What is an earthquake?

What is an earthquake?

An earthquake is the sudden shaking of the ground caused by energy being released inside the Earth. This happens when rocks in the Earth’s crust suddenly break or move along cracks called faults. When this movement happens, energy travels outwards as seismic waves, making the ground shake.

Earthquakes can be very small or extremely powerful. Some are so weak that people do not notice them, while others can destroy buildings, roads and bridges in just a few seconds.

Why do earthquakes happen?

Most earthquakes are caused by the movement of tectonic plates. The Earth’s lithosphere is broken into large plates that slowly move over the softer asthenosphere below. As plates move, they can become stuck because of friction. Pressure builds up over time, and when the rocks finally slip, the stored energy is released as an earthquake.

Earthquakes are common at plate boundaries:

• Destructive boundaries – plates move towards each other.
• Constructive boundaries – plates move apart.
• Conservative boundaries – plates slide past each other.

Earthquakes can also be caused by volcanic eruptions, landslides, mining activity and some human actions such as underground explosions.

Key earthquake terms

The place inside the Earth where the earthquake starts is called the focus (or hypocentre).

The point on the Earth’s surface directly above the focus is called the epicentre. This is often where the strongest shaking and most damage happen.

How are earthquakes measured?

Earthquakes are recorded using instruments called seismometers. These detect ground movement and help scientists measure the size, depth and location of an earthquake.

The strength of an earthquake is measured using the Moment Magnitude Scale (Mw). Each increase in the number means a much stronger earthquake.

Why are earthquakes dangerous?

Earthquakes can be devastating, especially in crowded cities or poorer countries where buildings may be less safe. Effects can include:

• collapsed buildings
• cracked roads and bridges
• injuries and deaths
• loss of electricity, gas and water
• fires caused by damaged pipes or cables
• landslides and tsunamis

Some earthquakes last only a few seconds, but their impacts can continue for months or even years.