What does an earthquake really measure?
When an earthquake occurs, it's common to hear two terms: 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 describing an earthquake, but each answers different questions. In this article, we explain their differences, how they are calculated, and why both are important for understanding earthquakes.
What is the Richter scale?
The Richter scale, also called the local magnitude scale (ML), was developed in 1935 by Charles F. Richter and Beno Gutenberg at the California Institute of Technology. It measures the energy released at the earthquake's focus (hypocenter) using seismic waves recorded by seismographs.
Key characteristics of the Richter scale
- Logarithmic scale: each one-point increase represents 10 times more wave amplitude and approximately 31.6 times more energy released.
- Single value: an earthquake has only one Richter magnitude, regardless of where it is measured.
- Typical range: from 0 to 9, although theoretically there is no upper limit. Earthquakes greater than 9 are extremely rare.
- Accuracy: works best for local earthquakes (up to 600 km away) and magnitudes between 3 and 7.
Concrete example: The Valdivia earthquake (Chile, 1960) had a magnitude of 9.5 on the Richter scale, the largest ever recorded. It released energy equivalent to 20,000 atomic bombs like the one dropped on Hiroshima.
What is the Mercalli scale?
The Mercalli scale (or Modified Mercalli Intensity scale, MM) was created by Italian seismologist Giuseppe Mercalli in 1902 and later modified. Unlike Richter, it does not measure the earthquake's energy, but rather its observable effects on people, buildings, and the ground at a specific location.
Key characteristics of the Mercalli scale
- Qualitative scale: ranges from grade I (not felt) to XII (total destruction).
- Variable value: the same earthquake can have different Mercalli intensities depending on distance from the epicenter, soil type, and building quality.
- Based on observation: determined through surveys, testimonies, and reported damage.
- Useful for areas without seismographs: allows evaluation of historical earthquakes or regions with little instrumentation.
Concrete example: The 1985 Mexico City earthquake (magnitude 8.1 Richter) had a Mercalli intensity of IX in Mexico City (severe damage), but only IV in areas far from the epicenter.
Key differences between Richter and Mercalli
To understand the differences, imagine a light bulb: Richter measures the bulb's power (energy), while Mercalli measures how much it illuminates each corner (effect).
Comparison table
- What it measures: Richter measures energy released; Mercalli measures effects and intensity.
- Scale: Richter is numerical and logarithmic (0-9+); Mercalli is qualitative (I to XII).
- Dependence: Richter is location-independent; Mercalli varies by location.
- Instrument: Richter uses seismographs; Mercalli uses human observation and damage.
- Usefulness: Richter is useful for comparing earthquakes; Mercalli serves to assess risks and plan construction.
Why are both scales still used?
Neither scale is better than the other; they complement each other. The Richter scale provides an objective measure of magnitude, ideal for seismic catalogs and scientific studies. The Mercalli scale offers practical information on how an earthquake affects communities, infrastructure, and the environment.
For example, a magnitude 6.0 Richter earthquake in an uninhabited area may have a low Mercalli intensity (III), while the same earthquake near a poorly constructed city may reach Mercalli intensity VIII. This explains why some moderate earthquakes cause disasters, while others stronger ones go unnoticed.
Practical tips for interpreting an earthquake
- Don't rely solely on magnitude: a magnitude 5 earthquake can be devastating if shallow and in an urban area.
- Check intensity maps: after an earthquake, agencies like the USGS or SHOA publish Mercalli maps showing the most affected areas.
- Prepare according to your region: in seismic areas, check the quality of your home and have an emergency plan.
- Use reliable apps: apps like Contingencias alert you about earthquakes and show real-time intensity data.
Conclusion
The Richter scale and the Mercalli scale are complementary tools that help us understand earthquakes from two perspectives: the energy released and their real effects. While Richter answers the "how much," Mercalli answers the "how" and "where." Knowing their differences allows you to better interpret seismic news and make informed decisions for your safety.
Remember: during an earthquake, stay calm, find a safe spot, and follow authorities' instructions. Information is your best ally.