50 Gilbert — The California Earthquake. 



In detail it comprises many small ridges and hollows, approxi- 

 mately parallel but otherwise irregularly disposed, and evi- 

 dently caused by splintery dislocation. Streams zigzag more 

 or less about the ridges, and the hollows contain many small 

 ponds and marshes. There are reports of long cracks which 

 appeared in different parts of the rift in connection with various 

 earthquakes of the last century, and it is inferred that each of 

 these cracks was the surface expression of a fault-slip similar 

 to that of 1906. It is further inferred that the rift as a whole 

 marks the outcrop of a long fault or fault zone, separating two 

 crustal tracts which are slowly moving past one another, with 

 gradual accumulation of strain and stress, and occasional relief 

 by local slipping when the stress at some point overpowers 

 the adhesion on the fault plane. The physiography of the rift 

 is illustrated by numerous excellent photographs, and by a local 

 contour map by F. E. Matthes. Although the rift has been 

 mentioned in various writings of earlier date, its description in 

 this volume practically adds a type of surface configuration to 

 physiographic science. 



In the discussion of the intensity of the shock, a distinction 

 is recognized between the elastic wave propagated from the 

 origin through the crust, with gradually diminishing magnitude, 

 and the phenomena of emergence, conditioned by the nature 

 of the surface formation. The intensity observed at the surface, 

 and expressed chiefly by damage to buildings and other struc- 

 tures, is called "apparent intensity," and this only is mapped. 

 The general map shows a long narrow belt of high intensity, fol- 

 lowing the fault, with peninsulas and outlying islands where 

 destructive effect was enhanced by the presence of incoherent 

 formations; but this elongation is less characteristic of the 

 lines limiting the areas of low intensity. The outer line, 

 touching the most remote points of sensible tremor, traverses 

 southern Oregon, central Nevada and southern California. 

 In view of the ideas recently advanced by W. H. Hobbs, 

 there is a careful review^ of the relation of local intensity to 

 the known major faults of the region, about forty in number. 

 In three cases it was thought possible that some portion of 

 the movement of dislocation was diverted from the main (San 

 Andreas) fault to the planes of intersecting faults. A special 

 intensity map of San Francisco, by H. O. ^W\)od, shows with 

 great detail the grades of violence; and its comparison with a 

 geologic map brings out forcibly the intimate relation between 

 effective intensity and the underlying formation. 



The subject is further elucidated by the report of an experi- 

 mental study by F. J. Rogers. By mechanical arrangements 

 similar to those employed by the Japanese commission in inves- 

 tigating the principles of earthquake-proof construction, bar- 



