What Does an Earthquake Really Measure?
When an earthquake occurs, we hear terms like "magnitude 6.2" or "intensity VII." But do you know what they really mean? Confusion between the Richter scale and the Mercalli scale is very common, even among people living in seismic zones. In this article, we clearly and practically explain the fundamental differences between both scales, how they are used, and why it is important to know them for natural risk management.
Richter Scale: The Earthquake's Magnitude
The Richter scale, also known as the local magnitude scale (ML), was developed in 1935 by seismologist Charles F. Richter. It measures the energy released at the earthquake's focus using instruments called seismographs. It is a logarithmic scale: each whole number represents a tenfold increase in seismic wave amplitude and approximately 31.6 times more energy released.
Key Features of the Richter Scale
- Objective and numerical: based on instrumental data, not human perception.
- Open-ended scale: theoretically has no upper limit, though practical values rarely exceed 9.5.
- Unique per earthquake: an earthquake has a single Richter magnitude, regardless of where it is measured.
- Useful for small to moderate earthquakes: works best for local events with magnitude below 6.5.
For example, the Valdivia earthquake (Chile, 1960) had a magnitude of 9.5 on the Richter scale, making it the most powerful ever recorded.
Mercalli Scale: The Earthquake's Intensity
The Mercalli scale, formally known as the Modified Mercalli Intensity Scale (MMI), measures the observable effects of an earthquake on people, objects, and structures. It was created by Italian seismologist Giuseppe Mercalli in 1902 and later modified. Unlike Richter, it does not use instruments but relies on direct observation of damage and perceptions.
Key Features of the Mercalli Scale
- Subjective and qualitative: depends on eyewitness accounts and visual assessments.
- 12-level scale: ranges from I (imperceptible) to XII (total destruction).
- Varies by location: the same earthquake has different Mercalli intensities in different areas.
- Very useful for damage assessment: helps plan emergency responses and reconstruction.
For instance, the San Juan earthquake (Argentina, 1944) reached a Mercalli intensity of IX in the city, with massive destruction of buildings, but in outlying areas it was only V (moderate).
Key Differences Between Richter and Mercalli
To understand it better, imagine the Richter scale as the power of an engine (energy released), while the Mercalli scale is like the damage that engine causes when crashing into different obstacles (effects on terrain and structures).
Quick Comparison Table
- What it measures: Richter measures energy released; Mercalli measures observable effects.
- Measurement basis: Richter uses seismographs; Mercalli uses human observation.
- Scale: Richter is logarithmic (open-ended); Mercalli is ordinal (I to XII).
- Variability: Richter is unique per earthquake; Mercalli varies by area.
- Practical utility: Richter is ideal for classifying earthquakes; Mercalli is key for assessing damage and risks.
Which Is More Important for Risk Management?
Both scales are complementary. The Richter scale allows seismologists to compare earthquakes and study patterns, while the Mercalli scale is essential for natural risk management, urban planning, and emergency response. For example, a magnitude 6.0 earthquake can cause very different damage if it occurs in a city with earthquake-resistant buildings or in a rural area with fragile homes.
Practical Tips for the Reader
- Don't rely solely on magnitude: a 5.5 earthquake can be devastating if it is shallow and close to populated areas.
- Check intensity maps: after an earthquake, agencies like INPRES (Argentina) or USGS publish Mercalli intensity maps.
- Prepare your home based on your area: if you live in a high-seismicity region, reinforce structures and have an emergency plan.
Conclusion: Two Sides of the Same Coin
The Richter scale and the Mercalli scale are not competing; they complement each other. Richter tells you how much energy the earthquake released; Mercalli tells you how it was felt and what damage it caused. Knowing both helps you better interpret the news, understand risks in your area, and make informed decisions to protect your life and property. At Contingencias, we monitor both parameters to offer you accurate and useful alerts for each seismic event.