How Earthquakes Happen
Written by Viktor Jonas · Reading time about 6 minutes
The focus lies at depth; the epicentre is merely the point directly above it. The waves spread outwards in circles.
Plates do not glide smoothly past one another. They snag. For years and decades the drive from below keeps pushing, the rock deforms elastically like a bent branch — and at some point it gives. Everything then jumps at once into the position it should have reached long ago. That jolt is the earthquake.
Hypocentre — the point at depth where the rupture starts. Also called the focus.
Epicentre — the point on the surface directly above the focus.
P wave — the fastest wave, a compression wave. It arrives first and only jolts briefly.
S wave — the slower shear wave. It shakes sideways and does most of the damage.
Fault — the fracture surface in the rock along which the blocks slip.
Why it jolts instead of sliding
The key is friction. Two blocks of rock pressed together under enormous load stick to each other. The plates keep moving regardless, but that motion is not released at the contact surface — it piles up as deformation in the surrounding rock. Think of a door handle you press down ever harder: nothing happens, nothing happens, and then it suddenly gives.
The moment static friction is exceeded, a rupture races along the fault at two to three kilometres per second. In a major earthquake that surface can be hundreds of kilometres long, and the rupture then takes one to two minutes. That is precisely why a big quake lasts longer than a small one: not because the shaking is stronger, but because the tear keeps running.
After the main shock the stress is not neatly gone. In the surrounding area it has actually increased, and the rock looks for a new balance. Hence aftershocks — at first every few minutes, then more and more rarely. For rescue teams they are the biggest hazard, because damaged buildings collapse on the second or third jolt.
Two waves and the head start
The rupture produces several kinds of wave, and they travel at different speeds. That difference is not just theory; it saves lives.
| Wave | Speed | Motion | Damage |
|---|---|---|---|
| P wave | about 6 km/s | compression, along | low, a short jolt |
| S wave | about 3.5 km/s | shear, sideways | high |
| Surface waves | about 3 km/s | rolling | highest for buildings |
In this example 8 seconds pass between the P and S arrivals.
Time difference per kilometre: 1/3.5 − 1/6 = 0.286 − 0.167 = 0.119 s/km
Distance: 8 s ÷ 0.119 s/km ≈ 67 km
With three stations the epicentre can be pinned down exactly.
Early-warning systems rest on exactly this. A sensor near the focus detects the harmless P wave, sends the alert ahead at the speed of light — and the city gains anything from seconds to half a minute before the destructive S wave arrives. In Japan that window is enough for high-speed trains to brake, lifts to stop at the next floor and open, and surgeries to pause.
💬 Mia asks Grandpa Theo
Why is a quake felt more strongly on one side of a city than the other?
Usually because of the ground. On solid rock the wave passes through. On loose sand or an old lake bed it slows down and grows larger — like water piling up into a wave as the sea gets shallower.
Can earthquakes be forecast like the weather?
No, and experts agree on that. We can say where and with what probability over the coming decades. Nobody can give you the day and hour. That is why the effort goes into construction and warning rather than prophecy.
Common mistakes
- Confusing magnitude and intensity
- Magnitude describes the energy at the focus; there is one value per quake. Intensity describes the effect at a place and differs everywhere.
- "Magnitude 8 is twice as strong as 4."
- The scale is logarithmic: one step up means roughly 32 times the energy. Between 4 and 8 lies about a million-fold difference.
- "Doorways are safe."
- Outdated advice from an era when houses were built differently. Today the recommendation is: drop, cover under a sturdy table, hold on.
- "Earthquake weather."
- There is no link between weather and quakes. The processes happen kilometres down in the rock.
A station records the P wave at 10:04:12 and the S wave at 10:04:27. How far away is the epicentre? Use 6 km/s and 3.5 km/s. Why is one station not enough to locate it?
Show solution
Time difference: 27 s − 12 s = 15 s
Difference per kilometre: 1/3.5 − 1/6 ≈ 0.119 s/km
Distance: 15 s ÷ 0.119 s/km ≈ 126 km
One station only gives the distance, that is a circle of 126 km radius around it. Only
three circles from three stations intersect at a single point — the epicentre.
Why quakes do not kill people, buildings do
The sentence sounds harsh but it is the heart of earthquake safety. A magnitude 7 event can cause a handful of deaths in a country with strict building codes and tens of thousands in another. In 2010 Haiti was hit by a 7.0 and Chile, weeks later, by an 8.8 — roughly 500 times the energy. Chile lost a few hundred people, Haiti more than a hundred thousand. The difference was not physics but construction.
Building for earthquakes mostly means letting the structure move without breaking. Properly reinforced concrete can deform and spring back. Unfired mud brick or poorly tied masonry cannot — it is stiff and brittle and collapses in on itself. Modern towers in Tokyo sit on sliding bearings or carry huge pendulum masses that swing against the motion.
Then there is the ground itself. Loose, water-saturated sand can lose its strength under strong shaking and behave briefly like a liquid. Buildings sink or tilt even though the structure itself is undamaged. That is why a soil survey belongs in the planning of any building in a seismic zone, just as much as the structural calculations.
Around half a million earthquakes are recorded worldwide every year. Roughly a hundred thousand of them can be felt at all, and about a hundred cause damage. The vast majority go unnoticed — the Earth rumbles away continuously.