Page:Popular Science Monthly Volume 77.djvu/160

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ARTHQUAKES are infrequent in the upper part of the Mississippi Valley, and observations on earthquake phenomena in this part of the world have a peculiar interest, not only on account of the special bearing they may have on seismological questions, but also on account of the light they throw on the psychology of an observant public which is unacquainted with seismic phenomena. The writer wishes to present some observations on the earthquake which occurred in this region on the twenty-sixth of May, 1909. They are based on notices which appeared in the public press, and which were secured from fifty daily and weekly newspapers immediately after the earthquake. The collected reports contained in all some three hundred observations on incidents which occurred during a few moments shortly before nine o'clock in the morning, when the earth waves rapidly traversed the states of Illinois, Iowa, Wisconsin, Michigan, Indiana, Missouri and Minnesota.

The reports collected indicate that the mesoseismal area of this earthquake lay in northern and north central Illinois, and reached slightly beyond the south boundary of Wisconsin. It appears also that there were no less than three epicentral tracts, one near Dubuque in Iowa; one near Waukegan, and another near Bloomington, in Illinois. At all of these places the shock was strong enough to slightly damage a few buildings. From this unusually large triangular mesoseismal area the earthquake waves spread in all directions, sensibly as far north as to Rochester in Minnesota and to Muskegon in Michigan, east as far as to Muncie in Indiana, westward to DesMoinesDes Moines [sic] in Iowa, and southward to Hannibal in Missouri, affecting an area of some five hundred thousand square miles.

In the central region, where the earthquakes are most complex, one report states that a distinct raise, or upward motion, was first felt, and that this was followed by a trembling. In other cases, houses and floors are said to have "heaved." In Beloit the houses are said to have been "jostled out of plumb." Violent shaking and rocking is also reported. Farther out from the central area there is a more frequent use by the reporters of such terms as "shaking," "rocking," "swaying" and "jarring," while toward the outer margin of the disturbed area houses are more often said to have been gently rocked and shaken, or to have "trembled" or "quivered," indicating the more gentle and