Introduction to Seismic Scales
When an earthquake occurs, the media often reports its magnitude on the Richter scale or its intensity on the Mercalli scale. Although both terms are frequently used interchangeably, they measure different aspects of the same phenomenon. Understanding their differences is crucial to correctly interpreting information and preparing for seismic events.
What is the Richter Scale?
The Richter scale, developed by Charles Richter in 1935, measures the magnitude of an earthquake. That is, the amount of energy released at the focus or hypocenter. It is a logarithmic scale: each one-point increase represents approximately 31.6 times more energy released. For example, a magnitude 7 earthquake is 31.6 times more energetic than a magnitude 6 one.
Key Characteristics
- It is based on the amplitude of seismic waves recorded by seismographs.
- It is an objective and unique measure for each earthquake.
- It has no theoretical upper limit, although the largest recorded earthquakes have been around magnitude 9.5.
- Currently, the moment magnitude scale (Mw) is used for large earthquakes, but it is still popularly called Richter.
What is the Mercalli Scale?
The Mercalli scale, created by Giuseppe Mercalli in 1902 and later modified as the Modified Mercalli (MM) scale, evaluates the intensity of an earthquake. It is a subjective measure of the effects and damage caused at a specific location. It is expressed in Roman numerals from I to XII.
Key Characteristics
- It is based on observations from people, structural damage, and changes in the terrain.
- It varies according to distance from the epicenter and local soil conditions.
- The same earthquake can have different intensities in different locations.
- It is useful for assessing the impact on communities and for seismic engineering.
Fundamental Differences Between Richter and Mercalli
The main difference lies in what they measure: Richter quantifies the energy released (magnitude), while Mercalli describes the perceived effects (intensity).
- Objectivity vs. subjectivity: Richter is based on instruments; Mercalli on human observations.
- Scope: Richter assigns a single value to the earthquake; Mercalli assigns multiple values depending on location.
- Scale: Richter is logarithmic and open-ended; Mercalli is ordinal and closed (I to XII).
- Use: Richter is for scientists and early warnings; Mercalli for damage assessment and emergency response.
Practical Examples to Understand Them
Imagine a magnitude 7.0 earthquake on the Richter scale. At the epicenter, the Mercalli intensity could be IX (destructive), but 100 km away, the intensity could be V (moderate). Thus, while Richter reports a single value, Mercalli reflects the variable experience in different places.
Another example: a deep magnitude 6.5 earthquake may cause less surface damage than a shallow magnitude 5.5 one. Richter does not distinguish depth, but Mercalli does capture the actual impact.
Why is it important to know both scales?
For risk management and citizen preparedness, both scales are complementary. Richter helps seismologists understand the source and issue early warnings. Mercalli allows authorities to assess response and communities to understand their vulnerability.
In applications like Contingencias, we use data from both scales to send accurate alerts and practical advice based on the expected intensity at your location.
Practical Tips for an Earthquake
- Learn about the magnitude (Richter) to gauge the event, but pay attention to the intensity (Mercalli) in your area.
- If you feel an earthquake, protect yourself first and then seek official information.
- Know the difference: a magnitude 6 earthquake at sea is not the same as an intensity VIII one in your city.
- Use alert applications like Contingencias to receive personalized notifications.
Conclusion
The Richter scale and the Mercalli scale do not compete; they complement each other. While one measures the energy of the earthquake, the other describes how we feel it and what damage it causes. Understanding both allows you to better interpret seismic information and make more informed decisions to protect yourself and your family.