Richter Scale vs Mercalli Scale: Key Differences

What Does an Earthquake Really Measure?

When an earthquake occurs, two terms inevitably appear in the news: magnitude and intensity. Although often used interchangeably, they represent very different concepts. The Richter scale and the Mercalli scale are the best-known tools for measuring earthquakes, but each answers different questions. While one quantifies the energy released, the other describes the effects on the surface.

Richter Scale: Earthquake Magnitude

Developed in 1935 by Charles F. Richter, this scale measures the magnitude of an earthquake, i.e., the seismic energy released at the hypocenter. It is a logarithmic scale: each one-point increase represents a tenfold increase in wave amplitude and approximately 31 times more energy released.

Key Features

• Objective: Based on seismograph data, not human perception.
• Accurate: Ideal for small to moderate earthquakes (up to magnitude 7).
• Limited: Loses accuracy for large earthquakes (magnitude above 7.5). For these cases, the moment magnitude (Mw) is used.

For example, a magnitude 6 earthquake releases 31 times more energy than a magnitude 5, and 1,000 times more than a magnitude 4. In South America, the 1960 Chile earthquake reached magnitude 9.5, the most powerful ever recorded.

Mercalli Scale: Earthquake Intensity

The Modified Mercalli (MM) scale measures the intensity of an earthquake based on its effects on people, structures, and the environment. Created by Giuseppe Mercalli in 1902 and later modified, it ranges from grade I (not felt) to XII (total destruction). It is a subjective and qualitative scale.

Key Features

• Subjective: Depends on human observations and reported damage.
• Variable: The same earthquake can have different intensities in different areas.
• Useful for emergencies: Helps prioritize response in the most affected areas.

For example, a magnitude 7 earthquake may have a Mercalli intensity of IX at the epicenter (severe damage) and only V at 100 km away (felt by almost everyone, no damage). In the 2016 Ecuador earthquake, the Mercalli intensity ranged from VIII in Pedernales to IV in Quito.

Key Differences Between Richter and Mercalli

Richter ScaleMercalli ScaleMeasures magnitude (energy)Measures intensity (effects)Objective (seismographs)Subjective (observation)Open-ended numerical scale12-degree scale (I to XII)Single value per earthquakeVaries by locationBest for small earthquakesBest for assessing damage

Which is More Useful for Risk Management?

Both scales are complementary. Richter magnitude (or moment magnitude) is essential for understanding the earthquake's energy and predicting its destructive potential. Mercalli intensity is key for immediate response: with intensity maps, emergency teams know where the most damage is.

In countries like Chile, Peru, or Argentina, civil protection agencies use both scales. For example, the Seismological Center of Chile reports magnitude, while the National Emergency Office (ONEMI) creates Mercalli intensity maps to coordinate aid.

Practical Tips for the Reader

• Don't confuse magnitude with intensity: A magnitude 6 earthquake can cause severe damage if it is shallow and occurs near a city.
• Consult official sources: Use apps like Contingencias to receive seismic alerts with magnitude data and intensity maps.
• Prepare according to your area: If you live in a seismic zone, have an emergency plan and a survival kit. The Mercalli intensity indicates the real risk in your locality.

Conclusion

The Richter scale and the Mercalli scale answer different questions: "How much energy did the earthquake release?" and "How did it affect us?" For a complete understanding of an earthquake, we need both. The next time you feel a tremor, remember that magnitude gives you the scientific data, but intensity tells you what really matters: the impact on your community.