THE EARTHQUAKE AS A NATURAL PHENOMENON. 5 
rectangular block after the earthquake, its t.»o parts dislocated by 
sliding horizontally along the fault plane, and the line AB made dis- 
continuous by an offset. 
To return, now, from the ideal to the actual, the sides and bottom 
of the earth block disappear. The depth to which the fault pene- 
trates is indefinite and unknown, and so is the extent of the lands on 
either side affected by the displacement. For nearly 200 miles there 
is a fracture on the face of the land, and everything traversed by the 
fracture is dislocated, the part on the southwest side having appar- 
ently moved tow r ard the northwest and the part on the northeast side 
having apparently moved toward the southeast. The total horizontal 
offset ranges in general from 2 to 16 feet, but at one place, affected by 
abnormal conditions, reaches 20 feet (PL I, A). The effect is also 
shown by a redw r ood tree (PI. II), which was situated at a place 
where the displacement was slight. The average offset is 10 to 12 
feet. Associated with the horizontal dislocation are vertical disloca- 
tions of minor and variable amount. This line of fracture, with the 
associated dislocation, is the surface expression of the fault which 
occurred on April 18, 1906. It is the visible trace of the fault across 
the surface of the land, and in the following pages will be called the 
"fault trace." « 
The fault trace itself is in some places inconspicuous, as in the flat 
meadow represented in PI. I, Z?, where one might readily walk across 
it without noticing that the ground had been disturbed. Its ordinary 
phase, however, includes a disruption of the ground suggestive of a 
huge furrow, consisting of a zone, between rough walls of earth, in 
which the ground is splintered and the fragments are dislocated and 
twisted. This phase is shown in PL III. In many places the fault 
trace sends branching cracks into bordering land, and locally its 
effect in dislocation is divided among parallel branches. 
The views of the fault trace given in PL VII represent it at a 
point not far from the head of Tomales Bay, where it traverses a 
hillside having a general slope toward the southwest. In the upper 
view w r e look toward the northwest; in the lower, toward the south- 
southeast. The horizontal displacement is here about 16 feet, the 
ground at the left, in each view, having moved from us, and the 
a In the case of an earthquake fault with important vertical displacement the surface 
expression is a small cliff or scarp, and to such a feature the name "fault scarp" has 
been given ; but this name does not serve to characterize the feature produced by the 
horizontal displacement of April 18, 1906. In default of an appropriate and established 
name the geologists who first traced the feature made tentative use of " furrow " and 
" rift ; " and the word rift was employed in the preliminary report of the California com- 
mission, in a popular article by the present writer, and probably in other places. The com- 
mission, however, in giving more deliberate attention to terminology, has determined to 
reserve the word rift for a moaning more in consonance witli earlier geologic usage and 
to substitute " fault trace " for the surface expression of the fault. . The present paper 
conforms to the nomenclature accepted by the commission. 
7171— Bull. 324—07 2 
