A body wave is a seismic wave that travels through the Earth’s interior, as opposed to along its surface. When an earthquake occurs, the rupturing fault releases energy in multiple forms. Body waves are the fastest of these, racing through rock, metal, and even liquid deep inside the planet. There are two types: P-waves (primary) and S-waves (secondary), and the differences between them have revealed much of what we know about what lies beneath our feet.
P-Waves: The First to Arrive
P-waves, also called compressional waves, work like sound waves. In fact, they are sound waves, just at frequencies too low for human hearing. The rock vibrates in the same direction the wave is traveling, creating a push-pull motion similar to compressing and releasing a spring. P-waves are the fastest seismic waves, which is why they’re called “primary.” They’re always the first signal a seismograph picks up after an earthquake.
Their speed depends on what they’re passing through. In loose sediment near the surface, P-waves travel around 1 km/sec. In the deep mantle, they can reach roughly 14 km/sec. At a typical crustal distance of 50 to 500 km from an earthquake, P-waves travel about 8 km/sec. One key property: P-waves can move through solids, liquids, and gases, which means they pass through every layer of the Earth, including the liquid outer core.
S-Waves: Slower but Stronger
S-waves, or shear waves, move rock perpendicular to the direction the wave is traveling. Picture shaking a rope side to side: the wave moves forward, but the rope itself moves up and down. This sideways motion makes S-waves more destructive than P-waves when they reach the surface.
S-waves travel at roughly 60% the speed of P-waves in rock, so they always arrive second. Their speeds range from about 1 km/sec in loose sediment to around 8 km/sec near the base of the mantle. At moderate distances, they average about 3.45 km/sec compared to the P-wave’s 8 km/sec at the same range.
The critical limitation of S-waves is that they cannot travel through liquids. S-waves need a rigid medium to propagate. Solids have shear strength, the internal force that holds rock together and resists being pulled apart sideways. Liquids lack shear strength entirely, so there’s nothing to transmit the side-to-side motion. This single physical constraint turned out to be one of the most important clues in earth science.
How Body Waves Revealed Earth’s Interior
Scientists can’t drill to the center of the Earth, but body waves gave them the next best thing: a way to map it. When seismographs around the world record the same earthquake, the pattern of which stations detect waves and which don’t creates what’s called a shadow zone. These silent spots told researchers what the planet is made of at every depth.
The S-wave shadow zone was the breakthrough. Beyond about 103 to 104 degrees of angular distance from an earthquake (roughly the far side of the planet), S-waves simply vanish. They never arrive. Since S-waves can’t pass through liquid, scientists in the early 1900s deduced that the Earth must have a liquid outer core blocking them. P-waves also bend and weaken when they hit the core boundary, creating their own smaller shadow zone, but they do continue through because liquids can still transmit compressional motion.
In 1936, Danish seismologist Inge Lehmann noticed something unexpected: faint P-waves were showing up inside the shadow zone where they shouldn’t have been. She interpreted these as reflections off a boundary about 5,000 km deep, proposing that inside the liquid outer core sat a distinct inner region with different properties. This was the discovery of the solid inner core, later confirmed through observations of the planet’s free oscillations.
Body Waves vs. Surface Waves
Earthquakes also produce surface waves, which travel along the Earth’s outer layer rather than through its interior. Surface waves are slower than both P-waves and S-waves, so they arrive last at a seismograph. Despite arriving last, surface waves typically cause the most damage during an earthquake. They carry more energy at the surface and produce larger ground motions, including the rolling and swaying that topples buildings.
Body waves lose energy as they spread out in three dimensions through the planet’s interior. Surface waves spread in only two dimensions along the surface, so their energy stays concentrated over longer distances. This is why a distant earthquake might produce only a faint jolt from the body waves but then a long, damaging roll when the surface waves arrive seconds or minutes later.
How Body Waves Are Used Today
The time gap between P-wave and S-wave arrivals is the standard tool for calculating how far away an earthquake occurred. Because the two wave types travel at different, known speeds, the longer the gap between their arrivals, the farther away the source. By comparing this gap at three or more seismograph stations, scientists triangulate the earthquake’s epicenter. This method, called the S-minus-P time technique, remains foundational in seismology.
Earthquake early warning systems rely on body waves in a more immediate way. Sensors detect the fast-moving P-wave as soon as it arrives and instantly transmit data to an alert center, where the earthquake’s location and magnitude are estimated. Because the damaging S-waves and surface waves are still en route, the system can push alerts to phones and infrastructure before the shaking hits. California’s earthquake warning system works on exactly this principle. The warning window is short, often just seconds, but that’s enough time to stop trains, open fire station doors, and get people under cover.
Body waves also serve as the primary imaging tool for the oil and gas industry and for geophysicists mapping structures deep in the crust. By generating controlled seismic pulses and recording how body waves bounce off underground layers, researchers build detailed pictures of rock formations, fault lines, and reservoirs without ever drilling a hole. The same physics that revealed the planet’s liquid core in the early 1900s now guides everything from resource exploration to volcanic hazard assessment.