Geoscience Reference
In-Depth Information
“In order of severity the quakes have been: tokio 5.8; Long Beach, 4.6; Santa
Barbara 3.4; San Francisco, 2.5.”
note—Anything man-built cannot withstand a velocity of 10.00.
Wood made clear that the paper's numbers were entirely meaningless. In
this context, it must have been a relief to Wood to be able to state, with the
support of Richter's new scale, that Long Beach was “not a great earthquake.
It was a fairly strong and moderately large shock in the local earthquake
class. It had about the same magnitude and violence as the Santa Barbara
shock in 1925, perhaps a little greater, perhaps a little less.”
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Richter's scale originated, then, not in a quest for geophysical truth, but
within this pragmatic effort to classify earthquakes in terms of the dam-
age they could be expected to cause. Richter's idea was straightforward: the
ratio of the amplitudes produced by a single shock at two stations should
be inversely related to the ratio of the distances of each station from the
epicenter. the scale defined a unit magnitude corresponding to the response
of a standard seismometer to a standard shock at a distance of one hundred
kilometers. this was not meant to solve the fundamental physical question
of what intensity measured—whether, for instance, acceleration, amplitude,
or duration of shaking. Indeed, Richter himself judged the scale according
to the pragmatic criteria of consistency. the only way to “demonstrate the
reality of the computed grades of magnitude” was to compare calculated
values of magnitude to the “observed effects of a number of representative
shocks.”
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Moreover, contrary to the air of precision with which earthquake
magnitudes are often cited, Richter himself had no expectation of produc-
ing an exact measure: “Precision in this matter was neither expected nor
required.” Indeed, he stressed the method's many sources of inexactness,
including “inhomogeneity in the propagation of elastic waves, of varying
depth of focus, of difference in mechanism of shock production, of the
ground at the several stations, and of the instrumental constants.” In other
words, instruments might respond nearly as contingently as people did.
equally important, Richter did not aim from the start to make his scale uni-
versal. It emerged firmly within Wood's tradition of regional seismology. It
took decades to generalize the scale beyond Southern California because of
the need to develop corrections for different soil conditions and instrument
types.
nevertheless, Richter's scale soon took on a life of its own, and its popu-
larity edged out alternative perspectives. As Byerly would later write, “It has
been a struggle throughout the years to keep the record of felt earthquakes.
the public, both scientific and lay, have become so enamored of epicenters
