Earthquakes happen when built‑up stress inside the Earth’s crust is suddenly released along cracks in the rock called faults, sending out seismic waves that shake the ground.

Simple explanation

Imagine Earth’s outer shell as a set of giant, slowly moving plates that fit together like a jigsaw puzzle.

Where these plates meet, they sometimes get stuck instead of sliding smoothly, so stress builds up in the rocks on both sides of the boundary.

When the rocks can’t hold any more stress, they suddenly break and slip, releasing stored energy as vibrations we feel as an earthquake.

The point inside the Earth where this break starts is called the focus (or hypocenter); the spot directly above it on the surface is the epicenter.

What actually causes the shaking

  • Tectonic plates are constantly but very slowly moving over the softer, semi‑fluid layer beneath them.
  • Their interactions at boundaries (colliding, sliding past, or pulling apart) create faults and zones of high stress.
  • Over time, rocks deform elastically, storing strain energy like a bent ruler.
  • Once the stress is greater than the rock’s strength, the fault suddenly slips, and the rocks “rebound” to a less‑strained shape (elastic rebound).
  • This rapid slip releases energy as seismic waves that radiate outward, causing the ground to shake.

Types of plate motions that create earthquakes

  • Convergent boundaries: Plates move toward each other; one may sink beneath the other (subduction), generating powerful deep and shallow earthquakes.
  • Transform boundaries: Plates slide horizontally past one another (like the San Andreas Fault), often producing frequent shallow but sometimes strong quakes.
  • Divergent boundaries: Plates move apart, and new crust forms; these zones usually have smaller, more frequent earthquakes.

Seismic waves and what we feel

When an earthquake occurs, several kinds of seismic waves spread through and along the Earth.

  • Body waves travel through the interior:
    • P waves (primary) arrive first and compress/stretch the ground.
    • S waves (secondary) arrive next and move the ground side‑to‑side or up‑and‑down.
  • Surface waves move along the surface and typically cause the strongest, most damaging shaking in populated areas.

How strong it feels depends on:

  • How much energy was released (magnitude).
  • How far you are from the epicenter.
  • Local ground conditions (soft sediments can amplify shaking).

Other ways earthquakes can occur

While most earthquakes are tectonic, a few other processes can trigger similar shaking:

  • Volcanic activity: Movement of magma and pressure changes in volcanoes can produce volcanic earthquakes near active volcanoes.
  • Human activities: Large reservoirs, mining, or deep fluid injection can sometimes induce small to moderate earthquakes by changing stress on existing faults.

In short, earthquakes occur when slowly built‑up tectonic stress is suddenly released along faults, sending out seismic waves that travel through the Earth and shake the surface.

Information gathered from public forums or data available on the internet and portrayed here.