GKHODETIC MEASUREMENTS OF EARTH MOVEMENTS.’ 
By Joun F. Hayrorp anp A. L. Batpwin. 
GENERAL STATEMENT. 
The Coast and Geodetic Survey has done much triangulation in California to serve as 
a control or framework for its surveys along the coast and other surveys. The results of 
all the triangulation, south of the latitude of Monterey Bay, together with the primary 
triangulation to the northward, have already been published.? In 1906 the results of 
the triangulation in California, from the vicinity of Monterey Bay northward, were being 
prepared for publication. The reports from various sources in regard to the effects 
of the earthquake of April 18, 1906, indicated that there had been relative displacements 
of the earth’s surface of from 2 meters (7 feet) to 6 meters (20 feet) at various points near 
the great fault accompanying the earthquake. These were relative displacements of 
points on opposite sides of the fault and had been reported along all parts of the fault for 
185 miles, from the vicinity of Point Arena, in Mendocino County, to the vicinity of San 
Juan, in San Benito County.’ The average relative displacement was said to be about 3 
meters (10 feet). Displacements of that size would so change the relative positions of 
points which had been determined by triangulation and so change the lengths and direc- 
tions of the lines joining them that the triangulation would no longer be of value as a 
means of control for accurate surveys. The value of the triangulation could be restored 
only by repeating a sufficient amount of it to determine definitely the extent and character 
of the absolute displacements. It was, therefore, decided to repair the old triangulation, 
damaged by the earthquake, by doing new triangulation. 
If the displacements of a permanent character had occurred in a narrow belt only, close 
to the fault, but a few triangulation points would have been affected. The available 
evidence, however, indicated that the movements probably extended back from the fault 
for many miles on each side, and that the new triangulation necessary for repel purposes 
must, therefore, cover a ads belt. 
The new triangulation to repair the damage was completed in July, 1907. In addition 
to serving this practical purpose, it has shown the character of the earth movements of 
1906, which were found to extend back many miles on each side of the fault. These are 
very interesting results from a purely scientific point of view. Moreover, there came to 
light, during the study of the movements of 1906, entirely unexpected evidence of earlier 
earth movements, probably in 1868, which also affected a large area. 
The purpose of this paper is to set forth fully the amount and nature of these two 
great displacements of large portions (at least 4,000 square miles) of the earth’s crust and 
to indicate the degree of certainty in regard to these displacements warranted by the 
evidence. 
EXTENT OF NEW TRIANGULATION. 
The new triangulation done during the interval July 12, 1906, to July 2, 1907, extends 
continuously northwestward from Mount Toro, in Monterey County, and Santa Ana Moun- 
tain, in San Benito County, to Ross Mountain, and the vicinity of Fort Ross, in Sonoma 
County. This new continuous triangulation, as indicated on map No. 24, extends over an 
area 270 kilometers (170 miles) long and 80 kilometers (50 miles) wide, at its widest part. 
It includes the station known as Mocho, about 11} miles northeast from Mount Hamilton 

1 Published by permission of the Superintendent of the Coast and Geodetic Survey. 
* See Appendix 9 of the Report of the Coast and Geodetic Survey for 1904, Triangulation in Cali- 
fornia, Part I, by A. L. Baldwin, Computer. 
: Preliminary Report, State Earthquake Investigation Commission, Berkeley, May 31, 1906. 
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