||Not felt by anyone.
||Vibration is felt only
by individual people at rest in houses, especially on
upper floors of buildings.
||The vibration is weak
and is felt indoors by a few people. People at rest feel
swaying or light trembling. Noticeable shaking of many
||The earthquake is felt
indoors by many people, outdoors by few. A few people
are awakened. The level of vibration is possibly
frightening. Windows, doors and dishes rattle. Hanging
objects swing. No damage to buildings.
||The earthquake is felt
indoors by most, outdoors by many. Many sleeping people
awake. A few run outdoors. Entire sections of all
buildings tremble. Most objects swing considerably.
China and glasses clatter together. The vibration is
strong. Topheavy objects topple over. Doors and windows
swing open or shut
||Felt by everyone indoors
and by many to most outdoors. Many people in buildings
are frightened and run outdoors. Objects on walls fall.
Slight damage to buildings; for example, fine cracks in
plaster and small pieces of plaster fall.
||Most people are
frightened and run outdoors. Furniture is shifted and
many objects fall from shelves. Many buildings suffer
slight to moderate damage. Cracks in walls; partial
collapse of chimneys.
||Furniture may be
overturned. Many to most buildings suffer damage:
chimneys fall; large cracks appear in walls and a few
buildings may partially collapse. Can be noticed by
people driving cars.
||Monuments and columns
fall or are twisted. Many ordinary buildings partially
collapse and a few collapse completely. Windows shatter.
||Many buildings collapse.
Cracks and landslides can be seen.
||Most buildings collapse.
||All structures are
destroyed. The ground changes.
The Richter Magnitude Scale
The Richter scale is not a physical device, but a mathematical
formula. The magnitude of an earthquake is determined from the
logarithm of the amplitude of waves recorded on a seismogram at
a certain period.
Earthquakes are recorded by a seismographic network. Each
seismic station in the network measures the movement of the
ground at the site. The slip of block of rock over another in an
EQ releases energy that makes the ground vibrate. That vibration
pushes the adjoining piece of ground and cause it to vibrate and
thus the energy travel out from the EQ in a wave. There are many
different ways to measure different aspects of an earthquake.
Magnitude is the most common measure of an earthquake's
size. It is a measure of the size of the earthquake source and
is the same number no matter where you are or what the shaking
feels like. The Richter scale measures the largest wiggle on the
recording, but other magnitude scales measure different parts of
the earthquake. Intensity is a measure of the shaking and
damage caused by the earthquake, and this value changes from
location to location.
Seismic waves are the vibrations from earthquakes that travel
through the Earth; they are recorded on instruments called
seismographs. Seismographs record a zig-zag trace that shows the
varying amplitude of ground oscillations beneath the instrument.
Sensitive seismographs, which greatly magnify these ground
motions, can detect strong earthquakes from sources anywhere in
the world. The time, locations, and magnitude of an earthquake
can be determined from the data recorded by seismograph
The Richter magnitude scale was developed in 1935 by Charles F.
Richter of the California Institute of Technology as a
mathematical device to compare the size of earthquakes. The
magnitude of an earthquake is determined from the logarithm of
the amplitude of waves recorded by seismographs. Adjustments are
included for the variation in the distance between the various
seismographs and the epicenter of the earthquakes. On the
Richter Scale, magnitude is expressed in whole numbers and
decimal fractions. For example, a magnitude 5.3 might be
computed for a moderate earthquake, and a strong earthquake
might be rated as magnitude 6.3. Because of the logarithmic
basis of the scale, each whole number increase in magnitude
represents a tenfold increase in measured amplitude; as an
estimate of energy, each whole number step in the magnitude
scale corresponds to the release of about 31 times more energy
than the amount associated with the preceding whole number
At first, the Richter Scale could be applied only to the records
from instruments of identical manufacture. Now, instruments are
carefully calibrated with respect to each other. Thus, magnitude
can be computed from the record of any calibrated seismograph.
Earthquakes with magnitude of about 2.0 or less are usually
called microearthquakes; they are not commonly felt by people
and are generally recorded only on local seismographs. Events
with magnitudes of about 4.5 or greater - there are several
thousand such shocks annually - are strong enough to be recorded
by sensitive seismographs all over the world. Great earthquakes,
such as the 1964 Good Friday earthquake in Alaska, have
magnitudes of 8.0 or higher. On the average, one earthquake of
such size occurs somewhere in the world each year. The Richter
Scale has no upper limit. Recently, another scale called the
moment magnitude scale has been devised for more precise study
of great earthquakes.
The Richter Scale is not used to express damage. An earthquake
in a densely populated area which results in many deaths and
considerable damage may have the same magnitude as a shock in a
remote area that does nothing more than frighten the wildlife.
Large-magnitude earthquakes that occur beneath the oceans may
not even be felt by humans.
Modified Mercalli Intensity Scale
The effect of an earthquake on the Earth's surface is called the
intensity. The intensity scale consists of a series of certain
key responses such as people awakening, movement of furniture,
damage to chimneys, and finally--total destruction. Although
numerous intensity scales have been developed over the last
several hundred years to evaluate the effects of earthquakes,
the one currently used in the United States is the Modified
Mercalli (MM) Intensity Scale. It was developed in 1931 by the
American seismologists Harry Wood and Frank Neumann. This scale,
composed of 12 increasing levels of intensity that range from
imperceptible shaking to catastrophic destruction, is designated
by Roman numerals. It does not have a mathematical basis;
instead it is an arbitrary ranking based on observed effects.
The Modified Mercalli Intensity value assigned to a specific
site after an earthquake has a more meaningful measure of
severity to the nonscientist than the magnitude because
intensity refers to the effects actually experienced at that
place. After the occurrence of widely-felt earthquakes, the
Geological Survey mails questionnaires to postmasters in the
disturbed area requesting the information so that intensity
values can be assigned. The results of this postal canvass and
information furnished by other sources are used to assign an
intensity value, and to compile isoseismal maps that show the
extent of various levels of intensity within the felt area. The
maximum observed intensity generally occurs near the epicenter.