Modified Mercalli Intensity Scale.
Mercalli' improved intensity scale (1902) served
striking the walls. Standing motor cars rock.
as the basis for the scale advanced by Wood and Neumann
Windows, dishes, doors rattle. Glasses clink.
(1931), known as the modified Mercalli scale and
Crockery clashes. In the upper range of IV, wooden
commonly abbreviated MM. The modified version is
walls and frame creak.
described below with some improvements by Richter
Felt outdoors; direction estimated. Sleepers wakened.
Liquids disturbed, some spilled. Small unstable
To eliminate many verbal repetitions in the
objects displaced or upset. Doors swing, close, open.
original scale, the following convention has been adopted.
Shutters, pictures move. Pendulum clocks stop, start,
Each effect is named at that level of intensity at which it
first appears frequently and characteristically. Each effect
may be found less strongly or more often at the next higher
VI. Felt by all. Many frightened and run outdoors.
grade. A few effects are named at two successive levels to
Persons walk unsteadily. Windows, dishes, glassware
indicate a more gradual increase.
broken. Knickknacks, books, etc., off shelves.
Pictures off walls. Furniture moved or overturned.
Masonry A, B, C, D. To avoid ambiguity of language, the
Weak plaster and masonry D cracked. Small bells
ring (church, school). Trees, bushes shaken visibly,
quality of masonry, brick or otherwise, is specified by the
or heard to rustle.
VII. Difficult to stand. Noticed by drivers of motor cars.
Masonry A. Good workmanship, mortar, and design;
Hanging objects quiver. Furniture broken. Damage
reinforced, especially laterally, and bound together by using
to masonry D, including cracks. Weak chimneys
steel, concrete, etc.; designed to resist lateral forces.
broken at roof line. Fall of plaster, loose bricks,
stones, tiles, cornices, also unbraced parapets and
Masonry B. Good workmanship and mortar; reinforced,
architectural ornaments. Some cracks in masonry C.
but not designed in detail to resist lateral forces.
Waves on ponds; water turbid with mud. Small slides
and caving in along sand or gravel banks. Large bell
Masonry C. Ordinary workmanship and mortar; no
rings. Concrete irrigation ditches damaged.
extreme weaknesses like failing to tie in at corners, but
neither reinforced nor designed against horizontal forces.
Steering of motor cars affected. Damage to masonry
C; partial collapse. Some damage to masonry B; none
Masonry D. Weak materials, such as adobe; poor mortar;
to masonry A. Fall of stucco and some masonry
low standards of workmanship; weak horizontally.
walls. Twisting, fall of chimneys, factory stacks,
monuments, towers, elevated tanks. Frame houses
moved on foundations if not bolted down; loose panel
Modified Mercalli Intensity Scale of 1931 (Abridged
walls thrown out. Decayed piling broken off.
and Rewritten by C.F. Richter)
Branches broken from trees. Changes in flow or
temperature of springs and wells. Cracks in wet
Not felt. Marginal and long-period effects of large
ground and on steep slopes.
General panic. Masonry D destroyed; masonry C
Felt by person at rest, on upper floors, or favorably
heavily damaged, sometimes with complete collapse;
masonry B seriously damaged. General damage to
foundations. Frames structure, if not bolted, shifted
III. Felt indoors. Hanging objects swing. Vibration like
off foundation. Frame racked. Serious damage to
passing of light trucks. Duration estimated. May not
reservoirs. Underground pipes broken. Conspicuous
be recognized as an earthquake.
cracks in ground. In alluviated areas, sand and mud
ejected, earthquake fountains, sand craters.
IV. Hanging objects swing. Vibration like passing of
heavy trucks; or sensation of a jolt like a heavy ball