Sounds of Earth’s Crust: Acoustic Patterns Beneath Our Feet Are Fingerprints of Rock Stability

The crust is the hard, outermost layer of the Earth, comprising less than 1% of the planet's volume. It includes various rocks, holes, cracks, and faults that resonate when pressed or stressed. A team of MIT researchers discovered that the rhythm and pace of these sounds can provide insight into the depth and strength of the rocks around us.

Thin Layer With Mixed Mode Behavior

The crust of the Earth can be compared to the skin of an apple. It can be 43 miles (70 kilometers) deep at its thickest point, just a tiny fraction of the planet's total diameter of 7891 miles (12,700 kilometers). Despite this thinness, the rocks that make up the crust vary greatly in their strength and stability.

Geologists assume that rocks near the surface are brittle and fracture easily, while those at greater depths are subject to immense pressures and heat that make the rocks flow. This implies that there must be something in-between, a phase where rocks transition from one to the other with both abilities to fracture and flow.

The "brittle-to-ductile transition" is not well understood. However, geologists believe it could be the point where rocks are strongest within the crust. This transition state is also crucial because it could be where the largest earthquakes nucleate.

Microscopic Defect Dynamics

Geologist Matěj Peč and his colleagues are listening to rocks to identify any acoustic patterns or "fingerprints" that can emerge when subjected to various pressures. In their research on rock stability, they tested cylinders of Carrara marble, which is popularly used in sculpture and building decor.

Each marble cylinder is placed in a vice-like device composed of aluminum, zirconium, and steel pistons that can generate extreme stresses together. The scientists put the vice in a pressurized chamber and subjected each cylinder to pressures similar to the one experienced throughout the Earth's crust.

As each rock is slowly crushed, ultrasound pulses are sent through the top of the sample, and the acoustic pattern that exited through the bottom is recorded. The researchers listen to naturally occurring acoustic emissions when the sensors are not pulsing.

The researchers discovered that the marble formed sudden fractures with sound waves resembling large, low-frequency booms at the lower end of the pressure range, where the rocks are brittle. Meanwhile, the sound resembled a high-pitch crackling at the highest pressures, where rocks are more ductile. The research team believes this crackling resulted from microscopic defects known as dislocations that spread and flow like an avalanche.

In other words, rocks are "singing" at higher and higher pitches the deeper you go. This suggests that rocks make noises when deformed across the brittle-to-ductile transition. Scientists like these noises to the individual microscopic defects that cause them.

According to Peč, these acoustic patterns observed in rocks can help experts estimate the types of cracks, fissures, and other defects experienced by the Earth's crust. Scientists can use them to identify unstable regions below the surface with potential earthquakes or eruptions. The result of the study can also help inform the surveyors in their efforts to drill for renewable geothermal energy.