P-Waves, S-Waves, and Surface Waves: The Physics of Shaking

Science

June 2026 · 5 min read · By Tremr

When a fault ruptures, it doesn't just shake the ground at a single point. It releases energy that radiates outward as waves — the same way a stone dropped in water sends ripples in every direction. Those waves travel through rock, through the soft crust, and eventually through the surface of the earth. What you feel during an earthquake is not the fault itself but the waves that carry energy away from it.

There are several distinct types of seismic waves, and they behave very differently. Understanding them explains some of the most puzzling aspects of earthquake experience: why a large distant earthquake can feel like a long, rolling swell rather than a sharp jolt; why basements sometimes experience different shaking than upper floors; and why an earthquake warning system can know a damaging quake is coming before the worst of the shaking arrives.

Seismogram recording of the 1964 Alaska earthquake showing distinct P-wave, S-wave, and surface wave arrivals — A seismogram from the 1964 Alaska earthquake — the distinct wave types are clearly visible as separate arrivals. Image: USGS / Wikimedia Commons

How Energy Releases from a Fault

A fault rupture begins at a point called the hypocenter (or focus) — the location underground where the rock first slips. Energy radiates from this point in all directions simultaneously. Different types of waves carry that energy through the earth at different speeds and in different ways, which is why a seismograph at a distant station records a series of distinct arrivals rather than a single burst of shaking.

The depth of the hypocenter matters enormously. Shallow earthquakes (less than 70 km depth) cause the most surface damage because the waves have less distance to travel and less rock to absorb their energy. Deep earthquakes (greater than 300 km) can be felt over enormous areas but typically cause less damage at any single location.

P-Waves: The First Arrivals

P-waves — "P" stands for primary, or compressional — are the fastest seismic waves and the first to arrive at any recording station. They travel through solid rock at roughly 5–8 kilometres per second, meaning they can cross a continent in a few minutes. P-waves are compressional waves: the rock alternately compresses and expands in the same direction the wave is travelling, like a coil spring being pushed from one end.

When P-waves reach the surface, they often manifest as a brief sharp jolt — a sudden vertical movement that can sound like a distant explosion or a truck driving past. In many earthquakes, people report hearing a low rumbling or boom just before the main shaking starts. That sound is P-wave energy entering the atmosphere as an airwave, sometimes at frequencies audible to humans.

P-waves can travel through both solid rock and liquid (like the Earth's liquid outer core), which makes them useful for mapping the interior of the planet. S-waves cannot travel through liquid, which is one of the key lines of evidence that Earth has a liquid outer core.

S-Waves: The Dangerous Ones

S-waves — secondary, or shear waves — travel more slowly than P-waves, at roughly 3–5 kilometres per second. They arrive after the P-waves at any given location. Unlike P-waves, S-waves cause the rock to move perpendicular to the direction the wave is travelling — like shaking a rope from side to side. This shearing motion is far more destructive than compression for most structures.

The damaging main shaking that people associate with earthquakes is primarily caused by S-waves and the surface waves that follow. Buildings can accommodate vertical motion (they're designed for gravity) far better than lateral (side-to-side) motion. S-waves deliver powerful lateral shaking that can cause walls to crack, joints to fail, and poorly built structures to collapse.

The time gap between P-wave arrival and S-wave arrival at any location is directly proportional to the distance from the earthquake. Seismologists use this gap — measured at multiple stations — to locate the epicentre with precision. A gap of 8 seconds means the station is roughly 65 km from the epicentre. This method, developed in the early 20th century, remains the foundation of earthquake location science.

Surface Waves: The Rolling Motion

Surface waves travel along the surface of the earth rather than through its interior. They are slower than both P- and S-waves but often have larger amplitudes — meaning they cause bigger displacement, particularly at greater distances from the earthquake. There are two main types:

Love waves — move the ground horizontally, side to side, with no vertical component. Named after Augustus Love, who predicted them mathematically in 1911.
Rayleigh waves — move the ground in a rolling, elliptical motion — up, forward, down, backward — like ocean swells passing under a boat. Named after Lord Rayleigh, who predicted them in 1885.

The rolling swell that people experience during large, distant earthquakes is typically Rayleigh wave energy. Because surface waves decay more slowly with distance than body waves (P- and S-waves), they dominate the seismic record at large distances. A M8.0 earthquake in Japan will still register on seismographs in Europe via surface waves that have circled the globe multiple times.

California's seismic monitoring network — one of the densest in the world, enabling earthquake early warning across the state

Why This Matters for Early Warning

The difference in speed between P-waves and S-waves is the physical foundation on which earthquake early warning systems are built. When a seismograph detects P-wave energy from a large earthquake, it can instantly estimate the quake's location and magnitude — and send an alert to areas that haven't yet been reached by the slower, damaging S-waves and surface waves.

The amount of warning time depends on the distance between the earthquake and the receiver. Right at the epicentre, there is no warning — P-waves and S-waves arrive almost simultaneously. At 50 km distance, there might be 10–15 seconds of warning. At 200 km, there could be 60–90 seconds — enough time to drop and cover, stop a surgery in progress, halt a train, or trigger automated shutdown of industrial processes.

Japan's earthquake early warning system — the most advanced in the world — can detect a P-wave, estimate the quake's parameters, and push alerts to millions of phones within seconds of rupture. California's ShakeAlert system covers the entire state. Both systems work because of one fundamental physical fact: the speed difference between P-waves and S-waves gives us a window, however brief, between detection and destruction.