AXD VALPARAISO EARTHQUAKES AND THEIR CAUSES. 45 



of tlie earth about 186 miles at its deepest point . . . . 

 Because of its greater density and the enormous superincumbent 

 pressure, the elastic properties of the deep-lying substance of the 

 earth seem to propagate vibrations with higher and higher velocities 

 the deeper the path. These considerations lead to the conclusion 

 that if earthquake vibrations follow the path of the chord the speed 

 of propagation should not be constant for all distances from the 

 origin, but should be greater as the distance from the origin 

 becomes greater. This has generally been found to be true in the 

 case of the preliminary tremors, and will doubtless be shown in this 

 earthquake when accurate reports from numerous stations are exam- 

 ined. From San Francisco to Washington the speed along the chord 

 is found to be 5-4 miles (8 "7 kilometres) per second. This is based 

 on Professor Davidson's time at San Francisco, viz., 5.12 a.m. 

 This result is perhaps a trifle faster than we might expect. 



" The strong waves do not seem to follow the path of the chord, 

 but rather travel along the surface at a slower rate, which is nearly 

 constant for all distances. In the present case the velocity is 3"1 

 miles per second for the first strong waves, or as low as '2"2 miles 

 per second for the maximum waves. Both of these speeds, 

 however, are a little high, perhaps." 



From records of an observatory of the United States Coast 

 and Geodetic Survey at Sitka, 1,455 miles distant by the great 

 circle from San Francisco, and about 1,447 miles by the chord, 

 the velocity of the fisrt tremors from this earthquake, supposed 

 to have come along the chord, was 5*6 miles, or 9'0 kilometres, 

 per second, the whole distance being traversed in four minutes 

 and eighteen seconds. 



Origin of this Earthquake from Eock Faulting. 



Three days after the San Francisco shock, Governor Pardee 

 of California appointed a commission of geologists to investigate 

 its results and causes. Of this commission Professors Andrew 

 C. Lawson and A. 0. Leuschner, both of the State University, 

 are respectively chairman and secretary ; and the other 

 members are Professors G. K. Gilbert of the United States 

 Geological Survey, H. F. Eeid of Johns Hopkins University, 

 J. C. Branner of Stanford University, George Davidson of the 

 State University, Charles Burkhalter of the Chabot Observatory, 

 and AVilliam W. Campbell, director of the Lick Observatory. 

 The commission have issued a preliminary report, noting chiefly 

 the evidence of a prolonged new fracture and dislocation of the 

 earth crust on an old fault plane, called the San Andreas lault, 

 which cuts nortli-north-w^estward diagonally across the Coast 



