130 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
ward of the fault is 1.16 meters (3.8 feet) and for the six to the westward is 2.3 times as 
great, namely, 2.71 meters (8.9 feet). 
Group 7. Southern part of primary triangulation. — In this group, extending south- 
ward from the line Mocho-Sierra Morena, there are nine points (see map 24) of which the 
positions were redetermined after the earthquake of 1906. Of these, one, Loma Prieta, 
had been formerly determined both before and after the earthquake of 1868; five others 
had been determined before 1868 but not after, and three had been determined after but 
not before 1868. (See tables 1 to 3.) In this group, therefore, but one point is available 
to show the displacement of 1868. 
The triangulation of 1854-1855, starting from the line Ridge to Rocky Mound near the 
Pulgas Base, consisted of a single chain of triangles with all angles measured, down to the 
line Loma Prieta-Gavilan. The Point Pinos Light-house and the Point Pinos Latitude 
Station were connected with this chain, and with checks, by observations in 1854, 1864, 
and 1866. 
The main triangulation of 1876-1887, from the line Mount Diablo-Mocho to the line 
Mount Toro-Santa Ana, consisted of a strong chain of figures with many checks, being 
substantially as shown on map 24 if Gavilan be omitted and all stations occupied. In 
this triangulation, however, no complete independent determinations with checks were 
made of Black Mountain, Santa Cruz Azimuth Station, Gavilan, Point Pinos Light- 
house and Point Pinos Latitude Station. 
The triangulation of 1906-1907 was made as shown on map 24. Two separate least 
square adjustments were made of the main scheme connecting the points Mount 
Diablo, Mocho, Sierra Morena,’ Loma Prieta, Mount Toro, Gavilan, and Santa Ana. 
In the first adjustment, it was assumed, as for the computations of other groups, that 
Mount Diablo and Mocho only remained unmoved during the earthquake of 1906. This 
first adjustment showed an apparent displacement of Santa Ana in 1906 of 3.26 meters 
(10.7 feet), in azimuth 288° (72° E. of 8.), but an examination in detail of the possible accu- 
mulated errors in the triangulation showed that this apparent displacement was probably 
due to errors of observation. The new primary triangulation is much weaker in the figure 
defined by the five points, Mocho, Loma Prieta, Mount Toro, Gavilan, and Santa Ana, 
than elsewhere for two reasons. First, the length must be carried without a check 
thru the triangle Loma Prieta, Mocho, Mount Toro, of which only two angles were meas- 
ured and this triangle is very unfavorable in shape for an accurate determination of 
length. Second, it so happened that the least accurate observations made in the pri- 
mary triangulation were in this triangle or in its immediate vicinity. 
In the second and adopted adjustment it was assumed that Santa Ana, as well as 
Mount Diablo and Mocho, remained unmoved during the earthquake of 1906. The 
astronomic azimuth had been observed at Mount Toro in 1885 and again after the 
earthquake of 1906. These two observations measured the absolute change in azimuth 
of the line between Mount Toro and Santa Ana and indicated it to be 2.5”, the later 
azimuth being the greater. This was utilized to strengthen the adjustment. 
In view of the evidence of stations farther north, the assumption that Santa Ana re- 
mained unmoved is reasonably safe. -Santa Ana is about 27 kilometers (17 miles) to the 
eastward from the point at which the fault disappeared near the village of San Juan. 
There is no station anywhere in the triangulation more than 6.4 kilometers to the eastward 
of the fault for which any displacement in 1906 was determined with certainty. 
If Santa Ana was displaced in 1906, the erroneous assumption introduces an error into 
the computed displacements at the stations Gavilan, Mount Toro, Point Pinos Light- 
house, and Point Pinos Latitude Station, of about the same amount as the actual dis- 
placement at Santa Ana. The error produced in the computed displacement at Santa 
Cruz Light-house and Santa Cruz Azimuth Station must be much smaller, and no error 
