight-pressurized rock slabs slip past or under each other. And nothing melts. Usually.
That's weird. It could be because the friction and thus the heat are much lower than you'd expect from rocks above ground, Goldsby said.
Earthquake friction works like this, Heaton said: It starts out high when there is little to no movement; then friction plummets to zero as the rocks move fast; then friction goes to high again when the rocks slow down.
That weird behavior of friction during an earthquake might be the reason there is little to no melting, Goldsby said. If friction is low when the rocks move fast, then much less heat is generated and no detectable melting occurs.
Maybe some other mechanism kicks in before the rocks get to their melting phase, Heaton said.
One explanation is "flash heating." Faults are stuck in place by very high forces. Once faults start sliding, if they slide fast enough, they become extra slippery at microscopic contact points, like skaters on ice. Heat is generated, but the result is a zero-friction, high-temperature cushiony flash of light or superheated gas called plasma that yields no detectable melted material, Heaton said. When the faults slow down, they stick tight again.
Another idea is that pressurized water in the rocks during a slip could decrease the stress on the fault and therefore the friction, Goldsby said. Faults might ride on a cushion of steam, allowing the fault to slide at low friction and the rock heat would not reach the melting point.
Ripple in the rug
The key to understanding earthquakes is actually not where they start but how the fracture spreads, and that has a lot to do with the weird behavior of underground friction, Heaton says.
The highest sliding velocities happen at the leading edge of a pulse of slip that runs through the Earth like a ripple in a rug, says Heaton, who described this fault behavior in a landmark paper 17 years ago.
Think of a fault as a rug that you want to move, he said. You can just pull the rug from the edge. That's the hard way to move it. The easy way to move a rug is to "put a little bump in it and move the bump and when you're done, you've offset the rug," he explained.
Friction is in a yin-yang arrangement with those slip-pulses, it turns out, Heaton said. "The slip in the pulse depends on the friction, but it turns out the friction turns on how fast the slip is happening," he said. "That's a math problem, a positive feedback system. They are notoriously unstable."
If you knew how big a pulse would be, you could predict an earthquake's magnitude, but the exotic behavior of friction underground botches all that up, Heaton said.
Still, the revolution in the field of earthquake physics has brought new insights, Goldsby said.
"I am not only hopeful but certain that we will learn even more about how earthquakes occur in the coming decade," he said. "This knowledge will help us understand how to mitigate the damaging effects of earthquakes and help prevent the loss of life, and may someday allow us to detect earthquake precursors."
Earthquake physics has undergone a revolution during the past decade, thanks to new insights from lab experiments, field studies of exhumed faults and better theories.
But the nature and behavior of the forces that keep faults from moving and then suddenly fail are still unknown.
And when faults do move, something is missing—there is little to no evidence of the extremely high levels of friction and melting that would be expected to follow above ground when two giant rocks slid against each other.
"There are many reasons to believe that something exotic is happening," said Caltech geophysicist Tom Heaton.
"The problem of frictional sliding in earthquakes is one of the most fundamental problems in all of Earth science," Heaton said. "It has been a 30-year mystery story of figuring out the basic physics of the earthquake problem."
Gentle earthquakes
Most earthquakes happen where tectonic plates meet and glide against each other. Quakes occur when the frictional stress of the movement exceeds the strength of the rocks, causing a failure at a fault line. Violent displacement of the Earth's crust follows, leading to a release of elastic strain energy. This energy takes the form of shock waves that radiate and constitute an earthquake.
One of the strangest things about earthquakes is how gentle they are, Heaton said.
For instance, some scientists thought they had figured out how to simulate mini-earthquakes in the lab. But when they scale up the energies observed in the lab to the size of real faults, the model would predict extensive melting on faults. And such models predictdevastation far beyond what killed more than 500 people this week in Peru, more than 80,000 people in the 2005 Pakistan quake or more than a quarter of a million people in the 2004 earthquake off the coast of Sumatra.
"Earthquakes would be so violent that no living thing could survive the shaking," Heaton said.
Therefore, no one has actually simulated anything close to a real earthquake yet.
A machine design problem
The simulation problem lies partly in the fact that it's very difficult to make lab machines generate all the environmental conditions that occur miles below ground during an earthquake—including high stress, high pressure, elevated temperatures and a slip rate of about a yard per second (about the pace at which we walk).
David Goldsby and his rock mechanics colleagues at Brown University have designed machines that can apply the high stresses of temblors to rock specimens so the geophysicists can study friction at depth.
"We can apply normal stresses as high as occur throughout the entire seismogenic zone of the Earth's crust, about 10 kilometers [6 miles] in depth," he said.
That's incredibly impressive and important for earthquake science, but it still leaves a lot of questions unanswered, because what happens inside the Earth is so strange in magnitude and physics.
"No apparatus in the world is yet capable of meeting all of these criteria," Goldsby said.
Normal friction
Above ground, friction is a steady, stubborn force that opposes motion. Friction generates heat, as people with cold hands know, and increases with the stress you put on objects. So the heat on faults during sliding should increase with depth in the Earth. The rocks should definitely melt where they meet.
But underground, during earthquakes, two huge, hard, weight-pressurized rock slabs slip past or under each other. And nothing melts. Usually.
That's weird. It could be because the friction and thus the heat are much lower than you'd expect from rocks above ground, Goldsby said.
Earthquake friction works like this, Heaton said: It starts out high when there is little to no movement; then friction plummets to zero as the rocks move fast; then friction goes to high again when the rocks slow down.
That weird behavior of friction during an earthquake might be the reason there is little to no melting, Goldsby said. If friction is low when the rocks move fast, then much less heat is generated and no detectable melting occurs.
Maybe some other mechanism kicks in before the rocks get to their melting phase, Heaton said.
One explanation is "flash heating." Faults are stuck in place by very high forces. Once faults start sliding, if they slide fast enough, they become extra slippery at microscopic contact points, like skaters on ice. Heat is generated, but the result is a zero-friction, high-temperature cushiony flash of light or superheated gas called plasma that yields no detectable melted material, Heaton said. When the faults slow down, they stick tight again.
Another idea is that pressurized water in the rocks during a slip could decrease the stress on the fault and therefore the friction, Goldsby said. Faults might ride on a cushion of steam, allowing the fault to slide at low friction and the rock heat would not reach the melting point.
Ripple in the rug
The key to understanding earthquakes is actually not where they start but how the fracture spreads, and that has a lot to do with the weird behavior of underground friction, Heaton says.
The highest sliding velocities happen at the leading edge of a pulse of slip that runs through the Earth like a ripple in a rug, says Heaton, who described this fault behavior in a landmark paper 17 years ago.
Think of a fault as a rug that you want to move, he said. You can just pull the rug from the edge. That's the hard way to move it. The easy way to move a rug is to "put a little bump in it and move the bump and when you're done, you've offset the rug," he explained.
Friction is in a yin-yang arrangement with those slip-pulses, it turns out, Heaton said. "The slip in the pulse depends on the friction, but it turns out the friction turns on how fast the slip is happening," he said. "That's a math problem, a positive feedback system. They are notoriously unstable."
If you knew how big a pulse would be, you could predict an earthquake's magnitude, but the exotic behavior of friction underground botches all that up, Heaton said.
Still, the revolution in the field of earthquake physics has brought new insights, Goldsby said.
"I am not only hopeful but certain that we will learn even more about how earthquakes occur in the coming decade," he said. "This knowledge will help us understand how to mitigate the damaging effects of earthquakes and help prevent the loss of life, and may someday allow us to detect earthquake precursors."
Atom Breaks Rules, Beats Friction
 |
 A molecule suddenly kicked into rapid rotation in a liquid rearranges the molecules around it, destroying its own friction.Credit: Stephen Bradforth, Richard Stratt, and Guohua Tao |
The molecule spins without causing friction [Video]. That shouldn't be possible, according to a chemical physics theory. The finding could alter the way scientists think about chemical reactions in liquids.
Researchers hit a drop of iodine cyanide and water with pulses from an ultraviolet laser, exciting one type of molecule to reconfigure into a small, peanut shape with a carbon atom on one end, a nitrogen atom on the other.
The molecule heated up to 8,000 degrees Fahrenheit (4,427 Celsius) and started spinning at a furious 270 trillion rotations per minute.
Outta my way
Within the first quarter-turn, the molecule created a shock wave that kicked away surrounding water molecules. The peanut molecule created a nearly frictionless zone for itself in the 10-trillionths of a second the reaction lasted.
"If you give it enough spin, it pushes all the guys around it away, and it builds itself a little bubble," said study coauthor Stephen Bradforth of the University of Southern California. "It's destroyed the friction in the liquid around it by completely reshaping its environment."
After the molecule completed about 10 rotations, the shock dwindled and the water molecules rushed back in.
Despite its fleeting nature, the reaction managed to smash the linear response theory, a chemistry model that states such a thing can't happen in a liquid environment.
"You can see molecules behave this way in gases, but not in liquids," said study coauthor Richard Stratt, a chemical theorist at Brown University.
Breaking other laws
The molecule's activity also runs against Newton's third law of motion, which states that for every action there is an equal, but opposite, reaction. In the new experiment, there water molecules are displaced, but they don't in turn do anything to the peanut molecule.
Friction is important in chemistry. Molecules rub, grind, and bang against each other they generate heat that speeds up reactions. Friction in gas reactions is reduced due to the relatively far distances between molecules, but the close proximity of molecules in liquid form makes friction nearly unavoidable.
Although the discovery has no immediate practical use, it changes the way scientists think about the 90 percent of all chemical reactions that take place in liquid, Bradforth said. One potential use could be to manipulate reactions by isolating molecules from their surroundings and reducing the production of useless byproducts.
"The main reason we're so excited by these results is that friction is how energy is shuttled around in chemical reactions," Stratt toldLiveScience. "If it doesn't operate or it operates differently than we always thought, that makes us wonder if there are entirely new ways we ought to thinking about how chemical reactions take place."
Greatest Mysteries: What Drives Evolution?
| Boobies were a species of bird that Charles Darwin found on the Galapagos Islands. Credit: |
From bizarre butterfly spots to rainbow-colored lizards to adaptations that allow squirrels and even snakes to "fly," physical innovations in the natural world can be mind-boggling.
Natural selection is accepted by scientists as the main engine driving the array of organisms and their complex features. But is evolution via natural selection the only explanation for complex organisms?
"I think one of the greatest mysteries in biology at the moment is whether natural selection is the only process capable of generating organismal complexity," said Massimo Pigliucci of the Department of Ecology and Evolution at Stony Brook University in New York, "or whether there are other properties of matter that also come into play. I suspect the latter will turn out to be true."
Flexible genes
Some scientists are proposing additions to the list of evolutionary forces.
"Over the past decade or two, scientists have begun to suspect that there are other properties of complex systems (such as living organisms) that may help, together with natural selection, explain how things such as eyes, bacterial flagella, wings and turtle shells evolve," Pigliucci told LiveScience.
One idea is that organisms are equipped with the flexibility to change their physical or other features during development to accommodate environmental changes, a phenomenon called phenotypic plasticity.
The change typically doesn't show up in the genes. For instance, in social bees, both the workers and guards have the same genomes but different genes get activated to give them distinct behaviors and appearances. Environmental factors, such as temperature and embryonic diet, prompt genetic activity that ends up casting one bee a worker and the other a guard.
If beneficial, this flexibility could be passed on to offspring and so can lead to the evolution of new features in a species. "This plasticity is heritable, and natural selection can favor different kinds of plasticity, depending on the range of environmental conditions the organism encounters," Pigliucci said.
Made to order
Self-organization is another evolutionary force that some experts say whips up complex features or behaviors spontaneously in living and non-living matter, and these traits are passed on to offspring through the generations.
"A classic example outside of biology are hurricanes: These are not random air movements at all, but highly organized atmospheric structures that arise spontaneously given the appropriate environmental conditions," Pigliucci said. "There is increasing evidence that living organisms generate some of their complexity during development in an analogous manner."
A biological illustration of self-organization is protein-folding. A lengthy necklace of amino acids bends, twists and folds into a three-dimensional protein, whose shape determines the protein's function. A protein made up of just 100 amino acids could take on an endless number (billions upon billions) of shapes. While this shape-shifting takes on the order of seconds to minutes in nature, the fastest computers don't have the muscle yet to pull off the feat.
The mechanism that triggers the final form could be a chemical signal, for instance.
Novelties in nature
The environment also could drive changes in an animal's appearance or phenotype, a phenomenon that intrigues many biologists.
For instance, Sean Carroll, a molecular biologist at the University of Wisconsin-Madison, discovered butterflies in East Africa have different colorings depending on when they hatch. Those hatching during the wet season emerge with brightly colored eyespots while their dry-season relatives wear neutral cryptic coats.
Biology has a pretty good understanding of how animals develop from a fertilized egg to a fully formed organism.
"We just don't understand how … the environment and [the] genetic blueprint interact during development," said Theunis Piersma of the Center for Ecological and Evolutionary Studies at the University of Groningen in the Netherlands.
Piersma's research on shorebirds called red knots has revealed the birds can morph their phenotypes depending on their migration routes.
When brought into captivity and placed in colder temperature environments, the shorebirds' flight muscles and organs shrink to reduce heat loss. The birds pass on to offspring the capacity to make these changes.
So the mystery is starting to clear around how diverse species with an array of features evolve. The field, which had relied in the past mostly on fossil records, got a boost with the development of genetic techniques and the integration of diverse sectors of science, connecting genetics, biology, ecology and computer science.
While scientists are shedding light on natural mechanisms that work to shape species, many questions in the field are brewing on the lab-bench. And the original question examined by Charles Darwin—what is the mechanism that causes new species to evolve—has yet to be fully explained. And another related question looms: How important are chance events, as opposed to natural selection, to shaping organisms?
Natural Disasters: Top 10 U.S. Threats
Why the China Quake Was So Devastating
Tsunami Earthquake Three Times Larger Than First Thought
Deadliest Earthquakes in History
San Andreas Fault Longer Than Thought
The Worst Natural Disasters Ever'http://www.livescience.com/9794-worst-natural-disasters.html
What's the Deadliest Natural Phenomenon?
| Drought and flooding are the most deadly natural phenomenon. Credit: sxc.hu |
Hurricanes, tornadoes, and earthquakes might seem like the most dangerous natural hazards you could ever face, but floods and droughts actually kill more Americans over time.
Better predictions for hurricanes and other tropical cyclones, as well as tornadoes, have reduced the death tolls from such events in recent decades. But flooding deaths are on the rise.
On average, U.S. flooding kills more than 100 people a year — more than any other single weather hazard, including tornadoes and hurricanes, according to the University Corporation for Atmospheric Research (UCAR). Most flood deaths are from flash floods, however, and about half of those are because people try to cross swollen streams or flooded roads. Victims often underestimate the power of water when driving into flooded areas, UCAR scientists note, adding that it takes only 18 inches of water to float a typical vehicle.
Flooding deaths have risen in recent decades, and the U.S. Congress's Office of Technology Assessment says that "despite recent efforts, vulnerability to flood damages is likely to continue to grow" because populations in flood-prone regions continue to grow.
Heat waves rarely make lists of the deadliest natural disasters, but in modern times their death tolls have surpassed other phenomena in the United States.
In both 1980 and 1988, for example, severe drought and heat ravaged the central and eastern parts of the country. Estimated deaths due to heat stress approaching 10,000 in each case and the economic toll each time reached tens of billions of dollars.
Over half of all deaths from natural disasters worldwide are due to drought and famine, according to the International Federation of Red Cross and Red Crescent Societies. Droughts can decrease the availability of potable water and can ruin crops, making food scarce.
Droughts and floods could take a higher toll in the future as global warming increases the prevalence of these events in certain areas, scientists say.
Voice of Reason: The Myth of Tsunami Survivors' Sixth Sense
Shortly after the Dec. 26, 2004 tsunami tragedy, stories and news reports appeared making claims that animals and aboriginal tribes had escaped the danger because they possessed a mysterious "sixth sense" that somehow warned them in time. For example: "no dead animals have been found as a result of the tsunami, confiming animals' sixth sense" and "no one has found dead animals in the aftermath of the earthquake and tsunami" (Note 1).
These reports are simply incorrect. Many news and eyewitness accounts described dead animals among the debris and carnage. The Washington Post, for example, reported, "In the coastal town of Velanganni...volunteers wearing face masks drove around in trucks Tuesday, picking up cattle carcasses..." (Note 2)
So dead animals were found--but were they found in fewer numbers than some might have expected? To begin with, it's unclear how many animals would be expected to have been killed by the tsunami, since coastal regions don't necessarily have a high concentration of large animals to begin with.
But for the sake of argument, let's say that fewer animals were killed than some might have expected. Is this evidence of a paranormal "sixth sense?"
The fact that animals often have keener senses than humans is obvious and well-documented. Dogs have a remarkable sense of smell, birds can migrate using celestial cues, and bats can locate food with echoes. (Though animals' other senses are often worse than those of humans -- elephants have very poor distance vision, for example.) Animals may sense unusual vibrations or changes in air pressure coming from one direction that suggests they should move in the opposite direction. Let's say a herd of animals are seen fleeing to higher ground. All that is needed is for one or two of them to skittishly sense danger and head out; the rest will follow, not necessarily due to some preternatural sense but simple herd instinct.
Similarly, members of primitive tribes who inhabit the Andaman and Nicobar Islands are said to have been forewarned: "They can smell the wind. They can gauge the depth of the sea with the sound of their oars. They have a sixth sense which we don't possess" (Note 3). (The "sixth sense" label is a bit of a myth anyway, since the human body has other senses, such as balance and pressure, that are not counted in the usual five.)
As with the animals, one needn't posit a mysterious sixth sense; the other five are more than capable of warning about impending danger. The fact that an ancient seafaring people (as opposed to, say, German tourists on holiday) might recognize the signs of an impending tsunami is hardly surprising. In the aftermath of the tragedy, many miraculous stories of courage and survival emerged, but tales of supernatural abilities aren't among them.
Tsunami Special Report
The earthquake and tsunami of Dec. 26, 2004 combined to create a disaster in Asia that is among the worst in recorded history. Though significant tsunamis are rare, it was not the first event of its kind.
Tsunamis have proven incredibly deadly in the past. In 1896 a tsunami caused 27,000 deaths in Japan. A combination of earthquake, fire and tsunami centered around Lisbon, Portugal killed 60,000 people in 1755.
Just two weeks before the Asian catastrophe, scientists revealed the threat of a megatsunami in the Pacific Ocean that could be triggered by a subsea landslide. In recent days, other researchers warned of the potential for a 9.0-magnitude earthquake and tsunami that could hit the West Coast of the United States. There is a warning system in the Pacific, and now officials in India plan to set one up. Also, the United States now is planning to set up a global tsunami warning system
Tsunamis from near-shore earthquakes can begin striking a coast within minutes, however.
For a tsunami triggered by an earthquake in the Cascadia fault zone off the U.S. West Coast, some residents would have about 15 minutes to get to high ground, says Robert Yeats, professor emeritus of geosciences at Oregon State University.
Should you worry? While coastal residents ought to be aware of the threat, experts say, there are many other things to worry about. Heart disease, accidents, war, famine, disease and shootings are far more likely killers than anything Nature dishes out.
Mystery Behind Monday's 'Great' Earthquake
Monday's 8.7-magnitude earthquake off the coast of Sumatra apparently did not generate a significant tsunami, despite originating in the same area as the Dec. 26 earthquake that unleashed towering killer waves across the Indian Ocean.
The reason remains a mystery for now.
|
