50 CRANDALL— SAN FRANCISCO PENINSULA. [January 4, 



small loss of life. Inasmuch as this area is directly affected, a few 

 remarks, of a general nature, are appended regarding the relations 

 of the general geologic structure to the earthquake. 



Relations to Structure. — As shown in the chapter on structure 

 the northern end of the peninsula has its main features determined 

 by three faults. The largest of these is the San Andreas-Portola 

 fault which is one of the main lines of weakness in the Coast Ranges. 

 The earthquake is supposed to have been caused by the movement 

 along this old fault line. This disturbance was felt violently from 

 Eureka, in northern California, to southern San Benito county. 

 This is along the northwest-southeast line of valleys, continuous with 

 Santa Clara Valley. Parallel to the fault zone the many towns in 

 these valleys seem to have felt the earthquake with approximately 

 equal intensities. Crossing the fault zone, at right angles, in a north- 

 east direction, there seems some slight movement along a parallel 

 line of weakness passing down through San Francisco Bay by 

 Alviso, and southeastward, but there is no plain proof of this move- 

 ment. The intensity decreases rapidly, going eastward from the 

 fault, toward the San Joaquin Valley. 



Movement along the San Andreas-Portola Fault. — The move- 

 'ment along the San Andreas-Portola fault line is quite apparent and 

 consists of a thrust of the ]\Ierced block from three to eight feet 

 toward the southeast, along the direction of the fault, with a slight 

 downthrow, perhaps an average of twelve inches on the southwest, 

 the first movement being the more important. 



Intensity. — In San Francisco the general intensity was slightly 

 less than that in some of the smaller towns near by. Substantial 

 structures in that city were only slightly injured, broken chimneys 

 and small cracks being the usual destruction. A considerable num- 

 ber of old and poorly made buildings fell with a slight loss of life 

 compared with the population of San Francisco. The small number 

 of deaths, however, was due to the hour at which it occurred. Dis- 

 astrous fires broke out immediately and caused loss of life much 

 larger than would have resulted from the earthquake, and with a 

 loss of property that was excessively large in proportion to the 

 damage done by the earthquake. 



Effect. — In certain localities, between Lafayette Hill and the 



