3D Structure of Earthquake-Induced Sound Waves

An Earthquake That Resonates into the Sky

On January 1, 2024, a magnitude 7.6 earthquake struck Japan’s Noto Peninsula. While its human and material consequences were immediately visible, scientists discovered a phenomenon far less obvious but equally fascinating: the propagation of sound waves through the atmosphere and into the ionosphere. Using a dense network of GNSS sensors, these waves were captured and modeled in three dimensions for the first time at such a high resolution.

Atmospheric Tomography of the Invisible

The ionosphere, the atmospheric layer between 60 and 1,000 kilometers above Earth, is known to be sensitive to both electromagnetic and mechanical disturbances. During an earthquake, pressure waves (acoustic waves) travel upward and alter the electron density in this layer. Led by Professor Kosuke Heki of Nagoya University, Japanese researchers used more than 4,500 GNSS stations to precisely measure these variations.

Using a method similar to medical CT scanning, the team reconstructed a 3D image of the disturbed electron density field, revealing—for the first time—the full shape of earthquake-triggered atmospheric sound waves.

A Major Scientific and Technical Breakthrough

Contrary to the traditional assumption that sound waves emanate from a single point, the study revealed that the entire fault line emits sound, acting as a large vibrating surface. This insight allowed researchers to estimate the speed of sound wave propagation in the atmosphere at about 0.8 km/s—significantly slower than seismic waves in Earth’s crust.

The modeling also showed concentric, ring-like structures in the ionosphere—a sort of “sonic bubble” that may affect radio and GPS signals for several minutes after a major quake.

Practical Applications for Safety and Science

This 3D visualization of acoustic waves has immense potential for disaster prevention. It could improve early warning systems by providing faster, more accurate data on earthquake origin and intensity. Additionally, the technology could help monitor volcanic eruptions, industrial explosions, or even nuclear tests.

The study also bridges multiple disciplines: seismology, space weather, atmospheric physics, and GNSS technology. It's a clear example of the value of a multidisciplinary approach to understanding large-scale planetary phenomena.

Towards an “Aerial” Geophysics?

By revealing how Earth’s movements affect the upper atmosphere, this study reshapes our understanding of earthquakes. The ground doesn’t just shake beneath our feet—it also resonates into the sky. And this "voice" of the Earth might become a valuable new tool for understanding its most violent events.

While ionospheric disturbances were once seen as a nuisance for satellite signals, they are now becoming precious indicators, adding a whole new dimension—literally—to geophysical research.