452 University of California Publications in Geology [Vol. 12 



on the line hh'. But the geodetic surveys show that after the earth- 

 quake in 1906 the western semicircle was at c and the eastern semi- 

 circle at c'. Therefore the fault trace apparently shifted from the 

 position 66' to the parallel position cc\ The actual direction of this 

 shift, if real, is not known, but if we express it in terms of the com- 

 ponent normal to the fault trace it measures .78 meters, as compared 

 with .88 meters at Farallon, .57 meters at Chaparral, and .58 meters 



Fig. 8 



Fig. 8. Tomales Bay group. A small circle located at A in 1856 on tlie San 

 Andreas fault had moved 2.75 meters to B by strain creep up to 1906. By slip 

 and rebound in that year one half of the circle moved 3.03 meters to C and the 

 other half 3.00 meters to D. 



at Tamalpais. The actual net displacement of points on the west side 

 of the fault by sudden movement in 1906 was therefore from B to c 

 or 3.03 meters in the direction 130°. Similarly the net displacement 

 of points on the east side was from B to c' or 3.00 meters in the direc- 

 tion 340°. The distance cc' is the measure of the total relative dis- 

 placement by the fault slip for points about 2 km. distant from the 

 fault on either side. This is true no matter what may have been the 

 azimuth of the transverse shift which moved the fault trace from 66' 

 to cc'. The relative displacement by fault slip alone thus geodetically 

 determined is 5.83 meters. The measure of the slip at the fault would 



