4 THE SAN FRANCISCO EARTHQUAKE AND FIRE. 
fault. As the ridges of the neighboring Coast Range have the same 
northwesterly trend, it is thought probable that a subterranean slip- 
ping here occurred on a fault associated with the valley. In that 
case the geologic event causing the earthquake included coincident 
or nearly coincident yielding on more than one fault plane of the 
Coast Range system, and various other peculiarities in the distribu- 
tion of intensity may be ascribed to local faulting. 
The region of high intensity, to which most of the destruction was 
limited, is a belt 20 to 40 miles wide, extending from the mouth of 
Eel River at the northwest to Priest Valley at the southeast (fig. 1). 
This belt includes the surface expression of the principal fault — a 
feature distinctively known as the fault trace— a large number of 
cracks, and many local and superficial dislocations of soil and rock 
variously characterized as landslides and slumps. 
THE FAULT TRACE. 
The plane of the earthquake-causing fault, ivhere it appears at the 
surface, is approximately vertical. The movement which took place 
along this plane was approximately horizontal. 
Fig. 2.— Diagrams illustrating the nature of the earthquake-making fault. 
As the statement of these relations is sometimes found confusing, 
they are here illustrated diagrammatically. The upper diagram in 
fig. 2 represents a rectangular block as if cut from the land, the thick- 
ness being 25 feet, the length east and west (right to left) 150 feet, 
and the width 100 feet. The dotted line NS indicates the surface 
outcrop of the old fault plane, trending northwest and southeast, 
this plane cutting the face of the block in the vertical line SD. AB 
stands for any straight line on the surface of the land — such as would 
be defined by a road, a fence, or a row of trees — crossing the fault 
outcrop at right angles. The lower diagram represents the same 
