168 REID ET AL. KEPOET OX NOMENCLATURE OF FAULTS 



Our determinations are almost always relative, and hence we can 

 rarely tell which side of the fault has moved; therefore the expressions 

 "upthrow" and "downthrow," which are used according to the side from 

 which the fault is viewed, are objectionable, as they suggest that. a par- 

 ticular side of the fault has actually been moved. They are in too gen- 

 eral use to be abandoned, but it should be definitely understood that they 

 refer merely to a relative and not to an absolute displacement. 



It is desirable to emphasize the fact that the definitions which follow 

 refer, with a few exceptions, to the part of the fault under consideration 

 and not necessarily to other parts nor to the fault as a whole. 



We give the word "displacement" no technical meaning, but reserve it 

 for general use; it may be applied to a relative movement of the two 

 sides of the fault, measured in any direction, when that direction is speci- 

 fied, or to the change in position of a bed, etcetera, caused by the fault 

 movement. The word "dislocation" will also be most useful in a general 

 sense. 



Teanslatokt Movements 

 gexeral discussiox of terms 



There are two ways of defining the displacement due to a fault ; we 

 may define the apparent relative displacement of a bed by naming the 

 distance between its two disrupted branches measured in any chosen direc- 

 tion, such as the vertical distance between the branches, measured in a 

 shaft, or the perpendicular distance between the lines of intersection of 

 the two branches with the fault plane ; or we may define the actual rela- 

 tive displacement of the two sides and its components, in important 

 directions. The apparent displacements are those usually measured di- 

 rectly ; the actual displacement must be worked out for a complete under- 

 standing of the fault. 



It is necessary to have different words to indicate displacements meas- 

 ured in these two ways. For this purpose we have adopted the following 

 technical words, qualifying words being added to indicate the component 

 of the displacement referred to : 9 



Slip, which indicates the relative displacement of formerly adjacent 

 points on opposite sides of the fault, measured in the fault surface. The 

 qualifying words relate to the strike and dip of the fault surface. 



Shift, which indicates the relative displacement of regions on opposite 

 sides of the fault and outside the dislocated zone. The qualifying words 

 relate to the strike and dip of the fault surface, with the exception of the 



The exact definitions of these words will be given further on. 



