46 WARREN UPHAM^ M.A.^ D.SC.^ F.G.S.A., ON THE SAN FRANCISCO 



Eanges and passes into the sea at Mussel Eock, about seven 

 miles south-west of San Francisco. 



Other researches of the commission are in progress, to be 

 published later, concerning the intensity of the shock at 

 varying distances from the fault line, and the speed of passage 

 of the shock and tremors around and also through the earth. 

 When so thorough a study shall be completed as was given by 

 Dutton and others to the Charleston earthquake, it may be 

 hoped that important inferences will be obtained relative to the 

 constitution of the earth's interior and its fluidity or solidity, a 

 question of profound interest to geologists and physicists. 



North of the Golden Gate, the San Andreas fault crosses 

 outlying parts of the coast in several places ; and south of 

 Mussel Eock it passes by San Andreas and Crystal Springs 

 lakes, and is traced to the vicinity of Mt. Pinos in Ventura 

 county. Its known extent is about 375 miles. South-eastward 

 from San Francisco, its course is remarkably straight, running 

 obliquely across the mountainous belt that lies between the 

 coast and the San Joaquin valley. This great fault plane, 

 nearly vertically cutting the earth crust to great depths, is 

 paralleled by several other ancient faults of similar character, a 

 few miles apart, traversing the San Francisco peninsula, but 

 movements of this earthquake appear to have affected only the 

 San Andreas fault. (See map.) 



The published report of the Earthquake Investigation 

 Commission says : 



" The cause of these movements in general terms is that stresses 

 are generated in the earth's crust which accumulate till they exceed 

 the strength of the rocks composing the crust and they find a relief in 

 a sudden rupture. This establishes the plane of dislocation in the 

 first instance, and in future movements the stresses have only to 

 accumulate to the point of overcoming the friction on that plraie 

 and any cementation that may have been effected in the intervals 

 between movements. 



"The carthc^uake of the 18th April, 1906, was due to one of 

 these movements. The extent of the rift upon which the movement 

 of that date took place is at the time of writing not fully known. 

 It is, however, known from direct field observations that it extends 

 certainly from the mouth of Alder Creek near Point Arena to the 

 vicinity of San Juan in San Penito County, a distance of about 

 185 miles. The destruction of Petrolia and Ferndale in Humboldt 

 County indicates that the movement on the rift extended at least as 

 far as Cape Mendocino, though whether the line of rift lies inland 

 or off shore in that region is still a matter of inquiry. Adding the 

 inferred extension of the movement to its observed extent gives us 



