How Is the Magnitude and Strength of an Earthquake Measured?

How Is the Magnitude and Strength of an Earthquake Measured?

Earthquakes are among the most dramatic and unpredictable natural phenomena on our planet. Yet when one occurs, scientists swiftly provide two critical numbers: the magnitude and the intensity (or strength/shaking) of the event. Although the terms “magnitude” and “strength” (or “intensity”) might seem interchangeable, they describe quite different aspects of an earthquake. In this long explainer article, we will unpack what each term means, how measurement works, why multiple scales exist, and what it all implies for us in a country like Bangladesh.

Magnitude vs Intensity: Two Complementary Concepts

Magnitude

• The magnitude of an earthquake is a number that characterises the overall size of the earthquake–how much energy was released at its source.
• Critically, magnitude is the same everywhere for a given event (ignoring rounding). There is one magnitude value for the event.
• Magnitude is usually derived from measurements of seismic waves recorded by instruments (seismographs).

Intensity (or Strength of Shaking)

• Intensity refers to how strong the shaking felt – or how much damage was done – at a particular location. It is not a single number for the event but varies by place.
• This depends on many factors besides magnitude: distance from the fault rupture, local geology/soil, building design, and depth of the quake.
• Intensity is usually measured using scales based on observed effects (for example, how people felt it, what damage occurred).

Why the distinction matters

This distinction is important because a moderate-magnitude quake in a vulnerable built-environment may cause more damage (high local intensity) than a larger magnitude event far from population or under deep crust. Simply quoting magnitude does not tell you how bad things will be on the ground at a given place.

How Magnitude is Measured: The Science Behind the Numbers

Measuring magnitude involves seismological instrumentation, wave analysis, and increasingly sophisticated physical modelling.

a) The Original “Richter” (Local Magnitude) Scale

• The famous Richter scale (or local magnitude scale, Mₗ) was developed by Charles F. Richter (and Beno Gutenberg) in the 1930s for Southern California.
• It is based on the logarithm of the amplitude of the seismic waves recorded by particular seismograph (the Wood-Anderson instrument) and an adjustment for distance.
• Because the scale is logarithmic: each whole number increase (say from 4.0 to 5.0) corresponds to a ten-fold increase in measured wave amplitude and roughly 31.6 (≈10^1.5) times more energy release.
• However, the Richter/local magnitude scale “saturates” (i.e., under-estimates size) for very large earthquakes (above about magnitude 7) or those at great depth or distance.

b) The Moment Magnitude Scale

• To address these limitations, seismologists developed the Moment magnitude scale (Mₚ or M₍w₎) (symbol MwM_wMw​).
• It is based on the concept of seismic moment (M₀), which depends on how much rock area slipped, how far it slipped, and the rigidity (strength) of the rock:

M0=μ×A×D M_0 = \mu \times A \times DM0​=μ×A×D 

where
μ\muμ = rigidity of rock,
AAA = area of fault that slipped,
DDD = average slip (displacement). 

• Then magnitude MwM_wMw​ is computed via a formula such as

Mw=23log⁡10(M0)−C M_w = \frac{2}{3} \log_{10} (M_0) - CMw​=32​log10​(M0​)−C 

(where CCC is a constant to calibrate with older magnitude scales). 

• Moment magnitude does not saturate for large quakes, and thus gives a more reliable “size” for the largest events.

c) Other Magnitude Types

• There are multiple other magnitude scales in use because of different purposes, instrument types, or regional conditions: e.g., body-wave magnitude (mₙ, m_b), surface-wave magnitude (M_s), energy magnitude (M_e).
• For example, energy magnitude attempts to quantify the actual seismic energy radiated by the earthquake.
• Despite these technicalities, for many practical public reports, the “magnitude” you see is typically the moment magnitude (or a number approximated to it) even if the term “Richter” is still used colloquially.

d) The Logarithmic Nature of Magnitude

• Because magnitude is logarithmic:
  • A magnitude 6 event releases about 32 times more energy than a magnitude 5 event (≈10^1.5).
  • A magnitude 7 event releases about 1,000 times the energy of a magnitude 5 event (≈10^(1.5×2) = 10^3).
• This means small numerical differences in magnitude correspond to very large differences in energy release.