Magnitude vs Intensity: What's the Difference?
Magnitude measures energy at the source; intensity describes what you feel. Learn the Modified Mercalli Scale, why the same quake feels different in different places, and how both concepts help you stay safe.
Magnitude: Energy at the Source
Magnitude is a single number that quantifies the energy released at the earthquake's hypocenter (the underground point of rupture). It is an objective, instrument-based measurement. A M6.5 earthquake releases the same total energy whether it occurs in a desert or beneath a dense city.
The modern standard is moment magnitude (Mw), based on seismic moment — a measurement derived from fault area, slip amount, and rock rigidity. Read more in our full guide to earthquake magnitude scales.
Intensity: What You Actually Feel
Intensity describes the local effects of an earthquake — how strongly it is felt at a specific location, and what damage it causes there. Unlike magnitude (one number per earthquake), intensity varies from place to place and is expressed on scales like the Modified Mercalli Intensity (MMI) scale.
A single earthquake will have many different intensities across the affected area. The epicenter region typically experiences the highest intensity. Intensity decreases with distance, but can be locally amplified by soft soils, sedimentary basins, or steep terrain.
The Modified Mercalli Intensity Scale
| MMI | Description | What You Experience |
|---|---|---|
| I | Not felt | Only detected by instruments. |
| II–III | Weak | Felt by few, especially on upper floors. |
| IV | Light | Felt by many indoors. Hanging objects swing. |
| V | Moderate | Felt by nearly all. Doors open/close, liquids disturbed. |
| VI | Strong | Felt by all. Some minor structural damage. Difficult to stand. |
| VII | Very Strong | Damage in poorly built structures. Plaster cracks. |
| VIII | Severe | Major damage to ordinary structures. Chimneys fall. |
| IX | Violent | Severe damage. Bridges destroyed. Underground pipes broken. |
| X–XII | Extreme | Most structures collapse. Total destruction. Permanent ground deformation. |
Why the Same Quake Feels Different in Different Places
Four key factors determine local intensity for a given earthquake:
- Distance from epicenter: Seismic waves attenuate (weaken) with distance. Intensity generally decreases as you move further from the epicenter, though this is not always uniform.
- Earthquake depth: Shallow earthquakes (0–20 km deep) produce the most intense surface shaking for a given magnitude. Deep focus earthquakes (70–700+ km) affect much larger areas but feel weaker locally.
- Local site conditions: Soft sediments and clay amplify ground motion dramatically compared to hard bedrock. Mexico City and parts of Seattle, Oakland, and Christchurch are built on amplifying soils.
- Building quality: While not a seismological factor, building vulnerability determines whether intensity causes minor damage or complete collapse. Soft-story buildings and unreinforced masonry are particularly vulnerable.
A Practical Example: Mexico City and Distant Earthquakes
The Mexico City phenomenon illustrates the magnitude/intensity distinction perfectly. The city is built on dried lakebed clay — ancient Lake Texcoco. When a M7–8 earthquake occurs 300–400 km away on the Pacific coast, the soft clay amplifies seismic waves 20–50 times.
The result: a distant M8.0 earthquake with a local epicentral intensity of VIII–IX can produce VII–VIII intensity shaking in Mexico City — potentially devastating — while feeling like II–III in a bedrock city at the same distance. This explains why the 1985 and 2017 earthquakes caused far greater destruction in Mexico City than at their Pacific coast epicenters.
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Frequently Asked Questions
- Can a low-magnitude earthquake have high intensity?
- Yes. A M4.5 earthquake at 3 km depth directly beneath a city can cause strong shaking (intensity VII–VIII on the MMI scale). The same M4.5 at 50 km depth would be felt far less strongly. Depth and soil conditions dramatically affect intensity.
- What is the Modified Mercalli Intensity (MMI) scale?
- The MMI scale (I to XII) rates the effects of an earthquake as experienced by people and structures. I = not felt; IV = felt by most indoors; VI = felt by all, minor damage; VIII = serious damage to ordinary buildings; XII = total destruction. It provides a human-centered complement to magnitude.
- Why does Mexico City shake so strongly from distant earthquakes?
- Mexico City is built on the soft lacustrine clay of former Lake Texcoco. This clay layer acts like jelly, amplifying seismic waves 20–50 times compared to firm bedrock. A M8.0 earthquake 400 km away on the Pacific coast can cause intensity VIII shaking in the capital due to this site amplification effect.
- How does Earthquake Globe use intensity data?
- Earthquake Globe shows magnitude for all events. The app's notification descriptions include felt intensity estimates ("Felt: Moderate / Strong") based on depth and proximity, giving you practical context beyond the magnitude number alone.
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