154 THE POPULAR SCIENCE MONTHLY 
OSERVATIONS ON THE EARTHQUAKE OF MAY 26, 1909 
By Proressor J. A. UDDEN 
AUGUSTANA COLLEGE, ROCK ISLAND, ILL. 
BK ARTHQUAKES are infrequent in the upper part of the Missis- 
sippi 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 Min- 
nesota. 
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 
Towa; 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 DesMoines 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 
