464 University of California Publications in Geology [Vol. 12 



was chiefly parallel to the San Andreas fault, but on the assumption 

 of transverse shift there was also a small component of movement 

 normal to the fault. 



Loma Prieta. — Let us next consider Loma Prieta, for which we 

 have surveys in 1854, 1884, and 1906p. The geodetic data are : between 

 1854 and 1884 the station had been displaced 3.03 meters in the direc- 

 tion 307° and between 1884 and 1906 an additional .97 meters in the 

 direction 303°. I shall assume that the persistent northerly strain 

 creep was in the same direction as that adopted for Red Hill, or 163°, 

 although it may have been a few degrees more to the west ; and the rate 

 of this strain creep I shall again take at .058 meters per year. 



In figure 13, let A be the position of the station in 1854, B its 

 position in 1884, and C in 1906p. By 1868a it would have moved to 

 D, or .812 meters in the direction 163°. In arriving at B in 1884 the 

 station must have moved by strain creep in the direction 163° from 

 the point to which it was suddenly shifted in 1868. In I8682J, there- 

 fore, Loma Prieta was at E, .058 X 16 = .928 meters from B. The 

 line DE is the measure of the sudden movement of 1868. This is 4.57 

 meters in the direction 320°. After 1884 the station continued its 

 northerly migration until 1906a, when it had arrived at F, 1.27 meters 

 from B in the direction 163°. From F it rebounded, at the time of 

 the earthquake, parallel to the San Andreas fault, which here has the 

 strike of 135°, and was therefore on the line hb'. But after the 

 earthquake it was found at C ; so that the line hb' was apparently 

 suddenly shifted parallel to itself to C. This shift, measured normal 

 to the fault, amounts to .40 meters, and is the same apparent transverse 

 shift that was recognized in the stations north of the Golden Gate. 

 These last two movements, one parallel to the fault, the azimuth of 

 which is known but the amount not, and the other, of which only the 

 component normal to the fault is known, together make up the net 

 sudden displacement of the station on April 18, 1906, from F to C, 

 or 2.10 meters in the direction 325°. Thus between 1854 to 1906p, 

 Loma Prieta moved first by strain creep from AtoD, then by rebound 

 from D to E, then by renewed strain creep from E to F, and again 

 by rebound from F to C. 



Gradient of displacement. — It is noteworthy that while the sudden 

 movement of Red Hill in 1868 was 1.68 meters to the southeast, that 

 of Loma Prieta was 4.57 meters, or 2.72 times as much. This fact 

 consists with the hypothesis that the sudden movement which caused 

 the earthquake of 1868 was confined to the region south of the Golden 



