Scientists find the sounds beneath our feet are fingerprints of rock stability

With a keen ear, one may detect a cacophony of booms and crackles as one descends into the Earth's crust. When squeezed and under stress, the fissures, pores, and flaws that run through rocks vibrate like strings. Furthermore, the cadence and rhythm of these noises can reveal information about the strength and depth of the nearby rocks, as discovered by a group of geologists at MIT.

According to MIT scientist Matęj Peč, "the deeper you go, the higher and higher pitches the rocks would be singing."

Peč and his associates are listening to rocks in order to detect any acoustic patterns, sometimes known as "fingerprints," that may appear under different conditions. Experimental investigations have now demonstrated that marble samples at low pressure produce low-pitched "booms," and under high pressure, the rocks produce a 'avalanche' of higher-pitched crackles.

Peč claims that by using these acoustic patterns in rocks, scientists may determine the kinds of fissures, cracks, and other flaws that the Earth's crust encounters at different depths. This information can then be used to detect unstable areas under the surface that may experience earthquakes or volcanic eruptions. The team's findings, which were released today in the Proceedings of the National Academy of Sciences, may also provide guidance for surveyors looking to drill for geothermal energy that is renewable.

Assistant professor Peč of MIT's Department of Earth, Atmospheric and Planetary Sciences (EAPS) says, "If we want to tap these very hot geothermal sources, we will have to learn how to drill into rocks that are in this mixed-mode condition, where they are not purely brittle, but also flow a bit." "But overall, this is fundamental science that can help us understand where the lithosphere is strongest."

Peč's MIT partners include graduate student Hilary Chang, professor emeritus of geophysics Brian Evans, lead author and research scientist Hoagy O. Ghaffari, and technical associate Ulrich Mok. Co-author and former EAPS postdoc Tushar Mittal is an assistant professor at Penn State.

Breaking and Flowing

It's common to compare the Earth's crust to an apple's skin. The crust can reach a depth of 70 kilometers at its thickest, which is a very small portion of the globe's 12,700-kilometer circumference. Nevertheless, the strength and stability of the rocks that comprise the planet's thin peel vary widely. Geologists deduce that rocks closer to the surface are more readily fractured and brittle than rocks down below the surface, where extreme pressure and heat from the core can cause rocks to flow.

Rocks must exist in an intermediate phase when they go from being brittle at the surface to being more ductile at deep. During this phase, rocks may possess characteristics of both the granite and honey phases, such as the ability to fracture like granite and flow like honey. Geologists think that this "brittle-to-ductile transition" may represent the place in the crust where rocks are at their strongest, but this is not fully understood.

"This transition state of partly flowing, partly fracturing, is really important, because that's where we think the peak of the lithosphere's strength is and where the largest earthquakes nucleate," Peč explains. "But we don't have a good handle on this type of mixed-mode behavior."

He and his colleagues are investigating the relationship between a rock's tiny flaws and its strength and stability, determining whether the rock is brittle, ductile, or something in between. The brittleness or ductility of a rock can be influenced by the size, density, and distribution of flaws such as tiny cracks, fissures, and pores.

However, it is not an easy undertaking to measure the minute flaws in rocks under settings that mimic the different pressures and depths found on Earth. For example, scientists are unable to map the minute flaws in rocks by using optical imaging techniques. In order to learn more about the pattern of the tiny cracks and fissures in the rock, the researchers resorted to ultrasonic imaging and the theory that sound waves passing through it should bounce, vibrate, and reflect off of it in certain ways.

When under stress, all of these faults will also produce their own noises, so they should be able to learn a lot by actively sounding through the rock as well as listening to it. They discovered that the concept need to function with megahertz frequencies of ultrasonic waves.

"This kind of ultrasound method is analogous to what seismologists do in nature, but at much higher frequencies," Peč explains. "This helps us to understand the physics that occur at microscopic scales, during the deformation of these rocks."

A difficult position for a rock

The group tested Carrara marble cylinders in their tests.

"It's the same material as what Michaelangelo's David is made from," Peč observes. "It's a very well-characterized material, and we know exactly what it should be doing."

The group set each marble cylinder within a vice-like device composed of steel, zirconium, and aluminum pistons, which when combined may produce extremely high tensions. After submerging the vice in a pressurized chamber, they applied pressures to each cylinder that are comparable to those that rocks in the Earth's crust encounter.

The scientists recorded the acoustic pattern that emerged from the bottom of the sample while they gradually crushed each rock by applying ultrasonic pulses through its top. The sensors were monitoring for any naturally occurring acoustic emissions while they weren't pulsating.

It was discovered that the marble did indeed break suddenly at the lower end of the pressure range, where rocks are fragile, and the sound waves looked like big, low-frequency booms. The acoustic waves at the greatest pressures resembled a higher-pitched crackling, consistent with rocks being more ductile. According to the scientists, tiny flaws known as dislocations caused this crackling, which subsequently multiplied and moved like an avalanche.

"For the first time, we have recorded the 'noises' that rocks make when they are deformed across this brittle-to-ductile transition, and we link these noises to the individual microscopic defects that cause them," Peč explains. It was discovered that as these flaws traverse this transition, they drastically alter in size and propagation velocity. It's more intricate than previously believed."

Scientists can predict how the Earth's crust would behave at different depths by using the team's characterizations of rocks and their flaws at different pressures. For example, they can predict how rocks could fracture during an earthquake or flow during an eruption.

How does the partial fracture and partial flow of rocks contribute to the cycle of earthquakes? Furthermore, what impact does it have on the flow of magma through a network of rocks?" Peđ states. "Those are larger scale questions that can be tackled with research like this."