December 12, 2013
One In Four Earthquakes Worldwide Come From Deep Within Earth
April Flowers for redOrbit.com - Your Universe Online
The long standing mystery of what drives a particular type of earthquake that occurs deep within the Earth and accounts for one in four quakes worldwide might have been solved by a team led by Stanford researchers. Their findings are published in an upcoming issue of Geophysical Research Letters.
Originating farther inside the planet than shallow earthquakes, these temblors are known as intermediate-depth earthquakes. Shallow earthquakes, which take place in the uppermost layer of the Earth's surface called the crust, account for the types of earthquakes that affect California and most other places in the world.
"Intermediate-depth earthquakes occur at depths of about 30 miles down to about 190 miles," said Greg Beroza, a professor of geophysics at Stanford.
The causes of shallow earthquakes are well understood, unlike intermediate-depth earthquakes. Part of the challenge to understanding intermediate-depth is that the mechanism for shallow earthquakes should not physically work for quakes at greater depths.
"Shallow earthquakes occur when stress building up at faults overcomes friction, resulting in sudden slip and energy release," Beroza said. "That mechanism shouldn't work at the higher pressures and temperatures at which intermediate depth earthquakes occur."
Understanding the mechanisms behind intermediate-depth quakes could help scientists forecast where they will occur and the possible risks to buildings and people.
"They represent 25 percent of the catalog of earthquakes, and some of them are large enough to produce damage and deaths," said Germán Prieto, an assistant professor of geophysics at the Massachusetts Institute of Technology.
Scientists have proposed two main theories for what may be driving intermediate depth earthquakes. The first theory proposes that water is squeezed out of rock pores at extreme depths. The liquid then acts like a lubricant to facilitate fault sliding. This theory agrees with the finding that intermediate quakes generally occur at sites where one tectonic plate is sliding, or subducting, beneath another.
"Typically, subduction involves oceanic plates whose rocks contain lots of water," Beroza said.
The second theory posits that as rocks at extreme depths deform, they generate heat due to friction. These heated rocks become more malleable, or plastic, sliding more easily against each other. This creates a positive feedback loop, weakening the rock further and increasing the likelihood of fault slippage.
"It's a runaway process in which the increasing heat generates more slip, and more slip generates more heat and so on," Prieto said.
To test both theories, the researchers studied a site near the city of Bucaramanga, Columbia, which boasts the highest concentration of intermediate quakes in the world. Approximately 18 intermediate-depth temblors rattle Bucaramanga every day. The majority of these are magnitude 2 to 3, weak earthquakes that are detectable only by sensitive instruments.
However, about once a month a magnitude 5 or greater earthquake occurs, strong enough to be felt by the city's residents. Prior studies have demonstrated that most of the quakes appear to be concentrated at a site located about 90 miles beneath the Earth's surface. Scientists call this site the Bucaramanga Nest.
The clustering of earthquakes in this region is highly unusual and makes the Bucaramanga Nest a "natural laboratory" for studying intermediate-depth earthquakes. Because the makeup of the Earth's crust and mantle can vary widely by location, comparison studies of intermediate quakes from different parts of the world are difficult.
However, in the Bucaramanga Nest, the intermediate quakes are so closely packed together that for the purposes of scientific studies and computer models, it's as if they all occurred at the same spot. Beroza said that this vastly simplifies calculations.
"When comparing a magnitude 2 and a magnitude 5 intermediate depth earthquake that are far apart, you have to model everything, including differences in the makeup of the Earth's surface," he said. "But if they're close together, you can assume that the seismic waves of both quakes suffered the same distortions as they traveled toward the Earth's surface."
The research team was able to measure two key parameters of the intermediate quakes happening deep underground by investigating seismic waves picked up by digital seismometers installed on the Earth's surface above the Bucaramanga Nest.
One of the parameters is called the stress drop. This parameter allowed the team to estimate the total amount of energy released during the fault slips that caused the earthquakes. The second parameter, radiated energy, is a measure of how much of the energy generated by the fault slip is actually converted to seismic waves that propagate through the Earth to shake the surface.
The researchers immediately noticed two things. First, the stress drop for intermediate quakes increased along with their magnitudes — meaning larger intermediate quakes released proportionally more total energy than smaller ones. The second thing they noticed was that the amount of radiated energy released by intermediate earthquakes accounted for only a tiny portion of the total energy as calculated by the stress drop.
"For these intermediate-depth earthquakes in Colombia, the amount of energy converted to seismic waves is only a small fraction of the total energy," Beroza said.
The researchers suggest that this means that intermediate earthquakes are expending most of their energy locally, likely in the form of heat.
"This is compelling evidence for a thermal runaway failure mechanism for intermediate earthquakes, in which a slipping fault generates heat. That allows for more slip and even more heat, and a positive feedback loop is created," said Sarah Barrett, a Stanford graduate student in Beroza's research group.