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correlated these laboratory results with field evidence from the Mino-Owari
earthquake of 1891, producing a scale of seven degrees—from a “strong”
earthquake that slightly cracks brick walls “of bad construction” (maximum
acceleration no more than 300 mm/sec 2 ) to the upper limit of a “violent”
earthquake that destroys all buildings, “except a very few wooden houses”
(maximum acceleration above 4,000 mm/sec 2 ). 28 Reid used Omori's scale
to translate damage reports into values of maximum ground acceleration.
He then used the method Dutton had introduced in 1886 to calculate the
approximate focal depth of the shock—a shallow fault of only twenty ki-
lometers. this estimate of the depth figured directly into his calculation of
the “work done by the elastic stresses.” 29 Reid thus worked primarily with
data at the high end of the intensity scale, where he could rely on architec-
tural damage as evidence.
In order to trace fault lines beyond the hardest hit regions, however, it
was essential to map the distribution of weaker effects. this meant working
at the more ambiguous, lower end of intensity scales. In Sacramento, in-
tensity was estimated between 6 and 7 on the Rossi-Forel scale (6: “general
awakening of those asleep,” “some startled persons leaving their dwellings”;
7: “general panic”), but at Santa Barbara it dipped down to 4 (“felt by per-
sons in motion”). effects were reported as far as 340 miles east of the fault.
“Farther east the most notable feature of the reports is that wherever the
effects of the earthquake were made evident, the physical signs, such as the
swinging of suspended objects, etc., were described almost to the exclusion
of direct physiological effects.” George Louderback of Berkeley explained
this contradiction to the Rossi-Forel scale by noting that the region was
sparsely inhabited and its residents largely asleep when the shock occurred;
“the few who were up were moving about at active work and were in general
not of a sensitive type.” A few people engaged in irrigation noticed slight
waves in the water, an odd sight on a still morning. But those who men-
tioned it to others were met with “sallies of wit at the expense of the report-
ers.” 30 there was thus no shortage of frustrations for researchers measuring
weaker intensities east of the Sierra nevada.
Indeed, it became evident in the course of the investigation that the com-
mission was uncomfortable with the very concept of intensity. Lawson's
introduction cautioned that intensity as measured by field observations
was to be regarded as mere “apparent intensity,” in contrast to the “real
intensity” corresponding to the “energy” of the “earth-waves.” “Inasmuch
as we have to deal primarily with observable effects and record these as a
basis for inference, it has been found convenient to use the term 'apparent
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