Home Seismic Waves
Earth ScienceNo one has drilled deeper than about 12 kilometres. Yet we know Earth has a solid inner core, a liquid outer core, a mantle and a crust — and every bit of that came from reading waves released when rock breaks.
Seismic waves are waves of energy that travel through the Earth, usually generated by earthquakes. There are four main types in two families. Body waves travel through the interior: P-waves (fast, compressional, arrive first) and S-waves (slower, shear, cannot pass through liquid). Surface waves travel along the surface: Love waves and Rayleigh waves, which are slower but cause the most damage. Because each type is blocked, bent or slowed by different materials, scientists use them both to measure earthquakes and to map the inside of the planet.
Seismic waves are waves of mechanical energy moving through the Earth or along its surface. They are elastic waves: they travel by making particles of rock vibrate about their resting positions and then return, passing the energy along without permanently moving the rock. Speed depends on the density and elasticity of whatever the wave is passing through, so the same wave speeds up, slows down and changes direction as it crosses between layers.
Most come from earthquakes, but volcanic eruptions, large landslides, meteorite impacts and human activity — mining blasts, underground explosions — all generate them too. Whatever the source, the physics is identical: a sudden release of energy sends elastic waves rippling outward through the surrounding rock.
The instruments that record them are seismometers, and the wiggling trace they produce is a seismogram. By comparing seismograms from stations around the world, seismologists reconstruct what happened during an earthquake and what the waves passed through on the way. Those recordings are largely open and freely shared, which is why earthquake information reaches the public within minutes.
Two terms worth fixing first. The focus (or hypocentre) is the point underground where the rock first ruptures. The epicentre is the point on the surface directly above it. News reports give the epicentre because it is what you can find on a map, but the waves themselves spread out from the focus below.
Earth's outer shell is broken into enormous slabs — tectonic plates — drifting on hotter, softer rock beneath. Where they meet they can lock together along cracks called faults, and stress builds for years or centuries as the plates try to move past each other and cannot.
Eventually the rock gives way. All the energy stored through those years of strain is released at once, and it radiates from the focus as seismic waves, exactly as ripples spread when a stone breaks the surface of a pond.
How much energy depends on how much rock breaks and how far it moves. A small quake slips a short section of fault by a few centimetres. In the largest ever recorded, more than a thousand kilometres of fault ruptured at once and the ground shifted by tens of metres. That is why earthquake energy spans such an enormous range — and why the scales used to measure it had to be logarithmic.
They radiate in three dimensions from the focus, passing deep through crust, mantle and core. Because their energy spreads through a volume, it thins out relatively quickly with distance. The two body waves — P and S — are the fastest seismic waves, so they always arrive first.
They form when body waves reach the surface and some energy becomes trapped in the shallow layers. Spreading in only two dimensions, their energy fades much more slowly, so they keep far larger amplitudes. Love and Rayleigh waves travel slowest and arrive last — and do most of the damage.
The four always arrive in the same order, because they travel at different speeds: P, then S, then Love and Rayleigh bringing up the rear. That predictable sequence is not a curiosity. As you will see, the gaps between arrivals are exactly what let scientists work out where an earthquake happened and how strong it was.
The difference between the four is entirely about how the rock moves as the wave passes. Pick a wave below and watch a block of ground deform under it, with one particle tracked so you can see the path it actually takes.
This animation needs a browser with canvas support. The description and the comparison table below cover the same ground.
P-waves are the fastest seismic waves and therefore the first to arrive anywhere, which is how they earn the name primary. They are also called pressure or compressional waves. A P-wave is a longitudinal wave: rock particles vibrate back and forth along the same direction the wave travels, squeezing together and pulling apart as it passes.
In crustal rock they travel at roughly 6 to 8 km/s, speeding up to around 13 km/s in the denser mantle. A P-wave crosses an entire city in well under a second. When one reaches the surface and enters the air it becomes an ordinary sound wave, which is why people sometimes report a deep boom at the very start of a quake.
Their single most important property is that they pass through solids, liquids and gases alike. Compression works on anything that can be compressed, and everything can. That is what lets P-waves cross the molten outer core — and it became the crucial clue in mapping the interior.
P-waves carry smaller amplitudes than the waves behind them, so on their own they produce only a mild jolt. That gentleness is useful: because the harmless wave races ahead of the destructive ones, detecting it buys just enough time to trigger an alert before the serious shaking begins.
S-waves arrive second, which is how they earn the name. They are also called shear waves. An S-wave is a transverse wave: particles move at right angles to the direction of travel, shaking the ground from side to side or up and down rather than along the path.
They travel at roughly 3 to 4.5 km/s in the crust, about 60 percent of the P-wave speed — a ratio that falls straight out of the maths separating the two wave families. Near the epicentre the two arrive almost together; at distant stations the gap grows wide. That widening gap turns out to be one of the most useful measurements in all of seismology.
The defining property of the S-wave is the exact opposite of the P-wave's great strength: S-waves cannot travel through liquid. Their shearing motion depends on the medium resisting a sideways push and springing back — shear strength, which only solids have. A liquid flows out of the way instead, and the wave dies. This single limitation is why S-waves vanish at the outer core, and that disappearance was the first proof part of the core is molten.
Because they carry larger amplitudes and shake sideways, S-waves do considerably more damage than P-waves. Buildings resist a vertical push far better than lateral shaking, so the S-wave arrival is usually the moment an earthquake starts to feel genuinely violent.
Love waves are named after the British mathematician A.E.H. Love, who described them mathematically in 1911. They form when S-waves reach the surface and become trapped in the shallow layers, bouncing within a softer surface layer sitting on faster rock below. That layered arrangement exists almost everywhere, so Love waves are a common feature of earthquakes.
A Love wave moves the ground horizontally, shearing it side to side perpendicular to the direction of travel, with no vertical motion at all. The effect is something like the slithering of a snake. Because that sideways shearing acts directly on the foundations of buildings, roads and bridges, Love waves are especially destructive to structures.
They travel at roughly 2 to 4.5 km/s — slower than body waves, slightly faster than the Rayleigh waves behind them. They are also dispersive: waves of different periods travel at slightly different speeds, so the wave train spreads out as it goes. Like all surface waves their motion is largest at the surface and fades with depth, which is exactly why anything built on the ground feels them so strongly.
Rayleigh waves are the slowest of all seismic waves, named after Lord Rayleigh, who predicted them mathematically in 1885 — years before any instrument could record one. They are often called ground roll, for the motion they create.
Unlike the purely sideways motion of a Love wave, a Rayleigh wave moves the ground in a rolling elliptical path: up and forward, then down and back, much like a wave rolling across the ocean. That combination of vertical and horizontal movement makes it feel like a deep swell passing beneath your feet. They are, in effect, both transverse and longitudinal at once.
They travel at roughly 2 to 4 km/s, and what they lack in speed they more than make up in size. Rayleigh waves typically carry the largest amplitude of any seismic wave, and for many earthquakes they produce the strongest shaking felt at the surface. When a source vibrates vertically at the surface, about two-thirds of the radiated energy goes into Rayleigh waves — far more than into body waves. In the very largest earthquakes they can circle the globe several times before fading.
Notice how speed, arrival order and damage potential all line up in the same pattern: the slower the wave, the later it arrives — and the more harm it does.
| Property | P-wave | S-wave | Love wave | Rayleigh wave |
|---|---|---|---|---|
| Family | Body | Body | Surface | Surface |
| Motion | Longitudinal (push–pull) | Transverse (shear) | Horizontal shear | Rolling (elliptical) |
| Speed in crust | 6–8 km/s in crust | 3–4.5 km/s in crust | 2–4.5 km/s | 2–4 km/s |
| Arrival order | First | Second | Third | Fourth |
| Through liquid? | Yes | No | No | No |
| Damage potential | Low | Moderate to high | High | Highest |
The most extraordinary thing about seismic waves is that they gave us a detailed map of a place no human will ever visit. Waves change speed and bend — refract — whenever they cross between materials, just as light bends entering water. They reflect off boundaries, and some stop entirely at a material they cannot cross. Recording those changes worldwide reveals what lies in between.
P-waves bend sharply through the liquid outer core; S-waves stop at its boundary entirely. The gaps they leave on the surface are the shadow zones.
The clearest evidence comes from regions where certain waves never arrive at all. There are two, and each tells a different part of the story.
The S-wave shadow zone covers everywhere beyond about 104 degrees of angular distance from the epicentre. Across that entire far region, no direct S-waves are ever detected. Since S-waves cannot cross liquid, their complete absence can mean only one thing: a liquid layer deep inside is blocking them. That is how the outer core was proved molten.
The P-wave shadow zone is different. P-waves can cross liquid, so they are not blocked — but between roughly 104 and 140 degrees they are bent so sharply by the core that they are refracted away, leaving a band where few direct P-waves land. The existence and exact width of that band revealed the size and depth of the core itself.
Piecing decades of such observations together produced the layered model used today. Each boundary was found by spotting a sudden change in wave speed, and several carry the name of whoever found it.
| Layer | Depth | State |
|---|---|---|
| Crust | 0 – 5 km (ocean) to ~40 km (continent) | Solid rock |
| Mantle | to ~2,900 km | Solid rock that slowly flows |
| Outer core | 2,900 – 5,150 km | Liquid iron and nickel |
| Inner core | 5,150 – 6,371 km | Solid, held by pressure |
The crust–mantle boundary is the Mohorovičić discontinuity, almost always shortened to the Moho. The mantle–outer core boundary is the Gutenberg discontinuity. The boundary into the inner core is linked to the Lehmann discontinuity. All three appear in the history below.
The method is elegant, and it rests entirely on the speed difference between P and S. Because P travels faster, the two spread apart as they go, like runners of different paces pulling away over a long race. Near the epicentre they arrive almost together; far away the gap is wide. Measure that gap at one station and you know the distance to the quake.
Distance from one station draws a circle of possible locations. Repeat at a second and third, and the single point where all three circles meet is the epicentre. The technique is called triangulation, and modern networks use far more than three stations to pin it down within seconds.
This is the same calculation a seismologist makes from a single seismogram.
S-waves must be slower than P-waves — otherwise the gap never opens.
Once a quake is located, the next question is how strong it was — and the size of the recorded waves is the answer.
The first widely used scale came from the American seismologist Charles Richter in 1935. More precisely called the local magnitude scale, it is based on the maximum amplitude recorded on a seismograph, adjusted for distance.
The crucial thing is that it is logarithmic. Each whole step up means ten times the ground motion — a magnitude 6 shakes ten times harder than a 5 and a hundred times harder than a 4. In energy the jump is steeper still: roughly 32 times per step. Which is why the difference between a 6 and an 8 is not modest but staggering, around a thousandfold.
The public still says Richter, but seismologists mainly use moment magnitude (Mw), introduced in 1979. The older scale has a weakness: for very large earthquakes it stops rising accurately — a problem called saturation — so it underestimates the giants.
Moment magnitude fixes this by measuring something physical: the actual area of fault that ruptured and how far it slipped. That ties the number to the real scale of the event, so it stays accurate at any size. It is also logarithmic and calibrated to agree with Richter for moderate quakes, which is why the two match closely in the range most people know. When you hear "magnitude 7.1", it is almost always a moment magnitude — even if the report says Richter.
Magnitude measures energy released at the source: one number for the whole event. Intensity, on scales such as Modified Mercalli, describes how strong the shaking felt at a particular place, varying with distance, local geology and construction. One earthquake has a single magnitude but many intensities.
A magnitude 8 or greater strikes somewhere about once a year, but the true giants are far rarer. Every one of the largest occurred at a subduction zone, where one tectonic plate is forced beneath another — mostly around the Pacific Ring of Fire.
The most powerful ever recorded struck near Valdivia, Chile on 22 May 1960, at moment magnitude 9.5. Roughly 1,000 km of fault tore. It released so much energy that the tsunami it generated crossed the entire Pacific at around 500 miles per hour, causing destruction as far away as Japan and Hawaii. About 1,655 people died in Chile and two million were left homeless.
The 2004 Sumatra–Andaman earthquake ruptured about 1,300 km of fault and unleashed a tsunami that killed roughly 230,000 people around the Indian Ocean — one of the deadliest natural disasters in recorded history. The 2011 Tōhoku earthquake off Japan, also 9.1, triggered a tsunami that killed nearly 16,000 people, left thousands more missing, and caused the Fukushima nuclear accident.
These events are energetic enough to affect the whole planet. The 1960 Chile and 2010 Chile earthquakes are both estimated to have very slightly changed Earth's rotation, shortening the length of a day by a tiny fraction of a second.
The science developed over nearly two thousand years, but almost everything we know arrived in a remarkable burst between 1900 and 1940 — people turning faint wiggles on paper into a map of a world nobody could see.
Inge Lehmann deserves the last word. Studying seismograms from a large 1929 New Zealand earthquake, she noticed faint P-waves arriving inside the shadow zone, where the liquid-core model said none should appear. Her 1936 solution was bold: the core is not one liquid ball but has a distinct solid inner core nested inside it. Leading seismologists confirmed it within a few years, and it remains the accepted picture. Lehmann lived to 104.
The harmless P-wave outruns the destructive S and surface waves, so a seismometer network can detect it, recognise a large quake is underway, and alert before the damaging shaking lands. Seconds to tens of seconds is enough to stop trains, halt surgery, shut gas valves and get people under cover. Japan's national system and ShakeAlert on the US West Coast already protect millions this way.
Reflection seismology makes its own waves — vibrating trucks on land, air guns at sea — and reads the echoes off rock layers to build three-dimensional images of what lies beneath. The global seismic survey market was worth roughly $9–11 billion in 2025 and is projected toward $15–18 billion by the mid-2030s, with oil and gas accounting for most of the demand.
Civil engineers use seismic analysis to design structures that survive shaking and to model how local soil amplifies it. Volcanologists track the small earthquakes that often precede an eruption, which is one of the most reliable warning signs available.
The global network enforcing the nuclear test ban works by spotting the distinctive seismic signature of an underground explosion, which differs measurably from a natural quake. The same physics has left Earth entirely: NASA's InSight lander recorded marsquakes, beginning the map of another planet's interior.
A station records the P-wave 40 s before the S-wave. P travels at 8 km/s, S at 4 km/s. How far away was the quake?
d/4 − d/8 = 40 → d/8 = 40 → d = 320 km.
How much more energy does a magnitude 7 release than a magnitude 5?
Each step is about 32×, so 32² ≈ 1,024 times more energy.
A P-wave travels through crust at 6,000 m/s with a frequency of 2 Hz. Find its wavelength.
λ = v/f = 6,000/2 = 3,000 m — a wavelength three kilometres long.
A magnitude 6 against a magnitude 4: how much bigger is the ground motion, and how much bigger the energy?
Amplitude ×10² = 100 times. Energy ×32² ≈ 1,024 times. The two scale very differently.
A quake is 120 km away. With P at 8 km/s and S at 4 km/s, how long between the P arrival and the damaging S-wave?
120/4 − 120/8 = 30 − 15 = 15 s of warning.
Shear modulus G = 0 in a liquid. What does the S-wave speed √(G/ρ) become?
√(0/ρ) = 0 — the wave cannot propagate at all, which is the shadow zone in one line.
P-waves, S-waves, Love waves and Rayleigh waves. P-waves and S-waves are body waves that travel through Earth's interior; Love and Rayleigh waves are surface waves that travel along the top. P-waves are fastest and arrive first, while Rayleigh waves are slowest and usually cause the most damage.
Body waves travel through the interior of the Earth and include P-waves and S-waves. They are faster and arrive first. Surface waves travel along the surface and include Love and Rayleigh waves. They are slower and arrive last, but because their energy is concentrated near the surface and spreads in only two dimensions, they keep much larger amplitudes and cause most of the destruction.
The P-wave, at roughly 6 to 8 km/s in the crust and up to about 13 km/s deep in the mantle. Being fastest, it always reaches a seismograph first — which is exactly why it is called the primary wave, and why early warning systems are built around detecting it.
Surface waves, and Rayleigh waves in particular. They travel slowest and arrive last, but they carry the largest amplitudes and concentrate their energy right where buildings are. Love waves, which shear the ground horizontally, are also severely destructive because that sideways motion acts directly on foundations.
An S-wave moves the ground perpendicular to its direction of travel, which only works if the material resists being sheared sideways and springs back. That property, shear strength, belongs to solids alone. A liquid simply flows out of the way, so the wave dies. This is why S-waves stop dead at Earth's liquid outer core.
Beyond about 104 degrees from an epicentre, no direct S-waves arrive anywhere — a region called the S-wave shadow zone. Since S-waves cannot cross liquid, their complete absence over that whole band can only mean a liquid layer deep inside is blocking them. The way P-waves are sharply refracted around the same region confirmed the core's size and depth.
Magnitude measures the energy released at the source and is one number for the whole earthquake. Intensity describes how strong the shaking felt at a particular place, which depends on distance, local geology and construction. One earthquake has a single magnitude but many intensities — which is why two towns the same distance apart can report very different experiences of the same quake.
The public still says Richter, but seismologists mainly use the moment magnitude scale now. The Richter scale saturates for very large earthquakes, meaning it stops rising accurately and underestimates the giants. Moment magnitude is based on the physical size of the rupture and how far it slipped, so it stays accurate at any size. The two agree closely for moderate quakes, which is why the names get used interchangeably.
By measuring the time gap between the P and S arrivals at a station, which gives the distance to the quake. That distance draws a circle of possible locations around the station. Repeat at three or more stations and the single point where the circles intersect is the epicentre. The method is called triangulation, and modern networks do it within seconds.
No. Volcanic eruptions, large landslides, meteorite impacts and human activity such as mining blasts and underground explosions all generate them. The international system that monitors for secret nuclear tests works by detecting them. Exploration companies also create small seismic waves deliberately, to image rock formations when prospecting.
Short for the Mohorovičić discontinuity, it is the boundary between Earth's crust and the mantle beneath. Andrija Mohorovičić found it in 1909 after noticing that seismic waves suddenly sped up at a certain depth. It lies roughly 5 to 10 km down beneath the oceans and up to about 40 km beneath the continents.
Seismic waves are waves of energy travelling through the Earth, generated mainly when stress stored in rock is suddenly released along a fault. There are four main types: P and S waves pass through the interior, while Love and Rayleigh waves travel along the surface and cause most of the damage.
Their behaviour depends on what they pass through, and that is precisely what makes them useful. P-waves cross solids, liquids and gases; S-waves cannot cross liquid at all. By tracking where each goes, where it bends and where it disappears into a shadow zone, scientists mapped the crust, mantle, liquid outer core and solid inner core — none of which anyone has ever seen.
They also measure earthquakes. The P–S gap gives the distance, triangulation from several stations gives the epicentre, and the size of the recorded waves gives the magnitude on logarithmic scales where each step is roughly 32 times more energy. The largest ever recorded, magnitude 9.5 in Chile in 1960, released energy that rippled across the whole planet.
From Zhang Heng's bronze seismoscope in 132 CE to Lehmann's discovery of the inner core, and on to today's early warning systems and multi-billion-dollar exploration industry, seismic waves turned the trembling of the ground into one of science's sharpest instruments. They remain our only way to read the interior of the world beneath our feet.
Why P-waves always outrun S-waves, and the maths behind it.
The physics of shear motion that governs S-waves and Love waves.
Compressions and rarefactions — how a P-wave actually moves rock.
Solve v = fλ for seismic wavelengths and speeds.
Test yourself, including a seismic and comparison section.
Every wave term on this site, defined and cross-linked.