An Interview with Charles F. Richter
Charles F. Richter - An Interview
U.S. Geological Survey, Reston, Va.
Charles F. Richter, renowned seismologist,
is a professor emeritus at the California Institute of Technology
(Caltech). He is best known to the public for the Richter
magnitude scale; but he is equally recognized in the
scientific community for many other contributions to seismology
including his books Elementary Seismology (1958) and
Seismicity of the Earth (coauthored in 1954 with Beno Gutenberg).
HS: How did you become interested in seismology?
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.
HS: What were the origins of the instrumental magnitude scale?
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. I suggested
that we might compare earthquakes in terms of the
measured amplitudes recorded at these stations,
with an appropriate correction for distance.
Wood and I worked together on the latest events,
but we found that we could not make satisfactory assumptions
for the attenuation with distance. I found a paper by
Professor K. Wadati of Japan in which he compared large earthquakes
by plotting the maximum ground motion against distance
to the epicenter. I tried a similar procedure for our stations,
but the range between the largest and smallest magnitudes
seemed unmanageably large. Dr. Beno Gutenberg then made the
natural suggestion to plot the amplitudes logarithmically.
I was lucky because logarithmic plots are a device of the devil.
I saw that I could now rank the earthquakes one above the other.
Also, quite unexpectedly the attenuation curves were roughly
parallel on the plot. By moving them vertically, a representative
mean curve could be formed, and individual events were
then characterized by individual logarithmic differences
from the standard curve. This set of logarithmic differences
thus became the numbers on a new instrumental scale. Very perceptively,
Mr. Wood insisted that this new quantity should be given
a distinctive name to contrast it with the intensity scale.
My amateur interest in astronomy brought out the term
"magnitude," which is used for the brightness of a star.
HS: What modifications were involved in applying the
scale to worldwide earthquakes?
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.
HS: In looking at worldwide earthquakes, did Gutenberg
explore other possible magnitude scales?
Yes, without any significant contribution from me,
he later worked out a version of the magnitude scale that used
the measured amplitudes and periods of body waves,
that is P (primary), S (secondary), and PP (P waves that
are reflected at the Earth's outer surface and then continue
on as P waves). He was in favor of using this scale
in preference to the surface-wave scale. Theoretically,
it is better. I had doubts at the time, and I still feel that
the body-wave scale is not satisfactory for general use because
it gives results comparable with Gutenberg's only if his
procedure is closely followed. Experience has shown that
misunderstanding and oversimplified misapplications can occur.
For instance, magnitude is sometimes assigned on the first
few waves of the P group rather than the largest P waves
as Gutenberg did. For deep-focus earthquakes,
it has been shown that Gutenberg's results were
distorted by the comparatively large loss of energy
in body waves from shallow earthquakes when they pass out
of and back into the crust.
HS: Many people have the wrong impression that
"the Richter magnitude is based on a scale of 10."
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.
HS: No doubt you are often asked about the difference
between magnitude and intensity.
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.
HS: There has been interest recently in reassessing what
is meant by the "size of an earthquake."
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.
The most remarkable feature about the magnitude scale was that
it worked at all and that it could be extended on a worldwide basis.
It was originally envisaged as a rather rough-and-ready procedure
by which we could grade earthquakes. We would have been happy
if we could have assigned just three categories, large, medium,
and small; the point is, we wanted to avoid personal judgments.
It actually turned out to be quite a finely tuned scale.
The work of Dr. Kanamori and others is developing into a
more detailed perspective of an individual earthquake.
We can now arrive at different values of "size" by varying
the quantity being calculated, the type of instrument used,
and the kinds of observations made.
This is very similar to astronomy where different magnitudes
are assigned to the brightness of an astronomical object,
depending on the range of wavelengths being measured.
For example, some stars put out large amounts of energy
in the infrared part of the spectrum, so that this can
produce a different relative magnitude rating than using
light energy from the middle of the spectrum.
This is quite analogous to the investigation of earthquake size
that Dr. Kanamori and his colleagues are pursuing.
HS: You are now more involved in assessing seismic risk?
Yes; a good deal of my consulting work consists
of site analysis for proposed developments. The problem
of assessing seismic risk starts at the geological-geophysical
end and winds up at the engineering end. We have a given site,
and we have to ascertain what disturbances will affect
the structures on that site as a result of an earthquake.
These are usually described in the form of a design earthquake
with a time history of the motion and shaking characteristics
that are expected. From here, it becomes an engineering problem;
the engineer considers the ground motion that will occur and
evaluates the requirements of the proposed structure
in the light of the local foundation conditions.
The first steps toward earthquake-resistant construction
were made by assuming a fixed acceleration due to the earthquake.
Emphasis was usually put on the horizontal acceleration factor,
for the simple reason that ordinary
structures have a built-in safety factor for the vertical component;
that is, gravity. As seismologists gained more experience
from earthquake records, it became obvious that the problem
could not be reduced to a single peak acceleration. In fact,
a full frequency of vibrations occurs. The key factor
is the length of time involved in shaking;
in other words, how long is the acceleration applied.
HS: Should more effort be put into measures that
actually reduce the earthquake hazard?
I usually point out that most loss of life and
property has been due to the collapse of antiquated and
unsafe structures, mostly of brick and other masonry.
In every area of the world where there is earthquake risk,
there are still many buildings of this type;
it is very frustrating to try to get rid of them.
At least there is progress of California toward building
new construction according to earthquake-resistant design.
We would have less reason to ask for earthquake prediction
if this was universal.
HS: I realize your views on prediction.
But would you be willing to predict what you see for seismology
over the next decade?
Nothing is less predictable than the development
of an active scientific field. In seismology, we have an
increasing number of people and resources at work,
so that it's entirely imaginable that there will be a
breakthrough in some quite unforeseen area. We now have
the capability to directly compare the Earth with other bodies
in the solar system; for instance, the violently erupting
volcanoes on Io (one of Jupiter's moons)
must radically affect our whole thinking about geophysical
and geochemical process in the interior of the planets and their
Abridged from the Earthquake Information Bulletin.
Vol. 12, No. 1, January - February, 1980.