How are earthquakes measured, and what is the difference between magnitude and intensity?
Measuring the Earth's Tremors
When the ground beneath our feet begins to shake, scientists known as seismologists use specialized tools and mathematical scales to quantify the event. Understanding how earthquakes are measured requires a look at two distinct concepts: the physical energy released (magnitude) and the observed effects on the surface (intensity).
In this guide, we will break down the instruments, the scales, and the terminology used to describe these powerful geological events.
Key Definitions
Before diving into the scales, it is essential to understand the basic vocabulary of seismology:
- Seismograph (or Seismometer): The actual instrument used to detect and record the motion of the ground.
- Seismogram: The graph or digital record produced by a seismograph, showing the arrival times and amplitudes of different seismic waves.
- Hypocenter (Focus): The exact point inside the Earth where the earthquake starts.
- Epicenter: The point on the Earth's surface directly above the hypocenter.
Magnitude vs. Intensity: The Great Distinction
One of the most common points of confusion is the difference between magnitude and intensity. While they are related, they measure fundamentally different things.
| Feature | Magnitude | Intensity |
|---|---|---|
| What it measures | The total energy released at the source. | The strength of shaking at a specific location. |
| Number of values | One single value per earthquake. | Multiple values depending on distance from the epicenter. |
| Scale used | Moment Magnitude Scale (MMS) or Richter Scale. | Modified Mercalli Intensity (MMI) Scale. |
| Data source | Calculated from seismograms. | Based on observed damage and human reports. |
The Scales of Measurement
1. The Richter Scale ($M_L$)
Developed in 1935 by Charles Richter, this was the first widely used scale. It is a logarithmic scale, meaning that each whole number increase represents a tenfold increase in measured amplitude and roughly 32 times more energy release. However, the Richter scale is less accurate for very large earthquakes or those occurring at great distances.
2. The Moment Magnitude Scale ($M_w$)
This is the modern standard used by the USGS and other global agencies. Unlike the Richter scale, which measures wave amplitude, the Moment Magnitude Scale calculates the total energy based on the area of the fault that ruptured and the rigidity of the rocks. It is much more reliable for measuring massive earthquakes (magnitude 8.0 and above).
3. The Modified Mercalli Intensity Scale (MMI)
This scale uses Roman numerals (I to XII) to describe the effects of an earthquake on people, buildings, and the natural environment.
- Level I: Not felt except by a very few under especially favorable conditions.
- Level VI: Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
- Level XII: Damage total. Waves seen on ground surfaces. Objects thrown upward into the air.
Real-World Examples
To put these numbers into perspective, let's look at some historical events:
- Valdivia, Chile (1960): The largest earthquake ever recorded, measuring 9.5 $M_w$. It caused massive tsunamis across the Pacific.
- San Francisco (1906): Estimated at 7.9 $M_w$. While the magnitude was lower than Chile, the intensity was extremely high (MMI XI) because it occurred directly under a major city.
- Haiti (2010): A 7.0 $M_w$ earthquake. Though the magnitude was "moderate" compared to the 1960 Chile quake, the lack of earthquake-resistant infrastructure led to catastrophic intensity and loss of life.
Common Pitfalls
The "10x" Misconception: Many people believe a magnitude 7 earthquake is twice as strong as a magnitude 6. In reality, because the scale is logarithmic, a magnitude 7 has 10 times the ground shaking amplitude and releases about 32 times more energy than a magnitude 6.
Confusing Magnitude with Damage: A high-magnitude earthquake in the middle of the uninhabited desert might have a very low intensity (MMI) because there are no buildings to knock down and no people to feel it. Conversely, a shallow, lower-magnitude quake directly under a city can have a devastating intensity.