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rim of the Kilauea crater. the setting was spectacular, and a hotel close by
the volcano had been drawing tourists since 1866.
117
But the position was
a marginal one, apparently one of the few open to Wood without a PhD.
Finding himself in a wilderness 2,500 miles from California's booming cit-
ies, Wood began to ask new questions. He began to wonder, for instance,
if the paucity of felt reports of earthquakes at Kilaeua was a function of
something other than the low population density. It seemed that observ-
ers nearby were not sensing tremors that registered on his instruments as
exceeding 1 cm/sec
2
—the value of ground acceleration that Holden had
postulated as the minimum perceptible to humans. Wood pointed out that
this minimum unit had “never been determined by psychological experi-
mentation.”
118
the question he raised was a fundamental one: what exactly
did humans feel when they felt an earthquake?
the answer, according to his analysis of twenty-nine earthquakes re-
corded in Kilauea, was that the relevant stimulus was not the acceleration
of the shock alone, but rather the acceleration in combination with the am-
plitude of the ground movement. ever since the work of Dutton, Holden,
and Mendenhall on the mechanical interpretation of intensity in the 1880s,
seismologists had assumed that the dependence of seismic intensity on the
amplitude of ground motion was as simple as the dependence of sound
intensity on the amplitude of sound waves. Wood was pointing to a more
complex relationship: the amplitude-dependence of the intensity of an
earthquake as judged by human observers actually varied with the accelera-
tion of the shock. For weaker shocks, amplitude might even become more
important than acceleration in determining perceptibility. Wood called for
further study—“if possible, by experimentation.”
119
A decade later, the Brit-
ish seismologist R. D. Oldham challenged Wood on this point. Oldham sus-
pected that Wood's value for the “minimum unit of seismic perceptibility”
was artificially low, due to the use of “skilled observers.” He cited evidence
that the sensitivity of observers rose with prior exposure to earthquakes or
with training.
120
Wood objected (in an unpublished manuscript) that the
observations had by no means come from “skilled observers, specially on
the look-out, and living in lightly framed timber dwellings raised clear of
the ground, especially suited, consequently, for the recognition of feeble
shocks.” On the contrary, the observations “were made and reported by
all
sorts of people
and most of the structures.”
121
Wood was gaining a sense of the differences between studying nearby
earthquakes and distant ones. It was a distinction he would often phrase in
terms of “regional” versus “world” seismology, or “local” versus “teleseis-
