What Does an Earthquake Really Measure?
When an earthquake occurs, we often hear two terms: Richter scale and Mercalli scale. Although both measure earthquakes, they do so in very different ways. While one focuses on the energy released, the other describes the visible effects. In this article, we explain their differences, how they are used, and why both are important for understanding seismic activity in South America.
Richter Scale: The Magnitude of the Earthquake
The Richter scale, also called the local magnitude scale, was developed in 1935 by Charles Richter and Beno Gutenberg. It measures the seismic energy released at the earthquake's hypocenter using data from seismographs. It is a logarithmic scale: each point increase represents 10 times more wave amplitude and approximately 31 times more energy released.
Key Features of the Richter Scale
- Objective and quantitative: based on instrumental records.
- Single value per earthquake: an earthquake has only one Richter magnitude, regardless of where it is measured.
- Limited range: works well for local earthquakes with magnitudes below 6.5. For large earthquakes, scales like the moment magnitude scale (Mw) are used.
- Open-ended scale: has no theoretical upper limit, although the largest recorded was 9.5 in Chile (1960).
Example: a magnitude 5.0 earthquake on the Richter scale releases 31 times more energy than a magnitude 4.0 earthquake.
Mercalli Scale: The Intensity of the Earthquake
The Mercalli scale, created by Giuseppe Mercalli in 1902 and later modified (Modified Mercalli Scale or MM), measures the intensity of the earthquake based on its effects on people, buildings, and the ground. It is a qualitative scale ranging from I (imperceptible) to XII (total destruction).
Key Features of the Mercalli Scale
- Subjective and descriptive: depends on human observation and structural damage.
- Varies by location: the same earthquake can have different intensity levels in different places.
- Fixed range: goes from I to XII, with no decimal intermediate values.
- Useful for damage assessment: helps plan emergency responses and building codes.
Example: a magnitude 6.0 earthquake may have intensity VII at the epicenter (moderate damage) and only III in a city 100 km away (felt by few).
Key Differences Between Richter and Mercalli
To better understand these scales, here is a direct comparison:
- What they measure: Richter measures energy released (magnitude); Mercalli measures effects (intensity).
- Basis: Richter is instrumental (seismographs); Mercalli is observational (testimonies and damage).
- Value: Richter gives a single number for the earthquake; Mercalli gives varying values depending on the area.
- Scale: Richter is logarithmic and open-ended; Mercalli is linear and limited to 12 degrees.
- Practical use: Richter is useful for scientists; Mercalli is more understandable for the public and civil protection agencies.
Why Both Scales Are Important in South America
South America is one of the most seismically active regions in the world, due to the subduction of the Nazca Plate beneath the South American Plate. Countries like Chile, Peru, Ecuador, and Colombia experience earthquakes frequently. Knowing both scales allows:
- Risk assessment: magnitude (Richter) indicates destructive potential, while intensity (Mercalli) shows the real impact on communities.
- Building design: seismic codes are based on historical intensities (Mercalli) to define resistance standards.
- Public alerts: emergency agencies use intensity to prioritize affected areas.
For example, the 2010 Chile earthquake (magnitude 8.8) had intensities ranging from IX in the epicentral area to IV in Buenos Aires. While the magnitude was a single number, the intensity allowed coordination of aid based on actual damage.
Practical Tips for Interpreting an Earthquake
When you hear about an earthquake, keep these points in mind:
- Don't rely solely on magnitude: a magnitude 6.0 earthquake can cause severe damage if it is shallow or occurs in an area with vulnerable buildings.
- Look for the intensity map: agencies like the USGS or the National Seismological Center publish Mercalli intensity maps showing the most affected areas.
- Learn about depth: shallow earthquakes (less than 70 km) tend to generate higher intensity than deep ones, even with the same magnitude.
In short, the Richter scale tells us how much energy the earthquake released, while the Mercalli scale describes how it was felt and what damage it caused. Both are complementary and essential for understanding and managing seismic risks in our region.