Earthquakes are primarily known for the ground shaking they cause, but they also generate a significant amount of acoustic energy. The answer to whether an earthquake makes noise is definitively yes, though the sound is often misunderstood or goes completely unnoticed by people. This acoustic energy is a byproduct of the massive seismic vibrations traveling through the earth and atmosphere.
Audible Sounds and Infrasound
The acoustic energy released by an earthquake exists across a wide frequency spectrum, which can be divided into two main categories based on human hearing. The most common acoustic signature of large quakes is infrasound, consisting of sound waves operating at frequencies below 20 Hertz (Hz). Since the lower limit of human hearing typically begins around 20 Hz, these extremely low-frequency waves are inaudible to people. Scientists can detect this powerful energy traveling thousands of kilometers through the atmosphere using specialized sensors.
The sounds that people actually hear are classified as audible acoustics, which generally fall in the low end of the human hearing range, sometimes extending up to about 70 Hz. These are the sounds often reported just before or during the initial moments of shaking, frequently described as a deep, low-pitched rumble, a booming sound, or something akin to a freight train passing nearby. This audible noise is a direct result of the seismic energy entering the frequency range that the human ear can register. The phenomenon of hearing a sound before the shaking starts is possible because the sound waves travel through the air, while the more destructive shaking waves travel through the ground at different, often slower, speeds.
How Seismic Energy Becomes Acoustic Energy
The conversion of energy from a seismic wave traveling through solid rock to an acoustic wave traveling through air is a physical process known as acoustic coupling or ground-air coupling. This mechanism begins with the fastest seismic waves generated by the rupture, which are the Primary waves, or P-waves. P-waves are compressional waves, meaning they push and pull the material they travel through in the same direction as the wave, much like sound waves in air. When these underground P-waves reach the Earth’s surface, they cause the ground to move vertically, acting like the diaphragm of a massive, natural loudspeaker.
In contrast, the Secondary waves, or S-waves, which are responsible for the most intense and destructive shaking, are shear waves that move material side-to-side or up-and-down perpendicular to the direction of wave travel. Because S-waves cannot travel through fluids like air, they do not efficiently couple their energy into the atmosphere to create sound waves. The acoustic energy generated is only a tiny fraction of the total seismic energy, with some models suggesting less than one percent of the energy is converted from acoustic to seismic.
Why Sound Perception Varies
The ability of a person to hear an earthquake sound is highly variable and depends on several external factors. One significant variable is the earthquake’s depth, as shallower quakes are far more effective at creating audible sound. An earthquake originating near the surface allows the P-waves to reach the ground-air boundary with more energy intact, leading to more robust ground displacement and a louder acoustic wave. The sound wave’s intensity also attenuates quickly as it travels away from the epicenter through the atmosphere, meaning people close to the rupture point are far more likely to hear the sound.
The local geology and soil composition also play a role, as different types of rock and sediment transmit P-waves to the surface with varying degrees of efficiency. Magnitude influences perception in a counterintuitive way: extremely large earthquakes often release a greater proportion of their acoustic energy as powerful, globe-spanning infrasound, which remains inaudible to humans. Conversely, smaller, very shallow events can sometimes be more easily recognized audibly because their frequency content is concentrated closer to the human hearing range.