154 THE POPULAR SCIENCE MONTHLY 



OSERVATIONS ON THE EARTHQUAKE OF MAY 26, 1909 



By Peofessou J. A. UDDEN 



ADGUSTANA COLLEGE, ROCK ISLAND, ILL. 



EARTHQUAKES 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 

 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 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 



