Geoscience Reference
In-Depth Information
independent character, that much slower undulations were observed . . .
a great deal of it is positive and unequivocal as to what seemed to be the
fact. the evidence suggests that there is a type of wave in the ground, in the
region of high intensity, which has not yet been sufficiently recognized, and
the origin of which is obscure.”
24
Based on the ratio of horizontal compres-
sion to vertical expansion at the earth's surface, the ordinary longitudinal
seismic waves could indeed be expected to produce surface waves of ap-
proximately 1.33 inches. However, “it is not necessary to believe that the
amplitudes of surface waves are nearly as large as they appear, for it must
be remembered that an observer being shaken by the strong vibrations of
a violent earthquake is in a difficult position to make good observations
on the phenomena about him, and particularly to distinguish between the
movements which are actually taking place and those which he apparently
sees, but which are really due to his own oscillations.”
25
the question of the
reality of visible surface waves and their “obscure” origin was a persistent
motivation for the analysis of felt reports over the following decades.
Reports of earthquake sounds also aroused skepticism at first. In an early
publication, McAdie doubted that these sounds were a genuine seismic phe-
nomenon: “in our judgment most of these sounds can be explained by the
noise due to violent shaking of dwellings.”
26
In the course of the investiga-
tion, however, the commission concluded otherwise. Of eighty-one people
reporting having heard a sound accompanying the shocks, forty stated that
the sound preceded the jolt, and “evidence as to the character of the sounds
is consistent and uniform.” these were low vibrations, “below the range of
audibility of some people.” this limit would explain the fact that the earth-
quake seemed not to have been heard in some places where its effects were
seen. the commission went so far as to inquire of a professor of psychology
at Johns Hopkins “in regard to the limit of sound.” they were told that indi-
viduals varied as to the lower limit and were referred to Helmholtz's classic
treatise on sensations of tone.
27
eventually, scientists were willing to believe
that San Francisco residents might indeed have seen the earth undulate and
heard it roar.
For the harder hit regions, the distribution of intensity could be ascer-
tained from surveys of structural damage alone. It was on this basis that
Harry Reid developed his seminal theory of elastic rebound. He employed
the “absolute scale of destructive earthquakes” that Fusakichi Omori had con-
structed from experiments with a shaking table. Assuming that most ground
motion is horizontal, Omori determined the ground acceleration necessary
to overturn or fracture brick columns of varying dimensions and quality. He
