physics

Seismic moment

Seismic moment is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Seismic moment rather than just read about it. In short: Seismic moment is a quantity used by seismologists to measure the size of an earthquake. The scalar seismic moment M 0 {\displaystyle M_{0}} is defined by the equation M 0 = μ A D {\displaystyle M_{0}=\mu AD} , where μ {\displaystyle \mu } is the shear modulus of the rocks involved in the earthquake (in pascals (Pa), i.e. newtons per square meter) A {\displaystyle A} is the area of the rupture along the geologic fau…

Key takeaways

Seismic moment belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Seismic moment to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Seismic moment from memory before moving on to harder problems.

Reference excerpt

Seismic moment is a quantity used by seismologists to measure the size of an earthquake. The scalar seismic moment M 0 {\displaystyle M_{0}} is defined by the equation

M 0 = μ A D {\displaystyle M_{0}=\mu AD} , where

μ {\displaystyle \mu } is the shear modulus of the rocks involved in the earthquake (in pascals (Pa), i.e. newtons per square meter)

A {\displaystyle A} is the area of the rupture along the geologic fault where the earthquake occurred (in square meters), and

D {\displaystyle D} is the average slip (displacement offset between the two sides of the fault) on A {\displaystyle A} (in meters).

M 0 {\displaystyle M_{0}} thus has dimensions of torque, measured in newton meters. The connection between seismic moment and a torque is natural in the body-force equivalent representation of seismic sources as a double-couple (a pair of force couples with opposite torques): the seismic moment is the torque of each of the two couples. Despite having the same dimensions as energy, seismic moment is not a measure of energy. The relations between seismic moment, potential energy drop and radiated energy are indirect and approximative. The seismic moment of an earthquake is typically estimated using whatever information is available to constrain its factors. For modern earthquakes, moment is usually estimated from ground motion recordings of earthquakes known as seismograms. For earthquakes that occurred in times before modern instruments were available, moment may be estimated from geologic estimates of the size of the fault rupture and the slip. Seismic moment is the basis of the moment magnitude scale introduced by the United States Geological Survey's Thomas C. Hanks and Caltech's Hiroo Kanamori, which is often used to compare the size of different earthquakes and is especially useful for comparing the sizes of large (great) earthquakes. The seismic moment is not restricted to earthquakes. For a more general seismic source described by a seismic moment tensor M i j {\displaystyle M_{ij}} (a symmetric tensor, but not necessarily a double couple tensor), the seismic moment is

M 0 = 1 2 ( M i j 2 ) 1 / 2 {\displaystyle M_{0}={\frac {1}{\sqrt {2}}}(M_{ij}^{2})^{1/2}}

See also Richter scale Moment magnitude scale

Sources

Worked examples

Example 1 — a first encounter with Seismic moment

Start with the simplest possible case. Write down what Seismic moment claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Seismic moment before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Seismic moment ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Seismic moment

In research: Seismic moment appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Seismic moment in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Seismic moment is common in secondary-school and first-year university syllabi. It links to neighbouring topics Moment (physics), Seismology measurement, Seismology stubs, so understanding it makes those chapters shorter.
In everyday life: Look for Seismic moment outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Seismic moment in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Seismic moment means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Seismic moment out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Seismic moment in simple terms?

Seismic moment is a quantity used by seismologists to measure the size of an earthquake. The scalar seismic moment M 0 {\displaystyle M_{0}} is defined by the equation M 0 = μ A D {\displaystyle M_{0}=\mu AD} , where μ {\displaystyle \mu } is the shear modulus of the rocks involved in the earthquak…

Why does Seismic moment matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Seismic moment?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Seismic moment.