MEASUREMENT OF FAULTS. 255 
to measure fault movements along certain arbitrary planes. The most 
valuable of these planes are the earth’s surface, which may be considered 
a horizontal plane, and vertical sections, into which available data are put, 
with the gaps in the chain of information often theoretically filled out. In 
such cases, where some dislocation is evident, but the information is so 
meager that it is not possible to know the fault so accurately as to estimate 
even approximately its total displacement or lateral or perpendicular sepa- 
ration, it is necessary to employ specific terms to designate the known or 
estimated dislocations, although the relations of these dislocations to the 
total displacement may be unknown. For this purpose the terms heave or 
offset, throw, and vertical separation may be used. The terms throw and 
vertical separation are applied to the dislocations of a fault as seen in a 
vertical section; the terms heave and offset, to the dislocation as seen in 
a horizontal section, such as the earth’s surface may be considered. 
The throw may be defined as the distance between the two parts of 
any body available as a criterion (such as a sedimentary bed), when these 
parts have been separated by a fault, the distance being measured along 
the fault plane as shown in a vertical section. 
The vertical separation is the perpendicular distance between the 
intersection of the two parts of any body available as a criterion, such 
as a sedimentary bed, with the plane of a vertical section, the lines of 
intersection being projected, if necessary, for the purpose of measurement. 
In perpendicular faults the vertical separation is identical with the throw. 
In all others it is less than the throw, but sustains a certain relationship 
to it, being one side of a right triangle of which the throw is the 
hypotenuse. Thus the vertical separation may vary from zero to the full 
amount of the throw. The throw is always a part of the total displace- 
ment, although with no definite relationship to it, and varies from zero to 
the full total displacement. 
The terms heave and offset may be used interchangeably to designate 
the perpendicular distance between the intersections of corresponding 
planes in the two parts of any body available as a criterion, such as a 
sedimentary bed, with a horizontal plane, such as the earth’s surface may 
be considered. Like the throw, the heave or offset is a part of the total 
displacement, but it has no definite relationship to it. 
