Geology & Earth ScienceSeismologyA-Level
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Seismic Moment Calculator

Physical measure of earthquake size based on fault rupture.

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Seismic Moment

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Overview

The seismic moment is a fundamental measurement used in seismology to quantify the total energy released during an earthquake rupture. It relates the physical size of the faulting process to the geological properties of the crust, specifically the area of the fault, the amount of slip, and the rock rigidity.

Symbols

Variables

= Seismic Moment, = Rigidity, A = Fault Area, D = Average Slip

Seismic Moment
N⋅m
Rigidity
Pa
Fault Area
m²
Average Slip

Apply it well

When To Use

When to use: Apply this equation when calculating the objective size of an earthquake or when determining the Moment Magnitude (Mw) scale value. It is particularly useful for large earthquakes where other magnitude scales like the Richter scale tend to saturate and lose accuracy.

Why it matters: This measurement provides a physically based assessment of seismic sources, allowing scientists to relate earthquake energy to observable geological deformation. It is essential for understanding plate tectonic motions and assessing long-term seismic hazards in high-risk zones.

Avoid these traps

Common Mistakes

  • Using intensity (visual damage) instead of physical source parameters.
  • Convert units and scales before substituting, especially when the inputs mix N⋅m, Pa, m², m.
  • Interpret the answer with its unit and context; a percentage, rate, ratio, and physical quantity do not mean the same thing.

One free problem

Practice Problem

A fault ruptures in the upper crust with a shear modulus of 3.2 × 10¹⁰ Pa. If the total rupture area is 150 km² and the average slip along the fault is 2 meters, what is the resulting seismic moment?

Rigidity32000000000 Pa
Fault Area150000000 m²
Average Slip2 m

Solve for:

Hint: Convert the fault area from square kilometers to square meters before multiplying.

The full worked solution stays in the interactive walkthrough.

References

Sources

  1. Wikipedia: Seismic moment
  2. Aki, K. (1966). Generation and propagation of G waves from the Niigata earthquake of June 16, 1964. Part 2.
  3. Stein, S., & Wysession, M. (2003). An Introduction to Seismology, Earthquakes, and Earth Structure. Blackwell Publishing.
  4. Stein, Seth, and Michael Wysession. An Introduction to Seismology, Earthquakes, and Earth Structure. 2nd ed. Wiley-Blackwell, 2003.
  5. A-Level Geology — Seismology