abstracts: geology 351 



For fully a year from this date small shocks occurred at intervals of a 

 fews days, but as there were no other destructive shocks the people 

 gradually became accustomed to the vibrations and gave little or no 

 further attention to them. 



There is geologic evidence of earthquakes in this region long antedat- 

 ing that of 1811. Cracks as large as any of those caused by the last 

 great disturbance have been seen covered by trees fully 200 years old. 

 Nor is the action apparently altogether recent, for post-Lafayette but 

 pre-Iowan faults (antedating the deposition of the loess), and apparently 

 being either a cause or accompaniment of earthquakes, have been ob- 

 served in Crowley Ridge, and Glenn has described sandstone dikes 

 filling old earthquake cracks in the Porters Creek formation of the 

 Eocene Tertiary. 



The area affected by the New Madrid earthquake may be subdivided 

 into an area of marked earth disturbances, an area of slight earth dis- 

 turbances, and an area of tremors only. The total area characterized 

 by disturbances of the first type is from 30,000 to 50,000 square miles; 

 of the second type, of over a million square miles. 



Godfrey Le Sieur, who witnessed the shock, described it as follows: 

 "The earth was observed to roll in waves a few feet high, with visible 

 depressions between them. By and by these swells burst, throwing up- 

 large volumes of water, sand, and coal." The movement in the area of 

 principal disturbance was from the southwest. 



The tectonic effects of. the earthquake were fissures, faults, land- 

 slides, uplifts, domes, and depressions. There were also extrusions of 

 gas, water, and of various rock-making materials. The general trend 

 and shape of the affected area, taken in connection with the direction of 

 the earth waves, points to a centrum of the original shock along a north- 

 east-southwest line extending from a point west of New Madrid to a 

 point a few miles north of Parkin, Ark. The centrum of the heavier 

 subsequent shocks seems also to have been along essentially the same 

 line. The location of the centrum of some of the later and lighter shocks 

 may have been elsewhere. Inasmuch as the center of activity of the 

 primary shocks is within the embayment area of the Mississippi Valley 

 and well removed from the surrounding areas of consolidated rocks, it 

 seems clear that the ultimate cause lies in forces operating beneath the 

 embayment deposits. The action may be associated either with the 

 processes of folding or warping or be incident to a depression and deepen- 

 ing of the basin. A. H. Brooks. 



