The California earthquake of April 18, 1906 : report of the State Earthquake Investigation Commission, in two volumes and atlas.
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 unsatisfactory 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, accumulate 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 Astronomical 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 × 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 altazimuth, 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.
In order to render the monuments thus established available for future measurements of displacement, it was necessary to have their present relative positions established with precision. This work was very kindly undertaken for the Commission by Mr. B. A. Baird, Assistant, Coast and Geodetic Survey, a report from whom follows, setting forth his methods and results:
Relative Positions of the Monuments
By B. A. Baird
Olema
Description of monuments. — The monuments at Olema are on Mr. Skinner's ranch, just a little north of the dwelling-house. The two piers west of the fault-trace are in an orchard on level ground, but the other two, which are just east of the road, are on a hillside, the northeast monument being about 15 feet higher than any of the others. In order to measure and observe between the northwest monument and the southeast monument, a trench about 3 feet deep had to be dug thru the embankments on both sides of the road and somewhat into the traveled portion as well. Some clearing of brush was necessary in order to make the northeast monument and the southwest monument intervisible.
The relative positions of the four monuments are shown in the diagram, fig. 44. The lengths of the three heavy lines were determined by measurement. The measurements of the other three lines were considered impracticable, on account of the great height of the northeast monument above the others, as compared with the short distances between them and it. By means of the three measured lines, however, a double determination is obtained, thru the observed angles, of each of the three lines not measured by the steel tape.
The lines were cleared sufficiently so that all four of the monuments could be occupied with a theodolite, and then all of the lines were observed, including the diagonals. In order that future movements may be readily detected by means of observed angles, the centering of the instrument was considered to be of the greatest importance, especially for such very short lines as these. A bronze plate had been constructed and set up on each monument, especially designed for supporting in position the Fauth 10-inch alt-azimuth instrument of the Civil Engineering Department of the University of California.
A sketch of the plate is shown in fig. 45. The spindle which screws into the central socket of the plate is shown in fig. 47, and the iron cap which protects the plate when the spindle is removed is shown in fig. 46. Referring to the sketch of the plate, it will be seen that there are three lugs, or foot-plates, standing upon and attached to it. In one is a groove (vertex of angle at bottom), and in one a hole (inverted cone), while the third has simply a smooth surface. This arrangement prevents any binding of the foot-screws of the instrument, and insures that it will always be set in the same position in successive measurements.
To doubly insure this, one of the lugs is marked to correspond with a particular support of the theodolite, the corresponding support being similarly marked. This arrangement further insures that even tho the center of the instrument does not correspond exactly with that of the point observed upon (for each monument), thus preventing a closing of the triangles within the accuracy of observation (that is, that the sum of the 3 angles of each triangle should equal 180°), the angles obtained will still correspond with each other in successive observations within the limit of observational errors.
For observing upon, and also for reference points in the base measurements, a spindle has been constructed for each plate. This spindle screws into a cup-like projection fastened upon the plate for a center-mark. Each spindle is numbered to correspond with a particular monument, the corresponding number being upon the plate.
Up to a distance of about 200 feet these spindles make excellent objects to observe upon, provided there is a suitable background. A background that can not be surpast is readily made by propping up behind the spindle the black iron cap which covers and protects the plate when not in use. When the distances are greater than about 200 feet, as is the case with the longer lines at Crystal Springs, the best object to observe upon can be made by whittling the end of a lead pencil to fit into the cup and then wrapping the pencil with a little white cloth. In this case the background should be the same as before. In any event, if tape measurements are contemplated, the spindles should be taken along, in order that they may be used as reference marks in those measurements.
Leveling record. — In the following tabulation, S. W. p means the top of the bronze plate on the southwest monument, alongside the spindle bowl or cup near the center. A corresponding point was taken on each monument to show the relative elevations to be retained for future reference.
S. W. s is the top surface of the spindle hub, screwed into the socket, made for marking the center on the same plate. These points were taken to show differences of elevation of points used in the tape measures in the base-lines, and are of no value beyond this.
In computing the elevations, the top of the southwest monument was arbitrarily taken as 10 feet, and the other elevations are corrected to correspond with this datum plane.
The spindle bowl is not in the center of the plate, owing to the position of the lugs, so that the point leveled upon representing the level of the plate is on the side of the spindle bowl next to the center of the plate.
The level used was a Troughton and Simms dumpy level with compass attachment.
| First Measures. | Second Measures. | Mean. | Base-line Elevations. | Differences of Elevations. | |
| S.W. p | 10.000 | 10.000 | 10.000 | ..... | ..... |
| S.W. s | 10.075 | 10.076 | 10.076 | 10.076 | ..... |
| Stake A | 7.465 | 7.462 | 7.464 | 7.464 | - 2.612 |
| N.W. p | 8.006 | 8.004 | 8.005 | ..... | ..... |
| N.W. s | 8.057 | 8.053 | 8.055 | 8.055 | + 0.591 |
| Stake B | 11.694 | 11.690 | 11.692 | 11.692 | + 3.637 |
| S.E. p | 11.389 | 11.390 | 11.390 | ..... | ..... |
| S.E. s | 11.455 | 11.456 | 11.456 | 11.456 | - 0.236 |
| N.E. p | 25.877 | 25.877 | 25.877 | ..... | ..... |
| S.W. s | ..... | ..... | ..... | 10.076 | - 1.380 |
The relative elevations of the four monuments, taking the center of the plate in each case, are as follows:
- Southwest monument: (assumed) 10.000 Feet
- Northwest monument: 8.005 Feet
- Northeast monument: 25.877 Feet
- Southeast monument: 11.390 Feet
Base-line measures. — B. A. Baird in charge, reading rear end of tape and recording. R. S. Badger, forward end of tape and reading thermometer. Charles Evans (laborer), steadying spring balance attached to end of tape and watching tension of 10.5 lbs. The tape used, a 100-foot steel tape, G. M. Eddy and Co., Catalogue No. 703; was stamped on reel "No. 1" for identification in future use. Its width is 0.272 inch; its thickness 0.010 inch; and its weight per foot 3.8324 grams or 0.00845 lb. This tape, on May 1, was compared with the standard tape at the University of California, a long level stretch on the "bleachers" being used for the purpose. The standard tension of 10.5 lbs. was adopted, and no difference in the lengths of the tapes could be detected.
The standard tape, N.B.S. No. 8, is marked only at zero and 100 feet. The comparisons were made between these marks, and the equality of zero to 50 feet and 50 to 100 feet was measured on the tape used in the base-measures, there being no measurable difference.
The constants of the standard tape, N.B.S. No. 8, are: Temperature of observation, 64.6° F.; Tape supported thruout entire length; tension, 10.5 lbs. avoirdupois; resulting values of spaces at 62° F., assuming coefficient of expansion = 0.0000063 per degree F. are zero to 100 feet = 100 feet 0.00 inch.
Formulæ and Constants Used in Base-Line Computations
Correction for Level = [Mathematical Equation] where
h = difference of elevation of ends of tape.
d = distance between supports.
Correction for Temperature = -l (T-t)e, where
l = length of line corrected for.
T = standard temperature = 62° F.
t = mean temperature of tape.
e = coefficient of expansion = 0.0000063 per degree F.
Correction for Sag = [Mathematical Equation] where
w = weight of tape per foot = 0.00845 lb. per foot.
P = standard tension of 10.5 lbs., the same as used in measures.
d and l same as above.
From the above, [Mathematical Equation]
The correction for pull, accounting for elasticity of tape, is not necessary, since the standard tension was used in the measures.
Level correction. — In taking the measurements, the center of the spindle, firmly screwed into the cup on the bronze plate, as shown in the diagram, fig. 47, was the reference mark on the monuments.
The ordinary correction for level, [Mathematical Equation] is not sufficiently accurate when the differences are large, and a second correction has been allowed for. In the corrections at Crystal Springs, even third approximations are necessary.
The computed values of the measured lines are summarized as follows:
| Forward. | Backward. | Mean. | |
| N.W. to S.W. Mon. | 110.1528 | 110.5115 | 110.512 |
| N.W. to S.E. Mon. | 120.5890 | 120.5878 | 120.588 |
| S.E. to S.W. Mon. | 79.3728 | 79.3725 | 79.373 |
Crystal Springs Lake
Description of stations. — These monuments are about 7 miles northwest of San Mateo on the eastern shore of Crystal Springs Lake, the reservoir of the Spring Valley Water Company. The location is about a mile southeast of Camp Sawyer, which is on the west side of the lake at a point where the lake is very narrow and is spanned by a bridge, close to the northern end.
As will be seen from the leveling record, and from the accompanying rough sketch, fig. 48, the ground is very uneven, and the measurement of a base-line was executed under considerable difficulty. The line measured, which was the only practicable one, crosses two ravines and comes up toward the southeast monument, over a very steep road embankment. Considerable clearing of brush was necessary in order to cross the ravine near the northwest monument. On account of the large differences of level between the base-line stakes in some places, the leveling had to be done with extreme care, there being at one place a rise of 11.5 feet in 50 feet, and, next to the road embankment, a rise of about 6 feet in 20 feet.
The plates, spindles, and caps are the same as those described under "Description of Monuments" at Olema. The distances between these stations being too great to observe upon the spindles with advantage, pencils were wrapt with white cloth and set in the spindle cups upon the plates, the bronze protecting caps being used for background, as in the Olema measurements.
Base-line measures. — The stakes, made long enough to stand above the grass, were lined in with the alt-azimuth instrument, and to avoid the possibility of errors they were all numbered on top with a blue crayon. The stakes were made of redwood, and the method of marking was to stick a pin straight down in the top of each at the marking edge of the tape. The tape used was marked to hundredths of feet the entire length, the thousandths being estimated. The measurements were so taken as to avoid estimating the thousandths, excepting on the last measure, the mark being arbitrarily placed at the nearest convenient tenth of a foot on the top of each forward stake.
As the diameter of the pins used was almost exactly the same as the width of the 0.1-foot marks on the tape, the marking could be done with exceptional accuracy, especially by holding the eye directly over the mark in such a way that there would be no parallax. The spring balance was fastened to the forward end of the tape, and steadied by means of a cord looped so as to slip up and down on a pole, held by a man who at the same time watched the tension. To avoid any pulling against the stakes, the height of the tape was regulated by means of the loop so as just to graze the top of the stake. All the marking was done at the forward end of the tape, the officer in charge at the rear end simply steadying on the mark of the previous measure and then reading the tape.
The lengths of the base joining the southeast and northwest monument resulting from the measurements are the following:
- First measure: 857.8020 Feet
- Second measure: 857.7988 Feet
- Mean: 857.800 Feet
Relative elevations. — By means of precise leveling the relative elevations of the four fixed monuments, taking the center of the top of the bronze plate in each case, were found to be as follows:
- Northwest monument: (assumed) 50.000 Feet
- Northeast monument: 86.513 Feet
- Southeast monument: 75.787 Feet
- Southwest monument: 46.113 Feet
Method of observing angles. — The instrument used was a 10-inch alt-azimuth theodolite, carrying two micrometers 180° apart. Each micrometer head is divided to represent seconds of arc, enabling the observer to estimate to tenths of seconds of arc at each reading. In taking the observations, each micrometer was read to correspond with two consecutive 5-minute divisions, one being back of the reference mark and one in front. The corrections for "run" at each station were based upon the observations themselves, the mean of all observations at the first two monuments being taken both at Olema and at Crystal Springs. In order to eliminate all possible instrumental errors, the observations were, in general, taken in four sets, having for the initial reading of each set, 0°, 90°, 45°, and 135°, respectively; making for the reversal of the telescope, without changing the setting of the circle, the corresponding readings of 180°, 270°, 225°, and 315°.
Thus, upon each station there were eight pointings of the instrument, representing eight portions of the circle equally divided. Since for each of these pointings there are two micrometers, each giving two readings of the thread, there were in reality 32 micrometer readings for each observed station. The above statements apply fully at Crystal Springs, but at Olema one micrometer was not in condition to use, so that the Olema observations, while constituting the same number of telescope pointings, represent for each observed station but 16 micrometer readings.
At Olema, on account of the very small distances between the monuments, large changes of focus were necessary for the different pointings. This, combined with the large differences of elevation, and lack of perfect centering of the instrument on the plates to correspond with the positions of the spindles, prevented the triangles from closing to a very high degree of accuracy. Still, when these discrepancies are reduced to errors of distance, they become practically inappreciable.
At Crystal Springs, where the lines are much longer, the closing of the triangles was very good. The correction for each angle, in order to make the sum of the angles of each triangle equal to 180°, was on the average only about one second of arc. This goes to indicate that this instrument, when properly used, is capable of excellent results. At Crystal Springs a least square reduction of the observations has been made, but the angles and distances thus computed are almost identical with those of the original computation.
At Olema, where the three lines having the least differences of level were measured, the diagonal between the northwest monument and the southeast monument (being best suited for computation) was taken as a base for computing the other two measured sides. The means of the computed and measured distances of these two sides, together with the direct measure of the above-mentioned diagonal, were taken as the best measures for computing the unmeasured sides. The lengths of the three unmeasured sides, therefore, depend not only upon the observed angles, but upon the lengths of the three bases, as indicated above. This method gives the measured distances and observed angles about equal weight, the angles being corrected for each triangle according to what is known as the "field adjustment." As above noted, however, it is very doubtful if the angles are entitled to as much weight as the measured distances, and hence, it was decided to retain the exact values of the three measured distances, and make a "least square" adjustment of the angles of the quadrilateral to correspond.
The three measured sides being assumed as fixed, the three angles of the triangle N.W. Mon., S.E. Mon., S.W. Mon., can each have but one value, and these values have been computed from the three sides. These three sides and the corresponding angles remaining fixed, an adjustment is made between the remaining angles and the three unmeasured sides, so as to fulfill all the geometrical conditions, giving at the same time the most probable values, according to the theory of "least squares."
Abstracts of horizontal angles. — In the abstracts of horizontal angles tabulated below, the first set of angles given under the heading "Observed" are the means of angles taken directly from the original records. The column headed "Field Adjustment" shows the angles as they appear in the field computations after the angles of each triangle have been corrected to sum up 180°, giving the same correction to each angle in a particular triangle. This adjustment, which is the usual one made in the original computations, does not account for the other geometrical conditions required for the rigid solution of a quadrilateral, but when the errors in the angles are small, the resultant distances, especially if short, will be very near the truth. The column headed. "Least Square Adjustment" shows the angles computed so as to fulfill all the geometrical conditions, giving their most probable values according to the theory of "least squares."
In future measurements, it will not be necessary to repeat the base-measurements unless the angles show some change, for by occupying all the stations, any change that could be detected by tape measurement will at once show in the angles. When, however, the angles indicate any change, then a remeasurement of at least one line will be necessary.
| Observed. | Field Adjustment. | Least Square Adjustment. | |||||||
| At N.E. Monument: | |||||||||
| S.E. Mon. to S. W. Mon. | 37° | 33' | 23.6'' | 24.6'' | 37° | 33' | 26.7'' | ||
| S.W. Mon. to N.W. Mon. | 57 | 24 | 50.4 | 61.8 | 57 | 25 | 21.6 | ||
| S.E. Mon. to N.W. Mon. | 94 | 58 | 14.0 | 22.1 | 94 | 58 | 48.3 | ||
| At S.E. Monument: | |||||||||
| S.W. Mon. to N.W. Mon. | 63 | 12 | 13.4 | 17.6 | 63 | 12 | 29.7 | ||
| N.W. Mon. to N.E. Mon. | 27 | 57 | 59.1 | 67.2 | 27 | 57 | 48.6 | ||
| S.W. Mon. to N.E. Mon. | 91 | 10 | 12.5 | 13.5 | 91 | 10 | 18.3 | ||
| At S.W. Monument: | |||||||||
| N.W. Mon. to N.E. Mon. | 25 | 38 | 40.6 | 52.0 | 25 | 38 | 41.9 | ||
| N.E. Mon. to S.E. Mon. | 51 | 16 | 20.9 | 21.9 | 51 | 16 | 15.0 | ||
| N.W. Mon. to S.E. Mon. | 76 | 55 | 01.5 | 05.8 | 67 | 54 | 56.9 | ||
| At N.W. Monument: | |||||||||
| N.E. Mon. to S.E. Mon. | 57 | 03 | 22.6 | 30.7 | 57 | 03 | 23.1 | ||
| S.E. Mon. to S.W. Mon. | 39 | 52 | 32.3 | 36.6 | 39 | 52 | 33.4 | ||
| N.E. Mon. to S.W. Mon. | 96 | 55 | 54.9 | 66.2 | 96 | 55 | 56.5 | ||
| Measured. | Computed. | Mean. | Least Square Adjustment. | |
| S.E. Mon. to N.W. Mon. | 120.588 | ..... | 120.588 | 120.588 |
| S.E. Mon. to S.W. Mon. | 79.373 | 79.373 | 79.373 | 79.373 |
| S.W. Mon. to N.W. Mon. | 110.512 | 110.508 | 110.510 | 110.512 |
| N.E. Mon. to N.W. Mon. | ..... | 56.768 | 56.768 | 56.769 |
| N.E. Mon. to S.W. Mon. | ..... | 130.191 | 130.191 | 130.187 |
| N.E. Mon. to S.E. Mon. | ..... | 101.584 | 101.584 | 101.582 |
| Observed. | Field Adjustment. | Least Square Adjustment. | |||||
| At N.W. Monument: | |||||||
| N.E. Mon. to S.E. Mon. | 92 | 01' | 49.2'' | 49.5'' | 92° | 01' | 49.9'' |
| S.E. Mon. to S.W. Mon. | 11 | 23 | 43.5 | 43.5 | 11 | 23 | 44.9 |
| N.E. Mon. to S.W. Mon. | 103 | 25 | 32.7 | 34.8 | 103 | 25 | 34.8 |
| At N.E. Monument: | |||||||
| S.E. Mon. to S.W. Mon. | 10 | 12 | 21.8 | 20.1 | 10 | 12 | 19.0 |
| S.W. Mon. to N.W. Mon. | 59 | 09 | 45.1 | 47.2 | 59 | 09 | 47.9 |
| S.E. Mon. to N.W. Mon. | 69 | 22 | 06.9 | 07.3 | 69 | 22 | 06.9 |
| At S.E. Monument: | |||||||
| S.W. Mon. to N.W. Mon. | 77 | 57 | 38.1 | 38.1 | 77 | 57 | 37.0 |
| N.W. Mon. to N.E. Mon. | 18 | 36 | 02.8 | 03.2 | 18 | 36 | 03.2 |
| S.W. Mon. to N.E. Mon. | 96 | 33 | 40.9 | 39.2 | 96 | 33 | 40.2 |
| At S.W. Monument: | |||||||
| N.W. Mon. to N.E. Mon. | 17 | 24 | 35.9 | 38.0 | 17 | 24 | 37.3 |
| N.E. Mon. to S.E. Mon. | 73 | 14 | 02.4 | 00.7 | 73 | 14 | 00.8 |
| N.W. Mon. to S.E. Mon. | 90 | 38 | 38.3 | 38.4 | 90 | 38 | 38.1 |
| Measured. | Field Computation (Base-Line). | Least Square Adjustment. | |
| S.E. Mon. to N.W. Mon. | 857.800 | ..... | 857.800 |
| N.E. Mon. to S.W. Mon. | ..... | 950.414 | 950.412 |
| N.E. Mon. to N.W. Mon. | ..... | 292.368 | 292.366 |
| N.E. Mon. to S.E. Mon. | ..... | 916.008 | 916.008 |
| S.W. Mon. to S.E. Mon. | ..... | 169.500 | 169.500 |
| S.W. Mon. to N.W. Mon. | ..... | 838.984 | 838.982 |
Courtesy of The Bancroft Library, University of California, Berkeley, CA 94720-6000; http://bancroft.berkeley.edu/
http://content.cdlib.org/view?docId=hb1h4n989f&brand=eqf
Title: The California earthquake of April 18, 1906 : report of the State Earthquake Investigation Commission, in two volumes and atlas.
By: California. State Earthquake Investigation Commission, Lawson, Andrew C. (Andrew Cowper), 1861-1952
Date: 1908-10
Contributing Institution: The Bancroft Library, University of California, Berkeley, CA 94720-6000; http://bancroft.berkeley.edu/
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