How Earthquake Magnitudes Are Calculated

Earthquake magnitude is a measure of the energy released during an earthquake. Modern monitoring agencies use several methods to calculate magnitude, each providing insight into the size and impact of a seismic event.

Moment Magnitude Scale (Mw)

The moment magnitude scale is the most widely used today. It considers:

• The area of the fault that slipped
• The amount of movement
• The strength of the rocks involved

Mw provides consistent measurements for both small and extremely large earthquakes.

Local Magnitude (ML)

Often known as the “Richter scale,” ML was developed in the 1930s. It measures the amplitude of seismic waves recorded on local instruments. While still used for smaller earthquakes, it becomes less accurate for large or distant events.

Body‑Wave and Surface‑Wave Magnitudes

These scales measure specific types of seismic waves:

• Mb (body‑wave magnitude)
• Ms (surface‑wave magnitude)

They help scientists understand how energy travels through Earth’s interior and across its surface.

Why Magnitude Matters

Magnitude helps determine:

• The potential for damage
• How widely an earthquake will be felt
• The energy released
• Comparisons between different events

Magnitude is one of the most important pieces of information in earthquake reporting.