PROVISION FOR MEASUREMENT OF FUTURE MOVEMENTS ON SAN ANDREAS FAULT. 
The extent of the movement on the San Andreas fault on April 18, 1906, was measured 
imperfectly and inexactly by offsets of fences, lines of trees, roads, pipes, dams, creeks, 
shore lines, etc. The distribution of the displacement in the immediate vicinity of the 
fault, the drag and compression of the soil, the uncertainty as to the former orientation 
of the lines offset, and other adverse conditions rendered the determinations unsatisfac- 
tory to a certain degree. With one exception, the measurements obtained in this way 
are suspected of being less than the true amount of relative displacement of the firm rocks 
below the surface materials. 
With the object of obtaining a more exact measurement of any future movements that 
may take place on the same fault, the Commission caused to be established two sets of 
piers or monuments in the Rift, in proximity to the fault-trace, upon which instrumental 
observations could be obtained as to the amount of displacement. This was not done in 
anticipation of the recurrence of a large movement in the near future, but because it 
was suspected that there might be slight movements at the times of minor earthquakes, 
such as are fairly common. Such slight movements might, in the course of years, accu- 
mulate to an important amount, and yet the individual increments of the displacement 
escape notice unless refined methods of measurement are resorted to. It is hoped that 
the establishment of the monuments and the redetermination of their relative positions 
from time to time will enable future observers to ascertain whether or not there is a small 
progressive movement on the San Andreas fault, in addition to the larger movements 
which cause more violent earthquakes, such as those of 1857 and 1906. Besides serving 
this purpose, the movements will also be useful in any effort that may be necessary in 
future to determine with precision the amount of a large displacement. 
The localities selected for the position of the two sets of monuments are Olema, Marin 
County, and Crystal Springs Lake, San Mateo County. These localities are about 40 
miles apart on the Rift, and the fault-trace at both was confined to a very narrow zone 
in 1906, thus permitting the piers to be more closely grouped than at many other localities 
which for other reasons might have been chosen. 
Each set of monuments consists of four concrete piers, established two on each side 
of the fault-trace of 1906. They are sunk in the ground to a depth of about 6 feet, and 
are founded either upon rock or upon a firm “‘hard-pan” arising from the decomposition 
of the underlying rocks. They rise from 2 to 3 feet above the surrounding surface. The 
establishment of the piers at Olema was intrusted to Mr. A. J. Champreux, of the Astro- 
nomical Department of the University of California, and those at Crystal Springs Lake 
were set in place by the officers of the Spring Valley Water Company, under the direction 
of its chief engineer, Mr. Hermann Schussler, who very kindly relieved the Commission 
of any expense connected with the operation. The piers at Olema are 13 inches square 
in cross-section, while those at Crystal Springs Lake are 18 inches square. To the summit 
of each of the piers is fixed a thick bronze plate 13 inches square, with suitable appliances 
for receiving a selected instrument in a constant position for successive measurements, 
and a device for determining a fixed point to which to measure. This plate is protected 
by a heavy iron cap, 14.5 x 14.5 inches,.lockt upon it, bearing the inscription: 
S.E.I.C. 
To measure 
earth movements 
1906. 
The instrument selected for the first and subsequent measurements is a 10-inch alt- 
azimuth, the property of the University of California, and the key of the protecting caps 
is at present in the safe keeping of the same institution. 
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