How Is the Magnitude and Strength of an Earthquake Measured?
Published: 22 Nov 2025, 01:58 am
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
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
(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.
How Intensity (Strength of Shaking) is Measured
While magnitude describes the size of an earthquake, intensity describes the shaking and impact at particular locations.
a) The Modified Mercalli Intensity (MMI) Scale
One of the most widely known intensity scales is the Modified Mercalli Intensity Scale (MMI).
It uses Roman numerals I through XII to describe effects from not felt (I) to nearly total destruction (XII).
Example: Level VI might correspond to “felt by all, some heavy furniture moved, slight damage” etc. Level X might be “many buildings destroyed, ground visible cracks”, etc.
Important to note: the intensity value in one town may be high (say VIII) while many tens or hundreds of kilometres away it may drop to III or IV, even though the magnitude is the same.
b) Instrumental Measures and ShakeMaps
Modern systems combine felt reports with instrument data (e.g. peak ground acceleration (PGA), peak ground velocity (PGV)) and geological conditions to compute maps of shaking intensity (often called “ShakeMap” in the US) soon after the quake.
These maps help emergency responders quickly see which areas likely had strong shaking and may be heavily damaged.
c) Factors Affecting Intensity
The intensity experienced at a location depends on many variables beyond magnitude:
Distance from the fault rupture: closer sites generally feel stronger shaking.
Depth of the earthquake focus: shallower quakes tend to produce stronger surface shaking for the same magnitude.
Local geology/soil type: soft sediment or unconsolidated soils tend to amplify shaking; bedrock tends to reduce it.
Directionality and rupture mechanism: how the fault ruptured (its geometry) changes how seismic waves propagate.
Building construction and vulnerability: though this is more about damage than pure shaking, local construction influences how shaking translates into damage.
d) Why the Same Magnitude Can Produce Different Intensities
Because of the combination of the above factors, two earthquakes of the same magnitude may have very different effects (intensities) at particular locations. Equally, a smaller magnitude quake under very unfavourable circumstances (very shallow, close to a city built on soft soil) may be more destructive than a larger magnitude quake far away under favourable geological conditions.
Bringing It Together: What the Terms Mean for Us
To make sense of these ideas, let’s consider how they apply practically, especially in regions like Bangladesh, which are tectonically active and densely populated.
a) What a Reported Magnitude Tells Us
When the seismological centre reports an earthquake of magnitude say M 6.5, we know:
It was a fairly large release of energy.
It occurred somewhere in the earth’s crust at some depth and ruptured some fault.
The event could produce damaging shaking at many tens or even hundreds of kilometres, depending on geology, depth, etc.
b) What Intensity Tells Us About Local Effects
However, for you living in a town or city, what you really care about is: How strongly did (or will) your location shake? That’s the intensity.
If you are near the epicentre and on soft soils, you might experience intensity VIII or IX (very strong shaking) and serious damage.
If you are farther away or on firmer bedrock, your intensity might be III or IV (light to moderate shaking), maybe just a scare.
So emergency planning and building codes focus more on intensity (shaking) than just magnitude.
c) Why Bangladesh Needs to Understand Both
Bangladesh sits in a complex tectonic region (e.g., the Himalayan thrust, the Bangladesh basin, and the Bay of Bengal margin). Some implications:
Even moderate-magnitude earthquakes under unfavourable conditions (very shallow, near dense settlement, on soft soil) can cause high intensities and major damage.
Conversely, a large magnitude earthquake far away (say off-shore) may produce modest intensities in Dhaka, though secondary effects (tsunamis, liquefaction) remain a concern.
Building design, land-use planning and emergency response must consider shaking intensity vulnerability (soil, distance, building types) as much as magnitude.
Public awareness efforts must emphasise: magnitude is one piece, but local shaking, building resilience and geology matter hugely.
The Measurement Process Step-by-Step
Here’s a simplified workflow of how seismologists measure magnitude and intensity:
For Magnitude
Seismic waves from the earthquake are recorded on seismographs around the world.
The amplitude of appropriate waves (P, S, surface waves) is measured; corrections are applied for instrument type, distance, crustal attenuation.
For moment magnitude: estimate seismic moment (slip amount × fault area × rock rigidity).
Compute MwM_wMw via the formula (e.g., Mw=23log10M0−CM_w = \frac{2}{3} \log_{10} M_0 - CMw=32log10M0−C).
Publish an initial magnitude (often preliminary within minutes) then refined values as more data come in.
Seismological agencies may provide magnitude types, depth, location (epicentre/hypocentre) and possibly an estimate of energy released.
For Intensity
Collect data on shaking: instrument measures (PGA/PGV) and rapid felt-report questionnaires (how people felt, observed damage).
Map the distribution of shaking: the areas closer to the rupture will often have higher values; but also amplify in certain soils.
Assign an intensity value (e.g., MMI VII) for each location or zone, describing observed effects (people felt it, objects moved, structural damage).
Produce “ShakeMap” or isoseismal maps (contours of equal intensity) showing how shaking varied spatially.
Use this information for emergency responders to prioritise damaged regions, and for engineering/hazard analysis.
Special Topics and Common Mis-understandings
Saturation & Why Older Magnitude Scales Were Limited
The Richter/local magnitude scale tends to under-estimate the size of very large earthquakes because it is based on wave amplitudes that saturate (stop increasing proportionally) for large ruptures.
The moment magnitude scale was introduced to overcome this limitation so that very large events (e.g., magnitude 8-9) can still be accurately compared.
Why Magnitude Doesn’t Fully Predict Damage
Even though magnitude indicates energy released, damage depends on intensity (shaking), which can vary widely by location.
Example: A magnitude 8 event very deep might generate less surface shaking (lower intensity) than a magnitude 6 very shallow event.
Also, local building standards, soil conditions, and population density matter enormously.
Misuse of Terms: “Magnitude” vs “Richter”
Many media outlets continue to say “Richter magnitude” even when the reported number is in fact a moment magnitude (M₍w₎). Technically the original Richter scale is outdated for large quakes.
It is better to refer simply to “magnitude” or specify “moment magnitude (M₍w₎)” or “local magnitude (Mₗ)”.
The “Energy” Behind the Numbers
Although magnitude is derived from wave amplitudes or seismic moment, scientists often convert magnitude into an estimate of energy release. For example, one formula is:
logE=5.24+1.44 Mw \log E = 5.24 + 1.44\,M_wlogE=5.24+1.44Mw
This helps illustrate how much “work” the earthquake did in the earth’s crust.
Micro-earthquakes and Negative Magnitudes
Very small earthquakes (magnitudes < 0) also exist. Magnitude scales are open-ended downward (you can have a magnitude -1 or -2 earthquake) though they cause no damage.
Extremes & Rare Cases
Because of geological diversity, two earthquakes with identical magnitude may produce very different intensities (and hence damage) in different locations—for example due to differences in depth or local geology.
For historical earthquakes (before seismographs), intensity scales (based on damage and eyewitness reports) are used to estimate magnitude (called “macro-seismic magnitude”).
Implications for Earthquake Preparedness and Engineering
Understanding the measurement systems is more than academic—it has real implications for how societies prepare for and respond to earthquakes.
a) Building Codes & Design
Engineers use the knowledge of likely shaking intensity (for given probable magnitudes and local geology) to design buildings resistant to expected forces.
In regions where soft sediments amplify shaking, building codes need to require stronger structural design.
b) Early-Warning and Rapid Response
Rapid determination of magnitude and shaking intensity allows early-warning systems to issue alerts, shut down critical infrastructure, and activate emergency procedures.
Mapping of shaking intensity helps prioritise inspections (bridges, dams, hospitals) and allocate resources.
c) Public Communication
When officials say “magnitude 7.2”, the public should not assume equal shaking everywhere. They also need to know how intense the shaking might be in their area given distance, soil and building type.
Education campaigns can help the public understand that even moderate magnitude quakes can be dangerous in unfavourable conditions.
d) Hazard Assessment & Land Use Planning
Seismologists and planners combine magnitude-frequency statistics (how often a magnitude X quake might occur) with intensity maps and local vulnerability to assess risk.
Zones with historically high intensity for moderately sized quakes may require stricter land-use controls or retrofitting programmes.
A Worked Example: From Fault Slip to Shaking in Your Town
Let’s walk through a simplified (and hypothetical) example to illustrate how all the pieces fit.
A fault beneath the earth’s crust slips over an area of, say, 20 km × 10 km (200 km²). The average slip is 1 m, and the rock rigidity is known.
The seismic moment M0M_0M0 is computed (rigidity × area × slip).
From the formula, seismologists find the moment magnitude MwM_wMw ≈ 6.8.
Instrument records worldwide confirm this magnitude as seismic waves reach seismographs and data is processed.
Meanwhile, you live in a city 80 km away from the rupture. The local soil is soft sediment. The quake is rather shallow.
Because of the relatively close distance, soft soil (which amplifies shaking), and shallow depth, the shaking you experience could be intensity MMI VII or VIII – strong enough to cause weak building damage. Meanwhile, 200 km away on hard rock, the intensity might only be MMI IV or V – light shaking, minor damage.
Engineers and emergency managers use this information to inspect vulnerable infrastructure in your city, issue warnings, advise retrofitting, and plan for after-effects (liquefaction, landslides).
This example underscores the chain: fault slip → seismic moment → magnitude → seismic waves → local shaking (intensity) → impact on structures and people.
Key Take-Away Points
Magnitude: a measure of the total energy released by an earthquake. One value for the event, regardless of where you are.
Intensity (or strength of shaking): a measure of how hard the earthquake shakes in a given location. Varies from place to place.
Magnitude scales are logarithmic: small changes in magnitude ≈ large changes in energy release.
The most reliable magnitude scale for large quakes is the moment magnitude (M₍w₎).
Intensity depends on many factors: distance, depth, geology, building design, soil type.
For earthquake risk, shaking intensity and local vulnerability matter just as much (if not more) than magnitude alone.
Understanding both helps engineers design safer buildings, helps emergency officials respond more effectively, and helps the public interpret what the numbers actually mean.
In a world where earthquakes continue to pose grave risks, especially in densely populated or geologically complex regions, the way we measure and communicate them matters enormously. When we hear “magnitude 7.5”, it is a headline number — but what really affects the community is how much the ground shakes beneath our feet, how our buildings respond, and how prepared we are.
বাংলা