THE SAN FRANCISCO EARTHQUAKE AND FIRE OF APRIL 
18, 1906, AND THEIR EFFECTS ON STRUCTURES AND 
STRUCTURAL MATERIALS. 
By G. K. Gilbert, R. L. Humphrey, J. S. Sewell, and Frank Soule. 
THE EARTHQUAKE AS A NATURAL PHENOMENON. 
By G. K. Gilbert. 
INTRODUCTION. 
The investigations to which the San Francisco earthquake has 
given rise are of two classes — the study of the natural phenomena 
constituting or associated with the earthquake, and the study of the 
relations of the San Francisco earthquake and future earthquakes to 
human activities. 
The principal studies of the earthquake as a natural phenomenon 
are under the direction of the California earthquake investigation 
commission, which was appointed by the governor of the State 
three days after the occurrence of the shock. The human or economic 
aspects of the subject have been studied chiefly by engineers and 
architects, of whom a number have acted in private capacity, while 
others have acted as the representatives of governmental or private 
organizations. In many of the architectural studies the earthquake 
and fire have been considered together, as the destructive effects of 
these two sources of danger were closely associated in the San 
Francisco disaster. 
This volume is devoted to certain economic aspects of the subject, 
and the present chapter on natural phenomena of the earthquake 
selects those features which seem contributory to an understanding 
of the papers which follow. A fuller account of tjie earthquake 
may be found in the preliminary report of the State commission 
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