Of the two ways to measure earthquake size, magnitude based on instrumental readings and intensity based on qualitative effects of earthquakes, only intensity can be applied to pre-instrumental earthquakes. The 1931 Modified Mercalli scale used in the United States assigns a Roman numeral in the range I - XII to each earthquake effect. The methodology is simple.
Another problem is that incomplete spatial coverage may lead to a mislocation of the earthquake or an underassessment of its size. This is easily visualized for offshore earthquakes or, in the case of the United States, inadequate population distribution at the time of the earthquake.
|Average peak velocity (centimeters per second)||Intensity value and description||Average peak acceleration (g is gravity=9.80 meters per second squared)|
| I. Not felt except by a very few under
especially favorable circumstances.
(I Rossi-Forel scale)
II. Felt only by a few persons at rest,
especially on upper floors of
buildings. Delicately suspended
objects may swing.
III. Felt quite noticeably indoors,
especially on upper floors of
buildings, but many people do not
recognize it as an earthquake.
Standing automobiles may rock
slightly. Vibration like passing of
truck. Duration estimated.
|1-2||IV. During the day felt indoors by many,
outdoors by few. At night some
awakened. Dishes, windows, doors
disturbed; walls make creaking
sound. Sensation like heavy truck
striking building. Standing
automobiles rocked noticeably.
(IV to V Rossi-Forel scale)
|2-5||V. Felt by nearly everyone, many
awakened. Some dishes, windows,
and so on broken; cracked plaster in
a few places; unstable objects
overturned. Disturbances of trees,
poles, and other tall objects
sometimes noticed. Pendulum clocks
(V to VI Rossi-Forel scale)
|5-8||VI. Felt by all, many frightened and run
outdoors. Some heavy furniture
moved; a few instances of fallen
plaster and damaged chimneys.
(VI to VII Rossi-Forel scale)
|8-12||VII. Everybody runs outdoors. Damage
negligible in buildings of good design
and construction; slight to moderate
in well-built ordinary structures;
considerable in poorly built or badly
designed structures; some chimneys
broken. Noticed by persons driving cars.
(VIII Rossi-Forel scale)
|20-30||VIII. Damage slight in specially designed
structures; considerable in ordinary
substantial buildings with partial
collapse; great in poorly built
structures. Panel walls thrown out of
frame structures. Fall of chimneys,
factory stack, columns, monuments,
walls. Heavy furniture overturned.
Sand and mud ejected in small
amounts. Changes in well water.
Persons driving cars disturbed.
(VIII + to IX Rossi-Forel scale)
|45-55||IX. Damage considerable in specially
designed structures; well-designed
frame structures thrown out of
plumb; great in substantial buildings,
with partial collapse. Buildings
shifted off foundations. Ground
cracked conspicuously. Underground
(IX + Rossi-Forel scale)
|More than 60|| X. Some well-built wooden structures
destroyed; most masonry and frame
structures destroyed with
foundations; ground badly cracked.
Rails bent. Landslides considerable
from river banks and steep slopes.
Shifted sand and mud. Water
splashed, slopped over banks.
(X Rossi-Forel scale)
|More than 0.60g|
|XI. Few, if any, (masonry) structures
remain standing. Bridges destroyed.
Broad fissures in ground.
Underground pipelines completely
out of service. Earth slumps and land
slips in soft ground. Rails bent greatly.
XII. Damage total. Waves seen on ground surface. Lines of sight and level distorted. Objects thrown into the air.