THE SAX FRANCISCO EARTHQUAKE 



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feet and have an average of about 10 feet. At one place (Fig. 5) a 



road was offset 20 feet, but in this case the underlying ground was wet 



alluvium and part of its movement may have been due to a flowing of 



the soft material. There was also 



some vertical change, but this was 



not everywhere in the same direction 



and its amount was comparatively 



small. At many points the land 



west of the fault appears to have 



risen one or two feet as compared 



with the land at the east. 



The surface manifestation is not 

 usually a simple crack, but a dis- 

 turbed zone a few feet broad, the 

 earth within the zone being split into 

 blocks which show more or less twist- 

 ing or rotation. In some places 

 the zone is slightly depressed below 

 the adjoining surfaces, and else- 

 whore slightly elevated. Other dis- 

 turbances of the surface were asso- 

 ciated with the earthquake, but the 

 track of the central fault has a 

 character of its own, a character with 

 which the field workers soon be- 

 came familiar, so that it could be clearly identified. It came to be 

 distinguished in their conversation and note-books as * the rift.' For 

 considerable distances the rift is single, but elsewhere it is more or 

 less divided, the parts lying within a few rods of one another and being 

 approximately parallel. There are also branches parting from the main 

 rift at various angles and gradually dying out in the adjacent country, 

 and in some of these the belt of disturbance is broad and complicated 

 (Fig. 7). There are also outlying cracks occurring within a mile or 

 two of the central rift and having irregular courses, and these mav 

 probably be referred to the same general system of rock strains. 



Other cracks are distinctly secondary in character; that is to say. 

 they are not due directly to the stresses and strains by which the fault 

 was made, but are results of the earthquake itself. The jar constituting 

 the earthquake, or in technical language the earthquake wave, as it 

 travels through rock and earth produces temporary compressions and 

 other strains, and these often occasion cracks at the surface. Where 

 the material is elastic such secondary cracks merely open and close, 

 leaving the ground with its original form ; but where it is inelastic and 

 incoherent, as in the case of young alluvial formations and artificial 



Fig. 6. Ordinaky Api'Eai;a.\ce of the 

 Earthquake Rift where it traverses 

 Firm Turf. 



