How does the Richer scale work? Earlier today (8 September) an earthquake in southern England registered 3.3 on the Richter scale. People in the area felt the windows in their houses rattle. But what does 3.3 mean and why do we use the Richter scale?

Firstly – what are earthquakes and what causes them? Put simply, an earthquake is a violent shaking of the ground normally caused by sudden movement along a geological fault.

When earthquakes happen underwater, they can lead to tsunamis such as the 2004 Boxing Day tragedy.

## Where did the Richter scale come from?

In 1935, Charles F Richter – a scientist at the California Institute of Technology – developed the scale as a way to measure earthquakes.

The Richter scale doesn’t calculate the damage caused by an earthquake – that’s covered by the Mercalli Scale.

To measure historical earthquakes that occurred before the introduction of the Richter scale, seismologists can use contemporary damage reports.

Looking at accounts of damage to buildings, the radius in which tremors were felt and even reported changes in soil can tell a lot about the scale of an earthquake.

## How does the Richter scale work?

The scale is used to rate the magnitude, or strength, of an earthquake. This is measured by the level of energy it releases.

That level of energy is measured by a seismograph – scientific instruments securely mounted to the ground. Whenever the ground starts to shake during an earthquake, the instrument’s case also moves.

As the case moves, a suspended instrument inside it remains stable. This is the seismometer and measures the magnitude of an earthquake.

The Richter scale is a base-ten logarithmic scale. This means each step of magnitude is ten times more intense than the last.

For example, an earthquake with a magnitude of two is ten times more intense than those with a magnitude of one, and so on.

The table below sets this out and describes how each level of an earthquake feels:

## Worst earthquakes in history

Many factors affect how bad an earthquake is and that goes further than simply its magnitude. For instance, the country the earthquake happens in and how close the epicentre is to water play huge roles in how deadly an earthquake is.

The deadliest recorded earthquake in history is the 1556 Shaanxi earthquake. It measured eight on the Richter scale and killed 830,000 people.

The 21st century has seen some of the worst earthquakes in history, with some arguing this is a result of climate change.

Three of the deadliest earthquakes this century include:

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