On this page
When an earthquake hits, the news gives you one number, the magnitude. But people in different towns can feel the same earthquake very differently. One notices a lamp swinging. The other watches a wall crack. Giuseppe Mercalli, an Italian volcanologist and Catholic priest, built his name on that second kind of measurement: rating an earthquake by what people felt and what got damaged.
What is the Mercalli intensity scale?
The USGS describes intensity as the effect of an earthquake on the Earth's surface, judged by key responses such as people waking up, furniture moving, chimneys being damaged and, at the top, total destruction.
Intensity is a ranking, not a calculation: the USGS calls the Modified Mercalli scale "an arbitrary ranking based on observed effects", written in Roman numerals from I to XII.
Mercalli was born in Milan in 1850 and died in Naples in 1914. He later took over the Vesuvius Observatory and tried to revive it before his death cut the work short, as the Vesuvius National Park records.
Did Mercalli invent the 12-level scale?
Not quite. A 2010 review by seismologists Roger Musson, Gottfried Grünthal and Max Stucchi in the Journal of Seismology traces the history:
At the end of the 1800s, intensities around the world were assessed with one recognised scale: the 10-degree Rossi–Forel scale.
Mercalli wrote two scales. His 1883 scale had six degrees. His 1902 scale kept the ten Rossi–Forel degrees but described each one in far more detail, and it was adopted as the standard by the Central Office of Meteorology and Geodynamics in Rome.
In 1904, Adolfo Cancani added two degrees at the top, "catastrophe" and "enormous catastrophe", making the first 12-degree scale.
August Sieberg rewrote the descriptions in 1912 and 1923, and Harry Wood and Frank Neumann translated and condensed his version into the Modified Mercalli scale of 1931.
The review notes that many articles online wrongly claim Mercalli invented the idea of intensity scales, or that his 1902 scale had 12 degrees. What Mercalli added was detail: everyday signs anyone could check. Here is his intensity V, in Charles Davison's translation quoted by Musson and colleagues: "Rather strong, felt generally indoors, but by few outside, with waking of those asleep, with alarm of some persons, rattling of doors, ringing of bells, rather large oscillation of suspended objects, stopping of clocks."
How is Mercalli intensity different from Richter magnitude?
The USGS puts it simply: magnitude is related to the seismic energy released at the source and is measured with instruments, while intensity is based on observed effects on people, buildings and nature, and it varies from place to place.
On the same USGS page, each whole-number step in magnitude means ten times the measured wave amplitude and about 31 times more energy. For the maths, read our post on Charles Richter and the logarithm behind earthquake magnitude.
What do the Mercalli levels I to XII mean?
Here are short versions of the USGS descriptions of the Modified Mercalli levels:
I: Not felt except by very few people
II: Felt by a few people at rest; hanging objects may swing
III: Felt noticeably indoors; vibration like a passing truck
IV: Felt indoors by many; dishes and windows disturbed
V: Felt by nearly everyone; some breakage
VI: Felt by all; slight damage
VII: Negligible to considerable damage, depending on construction
VIII: Considerable to great damage; partial collapse
IX: Considerable to great damage; buildings shifted off foundations
X: Most masonry and frame structures destroyed; rails bent
XI: Few structures left standing; bridges destroyed
XII: Total damage; objects thrown into the air
The low levels describe people; the high levels describe buildings. That is why intensity tells you where the damage is, not just how big the earthquake was.
How can you rate the shaking yourself?
After a tremor, once you are safe, note these four things in order:
People: Who felt it? Only you, lying still? Most people indoors? People outside too?
Objects: Did hanging lamps swing, dishes rattle, or things fall off shelves?
Building: Any new cracks in plaster, fallen tiles, or damaged chimneys?
Outside: Damage to nearby walls or roads?
Then pick the level in the list above that fits everything you saw. A worked example: you were sitting still, the ceiling fan swayed, nobody else at home noticed and nothing rattled. That fits II, not IV, because IV needs many people indoors to feel it and dishes, windows or doors to be disturbed.
The USGS Did You Feel It? project works the same way, turning reports from people who felt a quake into maps of what they experienced.
If logarithms still feel slippery, the Fermi Concept Library gives each concept its own lesson and shows its place in the map. For ideas that are easier to see than to read, try the Fermi Simulation Lab.
For the instruments before and after Mercalli, read about Zhang Heng's seismoscope and John Milne's horizontal pendulum seismograph.
