Geoscience Reference
In-Depth Information
What is really desired is some reliable indication which shall serve as a meas-
ure of the amount of energy in any given portion of the wave of disturbance
as it passes each locality. The means of reaching even a provisional judgment
are very indirect, and qualified by a considerable amount of uncertainty. . . .
In view of the precise methods which modern science brings to bear upon other lines
of physical research, all this seems crude and barbarous to the last degree.
But we
have no other resource. even if it were possible to obtain strictly comparative
results from such facts, and decide with confidence the relative measure of
intensity which should be assigned to each locality, we should have gained
measures only of a series of local surface intensities, and not of the real en-
ergy of the deeply seated wave which is the proximate cause of the surface
phenomena. notwithstanding the indirect bearing of the facts upon the real
quantities we seek to ascertain, and their apparently confused and distantly
related character, they give better results than might have been supposed.
When taken in large groups, they give some broad indications of a highly
suggestive character.
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Dutton believed he had found a way to use isoseismals to calculate the
depth of the shock's origin. He defined intensity as the energy of the seismic
wave per unit area of the wave front; from this definition it followed that
intensity varies inversely with the square of the distance from the origin.
This method was as ingenious as it was unreliable. Dutton had confused
intensity, the destructiveness of the earthquake shock as measured by the
Rossi-Forel scale, with the energy contained in the seismic wave. As the Brit-
ish geologist Richard Dixon Oldham explained, Dutton's formula for the
depth of the origin was virtually meaningless:
One of the most important problems of seismology is the determination of
the depth at which earthquakes originate. One method after another has been
proposed, only to be abandoned as its failure to give a true answer became ap-
parent, and Major Dutton has himself invented a method which he believes
to be sound, and which would be sound if its application were not vitiated
by a logical fallacy. in the formula he uses the term “intensity” in the sense of
energy per unit of wave-front, but in the application in the sense of a degree
of the Rossi-Forel scale, which, like every other scale proposed in place of it,
is miscalled a scale of intensity, being in fact a scale of acceleration, or, more
simply, violence of shock. The two are very different, and differ in their rate of
variation with distance from the origin, so that we are still left with no certain
and trustworthy method of determining the depth at which earthquakes orig-
inate, but have, on the other hand, a lesson in
the danger of misusing words.
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