THE EARTHQUAKE OF MAY 26, 1909 



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M i HNESOT/I 



Isoseismals of the Earthquakes in the Upper Mississippi Valley, May 26, 1910. 



more regularly undulating nature of the free and gradually vanishing 

 oscillations of the earth. 



The greater number of earthquakes are now known to be due to 

 slipping of enormous blocks of the earth., along fissures or joints of 

 great depths, and thus forming the dislocations known to geologists as 

 faults. In the case of the earthquakes with two maxima of disturbance, 

 the slipping occurs first at one point in such a fissure, and then at 

 another. There can be no doubt that this Illinois earthquake was of 

 the nature of such a compound slip, although the exact position of the 

 fault can not be correctly located from the data at hand. In most 

 descriptions of the shock no mention is made as to whether there was 

 one maximum or more. Such particulars were naturally overlooked. 

 The people of the upper Mississippi Valley are not trained in making 

 observations on earthquakes. Nevertheless, nine observers make men- 

 tion of more than one commotion. One account from each of eight 

 localities states that two distinct shocks were felt. These places are 

 Bushnell, Canton, Champaign, Chicago, Geneva and Sterling in Illi- 

 nois, and Davenport and Dubuque in Iowa. In the latter place the 

 first disturbance lasted about ten seconds, after which there was a short 

 pause and then again a shock of short duration. But the reports from 

 Chicago, Springfield and Champaign, which places lie on the other side 

 of the mesoseismal area, all agree in stating that the first shock was of 

 brief duration, and that the second lasted several seconds. One ob- 

 server is reported as having noted three distinct shocks, and this was 

 Professor W. H. Hobbs, at the time on a visit in Madison, Wis. He is 



