INSTRUMENTS FOR RECORDING AND MEASURING EARTHQUAKE SHOCKS
To attempt by the unaided senses a determination of the direction in
which earthquake shocks reach any certain spot, the velocity with
which they are travelling, their degree of intensity, their general
character, whether horizontal or vertical, or any peculiarities which
might show them to be exceptional would be futile for more reasons
than one. Even a skilled scientific observer, familiar with what has
already been discovered and eager to discover more, might in the
excitement of an earthquake become so excited himself as to make him
unable to take reliable observations.
But human ingenuity has succeeded in devising delicate instruments
capable of recording not only the exact time of the arrival of an
earthquake shock, but also of measuring the different parts of what
may seem to be a single shock, the direction in which the shocks reach
the place, as well as the variations of intensity in all the shocks.
Crude instruments to do some of these things have been in use from
very early times. According to Mallet among the more important
of these early instruments was the following: the instrument of
Cacciatore of Palmero. This consisted of a circular wooden dish, about
ten inches in diameter, placed horizontally, and filled with mercury
to the brim of eight notches at equal distances apart. Beneath each
notch was placed a small cup. On the passage of the earthquake waves
the vessel, being tilted in a direction dependent on the direction in
which the waves were travelling, would cause some of the mercury to
spill over into one or more of the cups, thus indicating by its amount
the intensity of the wave, and by the particular cup or cups that were
filled, the direction in which the waves reached the place.
Somewhat similar contrivances were of a vessel partly filled with
molasses, or other sticky liquid; or a cylindrical tub, the sides of
which were chalked or whitewashed and filled with some colored liquid.
In either of these cases, on the passage of the earthquake waves,
the vessels were tilted and showed by the height of the marks the
intensity of the waves, and by the position of the marks the direction
in which the waves first reached the instrument.
These instruments, though satisfactory for the study of earthquake
shocks a long time ago, when an earthquake was regarded as practically
consisting of but a single shock, or, at the most, of a very few
shocks, would be worthless for the study of earthquakes now, for it is
finally known that an earthquake consists of a series of many hundreds
of vibrations, differing greatly in their rapidity and intensity, and
following one another in a definite order.
The old forms of earthquake instruments were known as _seismoscopes_.
The word seismoscope is a compound word from Greek consisting of
the two words, seism and scope. It means literally any instrument
capable of seeing, or calling attention to, a seism, or _earth-shake_.
In other words, a seismoscope is any instrument capable of calling
attention only to an earth-shake.
Of course, neither of the rude seismoscopes just mentioned would be
able to give any valuable indications of the successive shakings to
which the vessel containing the viscid liquid had been subjected,
since the liquid would simply be splashed a number of times over the
same parts of the vessel. In order to get a record of the successive
shocks another form of apparatus must be employed, a form known as a
_seismograph_.
Concerning the complex character of the apparently single earthquake
shock, Professor Milne makes this highly interesting and picturesque
statement:
"An earthquake disturbance at a station far removed from
its origin shows that the main movement has two attendants,
one which precedes and the other which follows. The first
of these by its characteristics indicates what is to
follow, whilst the latter, in a very much more pronounced
manner, will often repeat at definite intervals, but with
decreasing intensity, the prominent features of what
has passed. Inasmuch as these latter rhythmical, but
decreasing, impulses of the dying earthquake are more
likely to result from reflection than from interference, I
have provisionally called them Echoes."
There are many different forms of instruments known as seismographs
that are capable of recording all of these vibrations, but there is
this objection to their use: that the records appear in so tangled a
form that it is practically impossible to decipher or untangle them.
This fact can be grasped by examining Fig. 51, which represents a
record of this kind.
[Illustration: FIG. 51. COMPLEX RECORD OF SEISMOGRAPH]
It is necessary, therefore, to employ a modified form of instrument
called a _seismometer_, able not only to record all the different
vibrations, but to record them in such a manner that they can be
easily recognized. Fig. 52, for example, shows results obtained by the
use of a seismometer, in which the different vibrations are separated,
and so recorded on a sheet of paper, as to be readily understood. Such
a record is called a _seismogram_, and represents a _long distance
seismogram_. Here the large arrow indicates the beginning of the
record. And herein, as can be clearly seen, what would appear to an
observer without an instrument only a single shock, lasting but the
fraction of a minute, in reality consists of the _preliminary shake_
as represented in ab and bc, the _principal shake_, as represented
at c, d1, d2, and d3, and the _final portions of the shake_ or the
"echoes" of Professor Milne, as represented from d3 to e.
[Illustration: FIG. 52. LONG DISTANCE SEISMOGRAM]
Except in a very general way there is for present purposes no need
of explaining the construction and operation of the seismometer
and seismograph. Suffice it to say, there are many forms of these
instruments, any of which are capable of recording the details of a
passing shock. The most important thing in either a seismograph or a
seismometer is to obtain what is known as a _steady point_; that is, a
point consisting of some object or mass that will remain practically
at rest, while everything around it, even the support which holds it,
is affected by the earthquake.
It is, of course, not very easy to obtain a steady point, but it can
be done; and it will be at once comprehended that if a plate or piece
of paper were attached to such a steady point or mass, and a pencil
or tracer had one of its ends resting on the plate, and its other end
attached to the support that vibrates with the earth, a tracing or
record would be drawn on the plate from which the character of the
motion of the end of the tracer, and, therefore, of the earth, would
be marked on the plate.
[Illustration: FIG. 53. VICENTINI VERTICAL PENDULUM]
Various devices have been employed for the steady points. The most
successful consists of a heavy mass of lead.
Fig. 53 represents a form of instrument invented by Professor
Vicentini of Italy. Here the steady point consists of a heavy leaden
bob, of 200, 400, or even 500 kilograms, suspended by three metallic
rods united above by a brass cap, hung on a steel wire to a bracket
fixed on the wall. This wire may have a length as great as fifty feet.
[Illustration: FIG. 54. VICENTINI PENDULUM AND RECORDER]
Fig. 54 represents the recording instrument. Here a tracer is provided
that is capable of multiplying the motion fifty times, or even eighty
times. A record is traced on a sheet of paper passing over a roller
which imparts a rapid motion to a sheet so as to make sure that the
different parts of the shock or vibration will be recorded on separate
portions of the paper.