The Physics of Earthquakes: How Seismic Waves Reveal Earth's Interior
Earthquakes are destructive — but the waves they produce are our best tool for mapping Earth's hidden interior. How P-waves, S-waves, and surface waves work and what they tell us.
Table of Contents
When the Earth Moves
Earthquakes are among the most powerful and destructive natural phenomena on our planet. The 2011 Tōhoku earthquake in Japan released energy equivalent to 600 million times the Hiroshima bomb. The 2004 Indian Ocean earthquake shifted the entire planet on its axis by several centimetres.
But earthquakes are also one of the most valuable tools in geophysics. The waves they generate travel through the entire Earth, and by studying how those waves behave, scientists have mapped our planet’s hidden interior in extraordinary detail — without ever drilling deeper than 12 kilometres.
Why Earthquakes Happen
Earth’s outer shell — the lithosphere — is divided into tectonic plates that float on the semi-fluid asthenosphere beneath. These plates move at speeds of a few centimetres per year, driven by convection currents in the mantle.
Where plates meet, enormous forces build up. Along transform boundaries like the San Andreas Fault, plates grind past each other horizontally. At subduction zones like the Pacific Ring of Fire, one plate dives beneath another. At divergent boundaries like the Mid-Atlantic Ridge, plates pull apart and magma wells up.
The rock along these boundaries deforms elastically under stress, storing energy like a compressed spring. When the accumulated stress exceeds the frictional strength of the fault, the rock ruptures and slips. The stored elastic energy radiates outward as seismic waves — and the ground shakes.
The Three Types of Seismic Waves
An earthquake generates several types of waves, each with different properties:
P-Waves (Primary Waves)
P-waves are compressional waves — they push and pull the rock in the same direction they travel, like sound waves in air. They are the fastest seismic waves, travelling through the Earth’s crust at about 6 km/s and through the mantle at up to 13 km/s. Crucially, P-waves can travel through any material: solid rock, liquid magma, and even the Earth’s liquid outer core.
P-waves arrive first at seismograph stations, which is why they are called “primary.”
S-Waves (Secondary Waves)
S-waves are shear waves — they move the rock perpendicular to the direction of travel, like shaking a rope up and down. They are slower than P-waves, travelling at about 60–70% of the P-wave speed.
The critical property of S-waves is that they cannot travel through liquids — liquids have no rigidity and cannot sustain shear forces. This single fact revealed one of the Earth’s greatest secrets.
Surface Waves
When body waves (P and S) reach the surface, they generate surface waves that travel along the Earth’s exterior. Love waves produce horizontal shearing, while Rayleigh waves create an elliptical rolling motion. Surface waves are slower but often more destructive because they concentrate energy near the surface where people live.
Mapping the Earth’s Interior
In the early 20th century, seismologists noticed something remarkable: for earthquakes on one side of the Earth, there was a “shadow zone” — a band between 104° and 140° from the epicentre where no direct P-waves arrived, and a much larger zone where no S-waves arrived at all.
The explanation: a liquid layer deep inside the Earth was refracting P-waves away from certain angles and completely blocking S-waves. This was the discovery of the liquid outer core, at a depth of about 2,900 km.
Later, faint P-wave arrivals within the shadow zone revealed that inside the liquid core sits a solid inner core — a ball of iron and nickel about the size of the Moon, at a temperature similar to the surface of the Sun but kept solid by enormous pressure.
Today, a global network of thousands of seismograph stations records every significant earthquake. Advanced computational techniques — seismic tomography — use arrival times from millions of recorded earthquakes to build three-dimensional maps of the Earth’s interior, revealing mantle plumes, subducting slabs, and lateral variations in temperature and composition.
Earthquake Measurement
The magnitude of an earthquake quantifies the energy released at the source. The modern moment magnitude scale (Mw) has replaced the older Richter scale for large earthquakes. It is logarithmic: each whole number increase represents roughly 32 times more energy. A magnitude 5 earthquake releases about 32 times less energy than a magnitude 6, and about 1,000 times less than a magnitude 7.
Intensity, measured on the Modified Mercalli scale, describes the shaking felt at a particular location and depends on distance, depth, local geology, and building construction — not just magnitude.
Early Warning Systems
While predicting earthquakes remains elusive, early warning is increasingly effective. Because electromagnetic signals travel at the speed of light — vastly faster than seismic waves — sensors near the epicentre can detect the first P-waves and transmit a warning to distant cities before the destructive S-waves and surface waves arrive.
Japan’s earthquake early warning system can provide 10–30 seconds of warning to Tokyo for offshore earthquakes. That is enough time to stop trains, open fire station doors, alert surgeons to step back from patients, and trigger automated shutdown of nuclear power plants.
Mexico City, Istanbul, and the US West Coast have deployed similar systems, with ongoing research into extending warning times through denser sensor networks and machine-learning detection algorithms.
Earthquakes and Fundamental Physics
Seismology connects directly to fundamental physics. The behaviour of seismic waves is governed by the same wave equations that describe sound, light, and quantum mechanical wave functions. The study of how waves propagate through heterogeneous media, scatter off boundaries, and interfere with each other is shared between seismology, acoustics, optics, and even gravitational wave astronomy.
In recent years, scientists have even explored using the Earth itself as a particle detector. Seismic sensors could potentially detect the incredibly rare interactions of high-energy neutrinos with rock deep inside the planet — connecting the physics of earthquakes to the physics of the cosmos.
The ground beneath us is never truly still. Every tremor, from the tiniest microseism to the largest megathrust earthquake, carries information about the hidden world beneath our feet.
Frequently Asked Questions
What causes earthquakes?
Earthquakes occur when accumulated stress in Earth's crust is suddenly released along geological faults — fractures where tectonic plates or rock masses meet. As plates move past each other, friction locks them in place, building strain energy over years or centuries. When the stress exceeds the friction, the rock slips abruptly, releasing energy as seismic waves.
What is the difference between P-waves and S-waves?
P-waves (primary waves) are compressional waves that push and pull rock in the direction of travel, like sound waves in air. They are the fastest seismic waves and travel through solids, liquids, and gases. S-waves (secondary waves) are shear waves that move rock side to side, perpendicular to the direction of travel. They travel only through solids — their inability to pass through Earth's outer core proved it is liquid.
How do seismic waves map Earth's interior?
Seismic waves change speed and direction when they encounter boundaries between different materials inside the Earth. By recording the arrival times of P-waves and S-waves at seismograph stations around the world, geophysicists can calculate the depth and properties of Earth's layers — the crust, mantle, outer core, and inner core — much like a medical CT scan uses X-rays to image the body.
Can we predict earthquakes?
Reliable short-term earthquake prediction remains beyond current science. We can identify high-risk zones, estimate long-term probabilities, and provide seconds to minutes of warning after an earthquake begins (early warning systems). But predicting the exact time, location, and magnitude of a future earthquake is not yet possible, despite decades of research.