What is an Earthquake?

An earthquake is a sudden, rapid shaking of the ground caused by the movement of rock beneath Earth's surface. This movement releases energy that travels outward as seismic waves, which we feel as the ground shaking.

What Causes Earthquakes?

The Earth's outer shell — the crust — is broken into large pieces called tectonic plates. These plates float on top of the semi-molten mantle and are constantly moving, albeit very slowly (a few centimetres per year). Where two plates meet, they can:

• Slide past each other (transform boundary — e.g. the San Andreas Fault in California)
• Collide head-on (convergent boundary — e.g. the Himalayan region)
• Pull apart (divergent boundary — e.g. the Mid-Atlantic Ridge)

At these boundaries, stress builds up over time as plates push and grind against each other. When the accumulated stress exceeds the strength of the rock, it ruptures — releasing energy in the form of seismic waves. This rupture point underground is called the hypocenter (or focus). The point directly above it on the Earth's surface is the epicentre.

Types of Seismic Waves

An earthquake generates several types of waves:

• P-waves (Primary waves) — The fastest waves, which compress and expand rock as they travel. They can pass through solid rock, liquid, and even air.
• S-waves (Secondary waves) — Slower than P-waves, they move rock perpendicular to the direction of travel. They cannot pass through liquids — a key fact scientists use to study Earth's interior.
• Surface waves — The slowest but most destructive waves. They travel along Earth's surface and are responsible for most of the damage felt in a large earthquake.

Where Do Earthquakes Happen?

About 90% of all earthquakes occur along a zone called the Ring of Fire, a roughly horseshoe-shaped band around the Pacific Ocean that includes Japan, the Philippines, New Zealand, the west coast of the Americas, and Alaska. Other major seismic zones include the Alpine-Himalayan belt (stretching from Italy through Turkey, Iran, and into China) and the Mid-Atlantic Ridge.

How Many Earthquakes Happen Every Day?

The USGS estimates that several million earthquakes occur worldwide every year — about 50 per day. The vast majority are too small to be felt. On average, around 17 major earthquakes (M7.0–7.9) and one great earthquake (M8.0+) occur every year globally.

Are Earthquakes Getting More Common?

Earthquakes are not becoming more frequent. What has changed is our ability to detect them. Modern seismographic networks can detect extremely small earthquakes that were previously undetectable, making the recorded number appear larger over time. The frequency of large, damaging earthquakes has remained relatively stable historically.