Decoding Mysterious Seismic Signals

For decades, scientists have been puzzled by mysterious seismic signals from deep within our planet. Now, U geophysicists have shed new light on this phenomenon, potentially uncovering a network of zones scattered throughout the globe’s interior.

When earthquakes happen, they send waves racing through Earth’s layers. Some of these waves get scattered when they hit unusual spots in the mantle—the thick layer of hot rock between Earth’s crust and outer core. These scattered waves, called PKP precursors, arrive before the main waves and hold clues about what’s happening deep inside our planet.

Way down in the lower mantle, there are regions called ultra-low velocity zones (ULVZs). These areas slow down seismic waves by a whopping 50 percent, acting like enigmatic speed bumps. “These are some of the most extreme features discovered on the planet. We legitimately do not know what they are,” says Michael Thorne, associate professor of geology and geophysics.

Using advanced techniques, Thorne’s team analyzed data from 58 earthquakes that struck New Guinea but were recorded in North America. This allowed them to pinpoint where seismic waves scatter along the core-mantle boundary, about 1,800 miles below the surface.

Their findings suggest ULVZs are more widespread than previously thought, particularly beneath North America. Thorne suspects these thin layers form where tectonic plates sink deep into the Earth.

“What we’ve now found is that these ultra-low velocity zones do not just exist beneath the hot spots. They’re spread out all across the core-mantle boundary beneath North America,” Thorne explains. “It really looks like these ULVZs are getting actively generated. We don’t know how.”

This discovery of widespread ultra-low velocity zones could change how we view Earth’s interior and how geological processes shape our planet.