Measurement of Earthquakes

 Earthquakes are measured using two primary frameworks: Magnitude (the total energy released) and Intensity (the shaking experienced at a specific location). The fundamental unit used to express this energy mathematically is the Moment Magnitude \(M_{w}\), which has largely replaced the older Richter scale. [1, 2]

Here are the specific scales and units used to measure earthquakes:

1. Earthquake Magnitude (Energy)

Magnitude measures the size of the earthquake at its source and is represented by a logarithmic scale. An increase of one whole number means roughly 32 times more energy is released. [1, 2]

• Moment Magnitude Scale (Mw): The modern standard used by seismologists worldwide. It measures the physical area of the fault rupture and the total energy (M₀ representing seismic moment) released during the quake. [, 2]
• Richter Scale: Developed in 1935, this scale measures the maximum amplitude of seismic waves. It is generally used for smaller, local earthquakes. [1, 2]

2. Earthquake Intensity (Shaking & Damage)

Intensity describes the severity of the earthquake's effects on people, structures, and the earth's surface at a given location. [1, 2]

• Modified Mercalli Intensity (MMI) Scale: Measures intensity using Roman numerals from I (not felt) to XII (total destruction).
• Rossi-Forel Scale: Another historical intensity scale that grades earthquakes from I to X based on observable effects and human perception. [1, 2]

3. Physical Ground Motion (Engineering)

For structural engineering and hazard assessment, seismologists measure the physical ground waves directly using an instrument called an accelerograph. [1]

• Peak Ground Acceleration (PGA): Measures the maximum acceleration of the ground during shaking, often expressed as a percentage of standard gravity (g).
• Peak Ground Velocity (PGV) & Displacement (PGD): Measures how fast and how far the ground moves during the seismic event. [1]

Further Exploration

• Understand the physics of fault lines via USGS Earthquake Glossary.
• View the exact Roman numeral definitions on the Michigan Tech Modified Mercalli Scale.
• Learn how the Richter scale operates mathematically through the Britannica Richter Scale Guide.