1921] Lawson: Tlie Mobility of the Coast Ranges of California 457 



This was done for each side of the fault separately. As a result I 

 found that a point close to the fault on the east side should have 

 suffered an absolute displacement of 2.23 meters to the southeast, 

 while a similar point on the west side should have suffered an absolute 

 displacement of 2.43 meters to the northwest. A curve constructed 

 in this way is superior to that suggested by Ilayford and Baldwin for 

 the purpose of estimating the relation of the amount of displacement 

 to distance from the fault. AVith these values we may construct a 

 diagram, figure 10, to illustrate the movements of the ground. Let 

 A be the position of a small circle bisected by the fault in 1891, B the 

 position of the same circle in 1906a by strain creep. After the slip of 

 1906 the western semicircle was found by the Geodetic Survey at C, 

 which is 2.43 meters from B in the direction 156^ ; and the ea.stern 

 semicircle was found at D, which is 2.23 meters from B in the direction 

 339°. There may be some doubt in this case as to the azimuth of the 

 fault, since in this vicinity it leaves the coast and is supposed to curve 

 easterly through about twenty degrees so as to connect with the similar 

 feature at Shelter Cove in Humboldt County. But whatever the 

 orientation may be the fault moved from A to B in the interval between 

 1891 and 1906. The absolute displacements of the two semicircles are 

 very nearly equal in amount and opposite in direction ; and it seems 

 probable that the average azimuth of the fault in this vicinity is 

 expressed by the line CD, the bearing of which is 157°. If this be so, 

 the apparent transverse shift is so slight that it is probably non- 

 existent. If the shift elsewhere is to be explained by the migration 

 of the ]\Iocho-Diablo base then this intluence is unfelt at Arena, and 

 it is worthy of note that the triangulation in the vicinity of Arena is 

 not shown by Hayford and Baldwin^^ to be directly connected with 

 that base, though it is doubtless indirectly tied to it. The positions of 

 the stations in the Point Arena group are dependent upon the Fislier- 

 Cold Spring base, which is also assumed to have been unaffected by 

 the crustal movements. But with the northerly strain creep at Arena 

 as a necessary precursor of the faulting and elastic rebound in 1906, 

 and the evidence of northerly creep of the station at Ukiah, it is 

 difficult to concede the assumption of immobility of the Fisher-Cold 

 Spring base. It may, however, have been advancing in the interval 

 1891-1906 and have sprung back in the latter year as far as it had 

 advanced, so that the effect in locating points near Arena would have 

 been the same as if it had not moved. 



13 Earthquake Eeport, Atlas, map 24. 



