Seismic waves: P waves, S waves, L waves

• Seismic waves are elastic waves set off when strain energy stored in rocks is suddenly released, most often by an earthquake, but also by volcanic blasts, landslides and underground nuclear or chemical explosions.
  • They are recorded by a seismograph; the trace is a seismogram, and the science that studies them is seismology.
• Seismology is the only direct physical probe of the deep Earth.
  • The deepest borehole (Kola, Russia) reached only about 12 km, and density, temperature and volcanic evidence give only indirect clues about the interior of the Earth.
• What seismic waves reveal depends on how fast they travel, which media they cross, and where they bend, bounce or vanish.

Origin of Seismic Waves

Why the Earth Shakes

• Rocks on either side of a fault are pushed in opposite directions by tectonic stress, but friction locks the fault.
  • Strain keeps building until the rock’s strength is exceeded and the fault slips abruptly.
  • The stored energy radiates outward in all directions as seismic waves (the elastic rebound idea).

Focus and Epicentre

• The focus (hypocentre) is the point inside the Earth where rupture begins and energy is released.
  • By focal depth, earthquakes are shallow (0–70 km), intermediate (70–300 km) or deep (300–700 km); the deepest foci lie at about 700 km, inside the mantle and well below the lithosphere.
• The epicentre is the point on the surface vertically above the focus; being nearest to it, it feels the waves first and usually most strongly.
• Indian example: the Jhajjar (Haryana) earthquake of July 2025 (M 4.4) had a focus only 10 km deep, so even a moderate event was felt across Delhi-NCR.

Types of Seismic Waves

• Body waves are generated at the focus and travel through the body of the Earth; they are of two kinds, P and S.
• Surface waves form when body waves reach the surface and interact with near-surface layers; they travel along the surface.
• Wave velocity depends on the elastic properties (incompressibility and rigidity) and density of the medium.
  • Velocity rises with stiffness and falls with density, so speed changes whenever the medium changes, and waves are reflected and refracted at every boundary.

Primary Waves (P Waves)

• Also called longitudinal or compressional waves; like sound waves, they move particles back and forth along the direction of travel.
  • This creates alternate compressions and rarefactions, pushing and pulling the material.
• They are the fastest seismic waves and the first to arrive at a station, hence “primary”.
• They pass through solids, liquids and gases, fastest in solids and slowed in liquids.
  • Velocity is Vp = √[(K + 4μ/3)/ρ], where K is incompressibility (bulk modulus), μ rigidity (shear modulus) and ρ density.
• They are relatively high-frequency and carry comparatively little destructive energy.
• Underground nuclear tests are told apart from earthquakes largely by their waves: an explosion pushes outward on all sides, so it sends strong P waves but weak S and surface waves.
  • The global monitoring network under the Comprehensive Nuclear-Test-Ban Treaty relies on this contrast.

Secondary Waves (S Waves)

• Also called transverse, shear or distortional waves; particles vibrate at right angles to the direction of travel, like a shaken rope, making crests and troughs.
  • The vibration may be vertical (SV) or horizontal (SH).
• They travel at roughly 60% of P-wave speed (Vp/Vs ≈ 1.7), so they arrive second, after a time lag.
• They move only through solids: velocity is Vs = √(μ/ρ), and because a liquid has zero rigidity (μ = 0), Vs falls to zero.
• The S–P time lag grows with distance; roughly, distance (km) ≈ 8 × S–P time (seconds) for nearby earthquakes, and lags from three stations fix the epicentre by triangulation.

Why S Waves Cannot Travel Through Liquids

• An S wave needs the medium to resist shearing and spring back, so the next particle is dragged sideways.
• Solid rock has this shear strength (rigidity); a liquid does not, just as water poured from a glass does not keep the glass’s shape.
• A liquid therefore cannot pass a shear disturbance on, and S waves die out at any liquid layer.

Surface Waves (L Waves)

• Also called long-period (L) waves: low frequency, long wavelength and large amplitude.
• Confined to the outer crust, their energy dies out rapidly with depth.
• They spread over a surface rather than a volume, so they lose energy slowly with distance and travel the longest distances of all seismic waves.
• Slowest of the three, they are recorded last, but they cause the largest ground displacement and most of the destruction.
• They are of two kinds, named after the scientists who described them mathematically.

• Long-period shaking is amplified in thick, soft sediment basins.
  • The 2001 Bhuj earthquake brought down multi-storey buildings in Ahmedabad, over 200 km away.
  • The Mandalay (Myanmar) earthquake of March 2025 (Mw 7.7) collapsed a high-rise under construction in Bangkok, about 1,000 km away, on the soft clay of its basin.
  • The same concern applies to the Indo-Gangetic alluvium under Delhi and Patna and to the Kathmandu basin.

P, S and L Waves Compared

Reading a Seismogram

• At a distant station, a seismogram shows three phases in order.
  • Preliminary tremors are small, weak swings from P waves.
  • Second preliminary tremors follow a short gap, from S waves.
  • Main tremors are the large, long swings of surface waves.
• Earthquake early warning exploits the head start of P waves: sensors detect them and issue alerts seconds before the damaging S and surface waves arrive.
  • Indian example: Uttarakhand Bhookamp Alert (IIT Roorkee, 2021) is India’s first earthquake early-warning mobile app.
• India’s National Seismological Network, run by the National Center for Seismology (NCS), expanded from 80 observatories in 2014 to 168 by 2025.

How Seismic Waves Reveal the Earth’s Interior

Curved Paths, Reflection and Refraction

• In a homogeneous Earth, waves would travel in straight lines at constant speed.
• Records show the opposite: travel times and paths are curved, concave towards the surface.
  • Velocity increases with depth, because rigidity and incompressibility rise faster than density, so rays refract gradually and bend back up.
• Abrupt jumps or drops in velocity mark boundaries, the discontinuities inside the Earth, where waves are sharply reflected (rebound) or refracted (change direction).
• Between about 100 and 250 km, S waves slow down in a low-velocity zone: rock there is near its melting point and partly molten, marking the weak asthenosphere beneath the rigid lithosphere.
• Velocity changes show changes in material and state; changes of direction, including the shadow zones, show where the layers lie.

The Emergence of Shadow Zones

• A shadow zone is a belt of the Earth’s surface where a given earthquake’s direct waves are not recorded; each earthquake has its own shadow zone, centred on its epicentre.
• Stations within about 103°–104° of the epicentre record both P and S waves.
• S-wave shadow zone: no direct S waves beyond about 103°.
  • This covers everything beyond that angle, nearly 40% of the Earth’s surface.
  • Cause: S waves cannot cross the liquid outer core.
• P-wave shadow zone: a band from about 104° to 140°.
  • Cause: at the core–mantle boundary (about 2,900 km), P-wave speed drops sharply, so rays bend inward and emerge beyond 140°.
• Faint P arrivals inside the P shadow are waves reflected and refracted at a faster, solid inner core.
• The S-wave shadow is much larger than the P-wave shadow.

Milestones in Reading the Interior

Evaluation and Current Understanding

• Strengths: seismic waves give the layered model (crust, mantle, outer and inner core), the depths of the discontinuities and the physical state of each layer.
  • Combined with the density needed for the Earth’s mean density (about 5.5 g/cm³) and with geomagnetism, they point to an iron–nickel core.
• Seismic tomography, like a CT scan built from millions of travel times, now maps 3-D velocity variations: cold subducted slabs, rising plumes and two huge low-shear-velocity provinces beneath Africa and the Pacific.
• The inner core is dynamic.
  • Repeating earthquakes from the South Sandwich Islands show it spun slightly faster than the mantle from 2003 to 2008, then drifted back more slowly (Wang, Vidale and colleagues, 2024).
  • A 2025 follow-up found its near surface changing shape.
• Limitations:
  • Seismic stations are unevenly spread, with few on the ocean floor.
  • Velocity alone cannot fix composition, which needs rock physics, meteorite analogues and magnetic data.
  • Some interpretations of deep structure remain non-unique.