What is the difference between earthquake magnitude and intensity?
Short answer
Magnitude measures the energy released at an earthquake's source and is a single value. Intensity describes the effects felt at a particular place and varies by location.
Checklist
- Magnitude belongs to the source; intensity belongs to a place.
- Magnitude is a single value; intensity varies by location.
- Moment magnitude is a unitless, logarithmic scale.
- For official earthquake information, check the announcements of the responsible authority.
In news reports, "magnitude" and "intensity" are often used interchangeably. They actually describe different things: one describes the earthquake itself, the other its effect at a given place.
What is magnitude?
Magnitude is a measure of the energy released at the earthquake's source, calculated from seismic records. A single magnitude value is given for each earthquake. It does not depend on where you felt it.
Unit
The most widely used scale today is moment magnitude (Mw). This scale has no unit; it is expressed as a plain number. It is logarithmic: one step up in magnitude means roughly a tenfold increase in ground-motion amplitude, and a much larger increase in released energy.
What is intensity?
Intensity describes the observed effects of an earthquake on people, structures and the environment at a specific location. Scales based on the Mercalli approach use Roman numerals. The same earthquake produces different intensity values in different places.
- Magnitude: Belongs to the source, a single value, calculated from instruments.
- Intensity: Belongs to a place, many values, based on observations and damage.
What changes intensity?
- Distance: Effects generally increase closer to the epicenter.
- Depth: Shallow earthquakes are generally felt more strongly at the surface.
- Ground: Soft, loose soils can amplify shaking.
- Building quality: Earthquake-resistant structures suffer less damage in the same shaking.
Frequently asked questions
Is the Richter scale still used?
The Richter scale was historically common. Moment magnitude is mostly preferred today because it is more reliable for large earthquakes.
Does an earthquake have only one intensity?
No. Intensity varies from place to place, so one earthquake can have many intensity values.
Does more magnitude always mean more damage?
Generally damage tends to increase, but depth, distance, ground and building quality determine the outcome.
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