........................................................................................
Charles Richter, Inventor of the Richter Magnitude Scale
Comparing the
sizes of earthquakes
byMary Bellis
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 stations.
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 value.
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
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.
NEIS Interview
The following is a
transcript of an NEIS interview with Charles Richter:
How did you become interested in seismology?
CHARLES RICHTER: It was really a happy accident. At Caltech, I was working on
my Ph.D. in theoretical physics under Dr. Robert Millikan.
One
day he called me into his office and said that the Seismological Laboratory was
looking for a physicist; this was not my line, but was I at all interested?
I
talked with Harry Wood who was in charge of the lab; and, as a result, I joined
his staff in 1927.
What
were the origins of the instrumental magnitude scale?
CHARLES RICHTER: When I joined Mr. Wood's staff, I was mainly engaged in the routine work of measuring seismograms and locating earthquakes, so that a catalog could be set up of epicenters and times of occurrence.
CHARLES RICHTER: When I joined Mr. Wood's staff, I was mainly engaged in the routine work of measuring seismograms and locating earthquakes, so that a catalog could be set up of epicenters and times of occurrence.
Incidentally,
seismology owes a largely unacknowledged debt to the persistent efforts of
Harry O. Wood for bringing about the seismological program in southern
California.
At
the time, Mr. Wood was collaborating with Maxwell Alien on a historical review
of earthquakes in California. We were recording on seven widely spaced
stations, all with Wood-Anderson torsion seismographs.
What
modifications were involved in applying the scale to worldwide earthquakes?
CHARLES RICHTER: You're quite rightly pointing out that the original magnitude scale which I published in 1935 was set up only for southern California and for the particular types of seismographs in use there.
CHARLES RICHTER: You're quite rightly pointing out that the original magnitude scale which I published in 1935 was set up only for southern California and for the particular types of seismographs in use there.
Extending
the scale to worldwide earthquakes and to recordings on other instruments was
begun in 1936 in collaboration with Dr. Gutenberg.
This
involved using the reported amplitudes of surface waves with periods of about
20 seconds.
Incidentally,
the usual designation of the magnitude scale to my name does less than justice
to the great part that Dr. Gutenberg played in extending the scale to apply to
earthquakes in all parts of the world.
Many
people have the wrong impression that the Richter magnitude is based on a scale
of 10.
CHARLES RICHTER: I repeatedly have to correct this belief. In a sense, magnitude involves steps of 10 because every increase of one magnitude represents a tenfold amplification of the ground motion.
CHARLES RICHTER: I repeatedly have to correct this belief. In a sense, magnitude involves steps of 10 because every increase of one magnitude represents a tenfold amplification of the ground motion.
But
there is no scale of 10 in the sense of an upper limit as there is for
intensity scales; indeed, I'm glad to see the press now referring to the
open-ended Richter scale.
Magnitude
numbers simply represent measurement from a seismograph record — logarithmic to
be sure but with no implied ceiling.
The
highest magnitudes assigned so far to actual earthquakes are about 9, but that
is a limitation in the Earth, not in the scale.
There is another
common misapprehension that the magnitude scale is itself some kind of
instrument or apparatus. Visitors will frequently ask to "see the
scale."
They're
disconcerted by being referred to tables and charts that are used for applying
the scale to readings taken from the seismograms.
No
doubt you are often asked about the difference between magnitude and intensity.
CHARLES RICHTER: That also causes great confusion among the public. I like to use the analogy with radio transmissions.
CHARLES RICHTER: That also causes great confusion among the public. I like to use the analogy with radio transmissions.
It
applies in seismology because seismographs, or the receivers, record the waves
of elastic disturbance, or radio waves, that are radiated from the earthquake
source, or the broadcasting station.
Magnitude
can be compared to the power output in kilowatts of a broadcasting station.
Local intensity on the Mercalli scale is then comparable to the signal strength
on a receiver at a given locality; in effect, the quality of the signal.
Intensity
like signal strength will generally fall off with distance from the source,
although it also depends on the local conditions and the pathway from the
source to the point.
There
has been interest recently in reassessing what is meant by the "size of an
earthquake."
CHARLES RICHTER: Refining is inevitable in science when you have made measurements of a phenomenon for a long period of time.
CHARLES RICHTER: Refining is inevitable in science when you have made measurements of a phenomenon for a long period of time.
Our
original intent was to define magnitude strictly in terms of instrumental
observations.
If
one introduces the concept of "energy of an earthquake" then that is
a theoretically derived quantity.
If
the assumptions used in calculating energy are changed, then this seriously
affects the final result, even though the same body of data might be used.
So
we tried to keep the interpretation of the "size of the earthquake"
as closely tied to the actual instrument observations involved as possible.
What
emerged, of course, was that the magnitude scale presupposed that all
earthquakes were alike except for a constant scaling factor. And this proved to
be closer to the truth than we expected.
Mary Bellis
· New
York-based film producer and director
· Singled
out by Forbes magazine for her writing on inventors.
· Known
in art and independent film circles by the name CalmX
· Creator
of computer-generated art
Experience
Mary
Bellis was a former writer for ThoughtCo, where she covered inventors for 18
years. She was a freelance writer, film producer, and director. In
addition, Forbes Best of the Web credited her for creating the number one
online destination for information about inventors and inventions. Her
writing has been reprinted and referenced in numerous educational books and
articles. She was known for her short independent films and
documentaries, including one on Alexander Graham Bell. She specialized in
making and exhibiting computer-generated art, while working as an animator,
journalist and an independent video game developer. She died on March 28,
2015.
Education
Mary
Bellis held a Master of Fine Arts in film and animation from the San
Francisco Art Institute.
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