2 THE SAN FRANCISCO EARTHQUAKE AND FIRE. 
above referred to ; and a monographic report is to be expected when 
the labors of this commission shall have been completed.® 
An earthquake is a jar occasioned by some violent rupture. Some- 
times the rupture results from an explosion, but more commonly 
from the sudden breaking of rock under strain. The strain may be 
caused by the rising of lava in a volcano or by the forces that make 
mountain ranges and continents. The San Francisco earthquake of 
April 18, 1906, had its origin in a rupture associated with mountain- 
making forces. A rupture of this sort may be a mere pulling apart 
of the rocks so as to make a crack, but examples of that simple type 
are comparatively rare. The great majority of ruptures include not 
only the making of a crack but the relative movement or sliding of 
the rock masses on the two sides of the crack; that is to say, instead 
of a mere fracture there is a geologic fault. After a fault has been 
made its walls slowly become cemented or welded together; but for 
a long time it remains a plane of weakness, so that subsequent strains 
are apt to be relieved by renewed slipping on the same plane of 
rupture, and hundreds of earthquakes may thus originate in the 
same place. 
A faulting may occur far beneath the surface and be known only 
through the resulting earthquake; but some of the quake-causing 
ruptures extend to the surface, and thus become visible. The New 
Madrid and Charleston earthquakes are examples of those having 
deep-seated origins; the shocks at Inyo and San Francisco, of those 
whose causative faults reached the surface of the ground. 
The San Francisco earthquake had its origin, wholly or chiefly, in 
a new slipping on the plane of an old fault. The trend of the fault 
is northwest and southeast, and it is known through a distance of sev- 
eral hundred miles. Visible evidence of fresh slipping — a surface 
trace, to be described presently — does not appear through its whole 
extent, but has been traced from San Juan at the south to Point 
Arena at the north (fig. 1), a distance of about 180 miles. Because 
the earthquake was severe in Priest Valley, 60 miles southeast of San 
Juan, it is thought that subterranean slipping on the old fault plane 
extended beyond San Juan. At Point Arena the visible fault trace 
passes under the ocean, and the line of its trend does not again touch 
the coast, so that its northwestern course and extent are in doubt. 
A fault trace which appears at Point Delgada, 75 miles to the north, 
may be part of its continuation or may represent a separate fracture. 
In a general way the intensity of the shock was greatest near the 
° The California earthquake investigation commission is composed of Andrew C. 
Lawson (chairman), A. O. Leuschner (secretary), G. K. Gilbert, H. F. Reid, J. C. Branner, 
George Davidson, Charles Burkhalter, and W. W. Campbell. Its work is organized under 
three committees— a committee on isoseismals, A. C. Lawson, chairman ; a committee on 
coseismals, A. O. Leuschner, chairman, and a committee on the geophysics of the earth- 
quake, H. F. Reid, chairman. 
