An earthquake report often carries two different numbers: “magnitude 5.4” and “felt with intensity V”. They are not the same thing. Magnitude relates to the energy released at the source, and each earthquake has one value (per agency and method). Intensity describes how the shaking was felt and what it did at a particular place, so one earthquake has many intensities.

Magnitude: ML, Mw and others

Local magnitude (ML) is today’s form of the scale Charles Richter defined in 1935. It is based on the largest wave amplitude on a seismogram and the distance. It is quick and practical for small and moderate earthquakes, which is why it appears often in the first automatic solutions of Kandilli and AFAD.

Moment magnitude (Mw), defined by Hanks and Kanamori in 1979, is based on the seismic moment: the slipped fault area, the amount of slip and the rigidity of the rock. For large earthquakes, amplitude-based scales such as ML “saturate” and understate the size; Mw does not, so large earthquakes are usually given in Mw.

Catalogues also contain types such as Md (duration) and mb (body wave). On our earthquake pages the source comparison table lists the magnitude and type each agency reports; small differences between AFAD, Kandilli, EMSC and USGS come from these methods and data.

How much is one unit?

Magnitude scales are logarithmic. As USGS explains, one unit more magnitude means about 10 times larger wave amplitude on the record and about 32 times more energy released. An M6 earthquake releases about 32 times more energy than an M5 and about 1,000 times more than an M4. The energy figure on our statistics page uses the same relation (log₁₀E = 1.5M + 4.8).

Intensity: Modified Mercalli (MMI)

The Modified Mercalli Intensity (MMI) scale runs from I to XII in Roman numerals and is defined by what people feel and how objects and buildings are affected, for example:

  • I–II: usually not felt, or felt only by people at rest on upper floors.
  • IV: felt indoors by most; windows, doors and dishes rattle.
  • VI: felt by all, many are frightened; heavy furniture may move, plaster may fall.
  • VIII and above: considerable damage to weak buildings.

Intensity depends mainly on magnitude, distance to the epicentre and the fault rupture, depth, and local soil. Soft soils can amplify shaking, so two neighbourhoods at the same distance can experience different intensities.

How we show intensity

The expected intensity rings on our earthquake pages come from the peak ground acceleration of the Akkar, Sandıkkaya and Bommer (2014) ground-motion model for Europe and the Middle East, converted to MMI with the relation of Worden et al. (2012). It is an estimate: the model assumes a reference site (Vs30 = 760 m/s), and real shaking can differ with soil and building. That is why we also collect “Did you feel it?” reports. Our personal alerts likewise use the expected intensity at your saved places, not the magnitude.

In short

  • Magnitude (Mw, ML): energy at the source; one value per earthquake, with small differences between agencies.
  • Intensity (MMI): shaking and its effects at a place; varies from place to place.
  • A large earthquake far away can be felt more weakly than a moderate one nearby.

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