EARTH FLOWS 643 



The following paper was then read : 



EARTH-FLOWS AT THE TIME OF THE SAN FRANCISCO EARTHQUAKE* 



BY ROBERT ANDERSOIJ^ 



[Ahstract] 



Landslides of different kinds resulted in great numbers from the California 

 earthquake of April 18, 1906. The term cartJi-flow is applied to landslides 

 having the nature of flows of surface material partially saturated with water 

 that were produced as a result of the earthquake by the sudden accession of 

 water to points on the surface. Underground conduits of water suffered dis- 

 turbances from the earthquake shock, and seepages occurred at the surface at 

 new points or, in increased amounts, at old points of moisture concentration. 

 In certain cases the water seems to have risen with a gush, as if actually 

 squeezed from tlie hills. The effect of this abnormal supply of water was a 

 saturation and loosening of the subsoil and a flowing movement of the sur- 

 face debris away from areas so affected. The original loosening was aided by 

 the vibratory movement of the shock, especially at points where some moisture 

 had already gathered in the ground, the intensity of the shock in moist ground 

 having been much greater than elsewhere. The water was the chief agent 

 in causing the movement of the loosened material, as a result of the weight 

 that it added and the semi-fluid nature that it gave to the mass. The process 

 was usually one of sapping and undei'mining, the surface sod remaining dry 

 and being broken into blocks and transjiorted in a comparatively little dis- 

 turbed position on a plastic substratum. At some of the earth-flows water 

 continued to flow after the earthquake where it had not flowed before. 



Earth-flows were of frequent occurrence in the Coast ranges, the writer 

 finding them numerous on the San Francisco peninsula and in the Santa Cruz 

 mountains within a score of miles of the fault, and what appeared to be flows 

 of similar origin at a much greater distance from the epicentrum. They were 

 formed on gentle as well as steep slopes, and both in previously dry drain- 

 age depressions and on convex hillsides. In the largest flows thousands of 

 tons of earth and rock detritus were removed and carried hundreds of yards, 

 leaving great cavities. In one case a hole ten feet deep was excavated over 

 an area of nearly an acre on a five-degree slope, and the material removed 

 was spread over two acres. 



Such earth-flows, as well as the related slumps caused by earthquakes, in 

 the case of which it is not easy to determine \vhether or not the slide or flow 

 of soil has been aided by a suddenly increased water supply, are of special 

 geologic importance for the reason that they aid in the initiation of drainage 

 lines and the furtherance of degradation. 



The following paper was read by E. B. Moore: 



RADIO-ACTiriTT OF THE THERMAL WATERS OF YELLOWSTONE NATIONAL 



PARK 



BY HERMAN SCHLUNDT AjVD RICHARD B. JfOORE f 



* This subject is treated more fully in the report of the Earthquake Commission being 

 published by the Carnegie Institution of Washington, 

 t Introduced by C. W. Hayes. 



