How Are the Magnitude and Strength of an Earthquake Measured?

Earthquakes rank among the most dramatic and unpredictable natural phenomena on Earth. Yet when one occurs, scientists promptly provide two crucial numbers: the magnitude and the intensity (or strength of shaking). Although these terms are often used as if they mean the same thing, they describe quite different aspects of an earthquake.

This explainer unpacks what each term means, how they are measured, why multiple scales exist, and what these measurements mean for a country like Bangladesh.

Magnitude vs Intensity: Two Complementary Concepts

Magnitude

• The magnitude of an earthquake expresses its overall size — specifically, the amount of energy released at the source.
• Magnitude remains the same everywhere for a particular earthquake.
• It is calculated using measurements of seismic waves recorded by seismographs.

Intensity (or Strength of Shaking)

• Intensity describes how strongly the earthquake was felt at a particular location and the level of damage sustained.
• Intensity varies from place to place, unlike magnitude.
• It depends on distance from the fault, local soil and rock conditions, building structure, and the quake’s depth.
• It is measured through observed effects such as damage and people’s experiences.

Why the distinction matters

A moderate-magnitude earthquake may cause severe damage in a vulnerable area if the intensity is high. Conversely, a larger earthquake may cause little harm if it occurs deep underground or far from settlements.

Thus, magnitude alone cannot tell us how badly the ground shook in any given place.

How Magnitude Is Measured: The Science Behind the Numbers

Measuring magnitude involves specialised instruments, seismic wave analysis, and sophisticated modelling.

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

• Developed in the 1930s by Charles F. Richter and Beno Gutenberg for Southern California.
• Based on the logarithm of wave amplitude recorded by a standard instrument (the Wood–Anderson seismograph), corrected for distance.
• Because it is logarithmic:

  • An increase from magnitude 4.0 to 5.0 means ten times greater wave amplitude and about 31.6 times more energy release.

• The Richter scale saturates for large earthquakes (above about magnitude 7), underestimating their true size.

b) The Moment Magnitude Scale (Mw)

To overcome the limitations of the Richter scale, seismologists developed the Moment Magnitude Scale, now the global standard.

It is based on seismic moment (M₀):

M0=μ×A×DM_0 = \mu \times A \times DM0​=μ×A×D

Where:

• $\mu$ = rigidity of the rock
• $A$ = area of fault surface that slipped
• $D$ = average displacement

Magnitude is then calculated with:

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

Moment magnitude does not saturate, making it reliable for very large earthquakes.

c) Other Magnitude Types

Various other magnitude scales exist for different purposes:

• Body-wave magnitude (m_b)
• Surface-wave magnitude (M_s)
• Energy magnitude (M_e)

In public reports, “magnitude” almost always refers to moment magnitude, though the term “Richter” is still used informally.

d) The Logarithmic Nature of Magnitude

• A magnitude 6 earthquake releases 32 times more energy than a magnitude 5.
• A magnitude 7 releases roughly 1,000 times more energy than a magnitude 5.

Small numerical changes therefore correspond to enormous differences in energy.

How Intensity (Strength of Shaking) Is Measured

Magnitude tells us how large the earthquake is; intensity tells us how strong it felt at different places.

a) The Modified Mercalli Intensity (MMI) Scale

• One of the most widely used intensity scales.
• Uses Roman numerals I–XII to describe observed effects:
  • I: Not felt
  • VI: Felt by all; some heavy furniture moves
  • X: Many buildings destroyed; ground cracks appear

Intensity varies widely within a single event.

b) Instrumental Measures and ShakeMaps

Modern systems integrate:

• peak ground acceleration (PGA),
• peak ground velocity (PGV),
• eyewitness reports,
• geological data

to produce ShakeMaps, visualising where strong shaking occurred. These maps guide emergency services.

c) Factors Affecting Intensity

Local intensity depends on:

• distance from the rupture,
• depth of the quake,
• soil and rock type,
• rupture direction,
• building structure and materials.

d) Why the Same Magnitude Produces Different Intensities

A shallow magnitude 6 earthquake near a city on soft soil may be far more destructive than a deep magnitude 7 quake under the ocean.

Bringing It Together: What These Terms Mean for Us

a) What a Reported Magnitude Tells Us

A magnitude 6.5 earthquake implies:

• a significant release of energy,
• fault rupture at some depth,
• potential to cause shaking felt across large distances.

b) What Intensity Tells You

Intensity answers:
“How strongly did my location shake?”

• Close to epicentre + soft soil → intensity VIII–IX
• Farther away + firm rock → intensity III–IV

Emergency planning focuses more on intensity than magnitude.

c) Why Bangladesh Must Understand Both

Bangladesh lies in a complex tectonic region.
Implications include:

• Even moderate earthquakes can be dangerous due to soft sediments and dense population.
• Large offshore quakes may cause mild shaking inland but can trigger tsunamis or liquefaction.
• Urban planning must consider local shaking amplification.

The Measurement Process Step-by-Step

To Measure Magnitude

1. Seismographs record seismic waves.
2. Amplitude, distance, and wave type are analysed.
3. Seismic moment is estimated.
4. Moment magnitude is calculated.
5. Initial magnitude is published, refined with more data.
6. Agencies report depth, epicentre, and energy release.

To Measure Intensity

1. Collect instrumental and eyewitness data.
2. Map shaking distribution.
3. Assign intensity levels (e.g., MMI VII).
4. Create ShakeMaps.
5. Use results to guide emergency response.

Special Topics and Common Misunderstandings

Saturation of Older Magnitude Scales

The Richter scale underestimates large earthquakes. Moment magnitude resolves this issue.

Magnitude Does Not Predict Damage Alone

Damage varies with intensity, soil type, building standards, and depth.

“Richter” Is Often Misused

Most reported magnitudes are NOT Richter magnitudes.

Energy Release

Magnitude can be converted to energy:

log⁡E=5.24+1.44Mw\log E = 5.24 + 1.44 M_wlogE=5.24+1.44Mw​

Micro-earthquakes

Magnitudes can be negative; such quakes are too small to be felt.

Historical Earthquakes

Before instruments existed, intensity reports were used to estimate magnitude.

A Worked Example: From Fault Slip to Shaking in Your Town

A simplified scenario:

1. A fault slips over 20 km × 10 km with 1 m displacement.
2. Seismic moment is calculated.
3. Moment magnitude ≈ 6.8.
4. Instruments confirm.
5. You live 80 km away on soft soil; quake is shallow.
6. Shaking intensity may reach MMI VII–VIII.
7. Emergency teams prioritise your area.

This chain is:

fault slip → seismic moment → magnitude → seismic waves → intensity → impact

Key Take-Away Points

• Magnitude: size/energy of the earthquake; one value only.
• Intensity: shaking strength; varies by location.
• Magnitude scales are logarithmic.
• Moment magnitude (Mw) is most reliable for large events.
• Intensity depends on distance, soil, depth, and building design.
• Understanding both enables safer construction, better response, and clearer communication.

As earthquakes continue to threaten lives—especially in densely populated, geologically complex regions—how we measure and communicate their effects is vital.

A headline like “Magnitude 7.5” tells us only part of the story. What truly determines impact is how the ground shakes, how structures respond, and how prepared communities are.

Understanding the difference between magnitude (size) and intensity (felt strength) allows individuals, engineers, and policymakers to grasp the full picture, make better decisions, and reduce disaster risks.