Understanding Earthquake Magnitude

When an earthquake occurs, the most widely reported number is its magnitude — a measure of the energy released at the earthquake's source. Understanding what magnitude means helps you interpret earthquake reports and assess risk.

The Richter Scale vs. Moment Magnitude

Most people have heard of the Richter scale, developed by Charles Richter in 1935. It was revolutionary for its time but has significant limitations — it was designed for a specific type of seismograph and works poorly for very large or very distant earthquakes.

Today, scientists almost universally use the moment magnitude scale (Mw), which measures the total energy released by an earthquake based on the area of fault that ruptured, how much it slipped, and the rigidity of the rock. For small and medium earthquakes, Richter and Mw values are very similar — which is why the general public still talks about "the Richter scale" even when that's not what scientists are using.

The Logarithmic Scale

Magnitude is logarithmic. Each whole-number increase represents:

• About 10 times more ground shaking (amplitude)
• About 32 times more energy released

So an M7 releases about 32 times more energy than an M6, and roughly 1,000 times more than an M5. An M9 releases about 1,000 times more energy than an M7. This is why large earthquakes are so catastrophically more powerful than small ones, even though the numbers look close.

Magnitude Scale Reference

• M0–1.9 (Micro): Not felt. Detected only by instruments.
• M2.0–2.9 (Minor): Rarely felt. Recorded by seismographs.
• M3.0–3.9 (Minor): Often felt near the epicentre; rarely causes damage.
• M4.0–4.9 (Light): Widely felt. Indoor items may rattle. Minor damage possible.
• M5.0–5.9 (Moderate): Felt by all. Significant damage to poorly constructed buildings.
• M6.0–6.9 (Strong): Destructive in populated areas. Strong shaking up to 160 km from epicentre.
• M7.0–7.9 (Major): Serious damage over large areas. Can cause widespread destruction.
• M8.0–8.9 (Great): Can destroy communities near the epicentre. Felt across entire regions.
• M9.0+ (Rare great): Devastating. Near total destruction near epicentre. Can trigger tsunamis.

Magnitude vs. Intensity: What's the Difference?

Magnitude is an objective measure of energy at the source — it doesn't change with distance.

Intensity describes how strongly shaking is felt at a particular location. The same earthquake will have a single magnitude but different intensities at different distances. The Modified Mercalli Intensity (MMI) scale (I to XII) is commonly used to describe intensity, from "not felt" (I) to "total destruction" (XII).