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A 3-D rendering of the tectonic plates (multicolored regions) in northern South America (coastline shown in yellow) shows the underlying Bucaramanga Nest, which experiences more intermediate-depth earthquakes than any place in the world. Image: German Prieto



"Intermediate-depth earthquakes occur at depths of about 30 miles down to about 190 miles," said Greg Beroza, a prof. of geophysics at Stanford and a co-author of a new study that will be published in Geophysical Research Letters.

Unlike shallow earthquakes, the cause of intermediate quakes is not well understood. Part of the problem 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."


"They represent 25% of the catalog of earthquakes, and some of them are large enough to produce damage and deaths," said study first author Germán Prieto, an asst. prof. of geophysics at the Massachusetts Institute of Technology.

A tale of two theories
There are two main hypotheses for what may be driving intermediate-depth earthquakes. According to one idea, water is squeezed out of rock pores at extreme depths and the liquid acts like a lubricant to facilitate fault sliding. This fits 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.


"It's a runaway process in which the increasing heat generates more slip, and more slip generates more heat and so on," Prieto said.


But about once a month one occurs that is magnitude 5 or greater—strong enough to be felt by the city's residents. Moreover, past studies have revealed that most of the quakes appear to be concentrated at a site located about 90 miles beneath the Earth's surface that scientists call the Bucaramanga Nest.

A natural laboratory
This type of clustering is highly unusual and makes the Bucaramanga Nest a "natural laboratory" for studying intermediate depth earthquakes. Comparison studies of intermediate quakes from different parts of the world are difficult because the makeup of the Earth's crust and mantle can vary widely by location.

In the Bucaramanga Nest, however, 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. This vastly simplifies calculations, Beroza said.

"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."

By studying seismic waves picked up by digital seismometers installed on the Earth's surface above the Bucaramanga Nest, the scientists were able to measure two key parameters of the intermediate quakes happening deep underground.



"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.


"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 study coauthor Sarah Barrett, a Stanford graduate student in Beroza's research group.

Source: Stanford Univ.