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REPORT OF THE SUB-COMMITTEE ON STATE INSTRUMENTAL RECORDS.
Number 43
Introduction
The sub-committee consisting of Director W.W. Campbell, Professor H.F. Reid and Professor C.F. Marvin, met at the invitation of Director Campbell at the Lick Observatory on Monday July 23rd, 1906, for the purpose of studying the various instrumental records that had been obtained within the State of California, not only of the great shock of April 18th, but of many after-shocks.
The records available for this study consisted chiefly of the records from the Ewing three component seismographs at the Lick Observatory and the Student's Observatory of the University of California.
These were supplemented by a number of static records obtained by means of the so-called Ewing duplex pendulum and other equivalent or at least similar forms of seismographs, in the hands of residents of the state, personally interested in seismology. Some of the after-shocks were recorded at the Student's Observatory at Berkeley by means of a special type of seismograph or tromometer brought directly from Japan by Professor Omori and kindly placed at the disposition of the Student's Observatory.
DESCRIPTION OF INSTRUMENTS AND INSTALLATION.
(supply if considered necessary)
All of the instruments within the State except the Omori tromometer, which latter was not set up until several
The analysis and study of records from distant points all over the world is deferred until full reports are received. These will be reported upon later by Professor H.F.Reid.
RECORDS OF THE GREAT EARTHQUAKE.
The only fairly complete record of the great shock was that obtained at the Lick Observatory on the Ewing three component recorder. Even in this case the strong motion caused all the pens to be thrown off the recording plate and the steady mass for the vertical motion to be thrown off its pivots. As the heavy motion subsided the pens recording the two horizontal components of motion returned upon the recording plate and completed their records. The details of these records are more fully set forth in Table I.
The pen of the Ewing duplex pendulum at the Lick Observatory was also thrown off the plate during the strong motion; nevertheless a comparatively good record was obtained of the moderate motion at the early stages of the disturbance. The beginning of the record is sharply defined and the first motion of the ground was directed South 60° East for a linear
At the Student's Observatory, Berkeley, the Ewing three component recorder was quickly disarranged by the strong motion and the recording plate elevated so that it failed to revolve. The pen recording vertical motion showed maximum displacements just within its range, from which we may fairly conclude that the maximum vertical motion was about 45 mm. The mechanisms recording the horizontal motion were seriously deranged and no reliable record obtained.
The Ewing duplex pendulum record at Berkeley was rendered almost illegible from the long continuance of the motion and the fact that the range of the instrument was greatly exceeded and caused the stops to strike against the steady mass and set it in violent motion.
A careful scrutiny of the record indicates that the first motion of the ground was directed to the East about 5° South, and for a distance of 10 mm., then sharply Northward 16 mm., turning in a rounded curve westwardly for about 12 mm., thence irregularly Southeast 35 mm., thence West South West
The records from duplex pendulums at the Chabot Observatory, Oakland, Mills College, Alameda Co., and the University of the Pacific at Santa Clara, are all greatly confused owing to the violence of the motion. The beginnings are not discoverable, in some cases the records are fragmentary and the motions cannot be followed.
LOS GATOS, SANTA CLARA CO.
Mr. Irving H. Snyder, located only about 4 miles from the fault line, submitted a static record of a pair of steady masses carried on rockers cradle fashion. Each rocker rested on a smooth plate and served to register one component only of the horizontal motion. The motion from the two rockers however was combined by means of suitable levers into a composite record, similar in all respects to the static records made by the Ewing duplex seismograph.
The first motion by this instrument was apparently East, about 6 mm., but the lines of the record quickly became greatly confused, the rockers were soon shifted by the great violence of the motion and the record became illegible.
VETERAN'S HOME, YOUNTVILLE, NAPA CO.
Mr. F. M. Clarke furnished prints of records made by his seismograph, which consisted of a simple pendulum 40" long with a bob weighing 18 pounds. The recording pen consisted of a long steel hatpin working smoothly in a hole directly through the middle of
The beginning cannot be made out; the first motion is reported by Mr. Clarke to have been North and South, as observed by the swinging of a pendent electric light bulb. The lines of the record indicate strictly linear motion back and forth through the neutral or central point. Some Irregularly curved motion of small amplitude appears at the center. Beginning with the North and South direction the motion shifted steadily to the East and West directions, with numerous intermediate vibrations. There is a noticeable absence of motion in the direction Northwest and Southeast.
CARSON CITY, NEVADA.
The record submitted by Professor C.W. Friend probably from a duplex pendulum shows a great confusion of lines, with groups of motion in all directions. The ranges of the motion are as follows:
- East & West
- 21 mm.
- North & South
- 21 mm.
- North & South
- 24 mm.
- One maximum swing N. 10° E. to S. 10° W.
- 25 mm.
| Time 120th Meridian | April 18th, 1906. |
| 5h 12m 12s A.M. | Beginning of perceptible motion accurately observed by a number of astronomers. |
| 5 12 45 | Beginning of strong motion as observed by R.G.A. |
|
5 12 45 |
Beginning of Ewing record - Moderately strong motion. |
| Time Ewing Record | Period secs. | Ampl. mm. | |
| h. m. sec. | |||
| 5 12 45
- - 49 |
4.2 | 5.8 | One moderately strong well defined wave at beginning. |
| 5 13 32
- - 42 |
10. | ? | One great wave, pen off the plate. |
| 5 13 42
- - 52 |
10 | 52 | One great wave partly recorded. |
| 5 13 52
5 14 00 |
4 | 20 | Two moderate waves and record fully resumed. |
| 5 14 00
- - 10 |
5 | Two waves | |
| 5 14 40
- - 51 |
5.5 | ||
| 5 14 57
5 15 12 |
5 | 12 | Three waves fairly uniform. |
| 5 15 39
- - 44 |
5 | 6 | One wave. |
| 5 16 00
- - 9.5 |
9.5 | 3 | One wave. |
| 5 16 20.5
- - 25 |
4.5 | 4 | One wave. |
| 5 16 34 | Record ended by automatic stopping of plate and thereafter amplitude did not exceed 0.9 mm. |
| Time. Ewing Record | Period secs. | Ampl. mm. | |
| h. m. sec. | |||
| 5 12 45
- - 49 |
3.6 | One wave. | |
| Pen went off plate at 5:12:56 - made one swing 5:13:12; then off plate until 5:14:31 when record was resumed. | |||
| 5 14 34
- - 39 |
5 | 4 | One wave. |
| 5 14 50
- - 56 |
6 | 4 | One wave. |
| 5 15 24.5
- - 29 |
4.5 | 5 | One wave near end of appreciable motion. |
| Time. Ewing Record | Period secs. | Ampl. mm. | |
| h. m. sec. | |||
| 5 12 12 | 3 | 9 | One imperfect wave at beginning has period of about 3 secs. ampl. 9 mm; first motion directed downward. Steady mass thereafter thrown off pivots and record interrupted. |
The amplitude is the displacement of the pen from its neutral position divided by the magnification.
It will be noticed in the foregoing table that the Ewing record started at 5:12:45, whereas the strong motion as personally observed by R.G.A. also began at 5:12:45, from which it would appear that the Ewing did not start until the arrival of the strong motion. The record however, itself shows that the strongest motion set in about 10 to 15 seconds after the record started. This seeming discrepancy is easily understood however when it is observed that the first motion recorded by the Ewing is abundantly powerful enough to be really alarming, especially when preceded as it was by distinct seismic activity for a period of fully 33 seconds and of sufficient power to attract the attention of many observers. Moreover there are possible out standing errors in both the times that if known might lessen the slight discrepancy. For these reasons the times of the several principal waves of the record whose periods and amplitudes are tabulated are stated in times deduced from the record sheet.
II AFTER SHOCKS.
A large number of after shocks have been observed personally since the primary disturbance and a full list of these will be given at the proper place. The data herein submitted refer only to shocks which were recorded on the seismographs at Mount Hamilton or at Berkeley.
As has already been stated, the Ewing seismograph
It has furthermore been observed at the Student's Observatory that the slight eminence of land on which the Observatory is located seems to be less sensitive to disturbances than nearby locations in slight valleys etc. It is scarcely practicable with the limited evidence at present available to arrive at any definite conclusions on these points, but it has sometimes happened that very slight shocks have neither been recorded not felt by individuals at the Observatory when distinctly felt in the near vicinity.
In the data given below the shocks are listed in chronological sequence, under the station at which they were recorded. It will be observed at once from a comparison that the same shock was recorded at Berkeley and at Mt. Hamilton in only one case, namely on May 17th. Four additional shocks were recorded at the Lick Observatory up to this date, namely
It is also worthy of note that the period of the waves recorded at the Lick Observatory are generally longer than at Berkeley, although there is not a corresponding difference in the amplitude of motion. We might conclude from this that Mt. Hamilton was in general farther from the origin or origins of the several shocks than Berkeley.
AFTER-SHOCKS RECORDED AT LICK OBSERVATORY.
Lat. 37° 20' 26" N. Long. 121° 38' 43° W.
Time: 120th meridian West.
April 18. 8h 14m 39s A.M.
Very slight shock just felt. East & West motion. Period of three small waves at beginning 2.1 sec. each, amplitude 0.08 mm. North & South motion. Periods and amplitude about the same.
Vertical component out of order.
April 18. 9h 16m 52s.A.M.
East and West motion record confused. North and South motion period 0.4 sec. amplitude 0.08 mm.
Vertical component not yet in order.
April 18. 2h 23m 10s P.M.
East and West motion: Begins with relatively large waves, dying down and ending after 2-1/2 minutes. Period 2.3 secs. near beginning, amplitude 0.26 mm. subsequent waves quite regular attaining period of about 4 secs after one minute. North & South motion - less regular, period near beginning about 2.4 secs. amplitude 0.45 mm.
Vertical component not in action.
May 17. 8h 21m 22s P.M.
East & West motion. Period of three small waves shortly after beginning about 0.75 sec. amplitude 0.13 mm.
North & South motion. Period of four waves near beginning 0.25 sec. Amplitude 0.08 mm.
Vertical motion. Period of well defined waves about 2.7 sec. amplitude 0.3 mm.
Strong motion lasted 15 secs. total duration about 1-1/2 minutes.
June 15. 6h 11m 50s A.M.
East & West motion. Period of 5 waves near beginning. 3 secs. amplitude 0.13 mm.
North & South motion. Period of 4 waves near beginning 2.4 amplitude 0.07 mm.
Vertical motion. Period of one wave 4.2 sec. amplitude 0.3 mm.
Note: The constants of the Ewing seismograph at the Lick Observatory are as follows:
| Seco. | ||
| Magnifying factor N.S. motion ... | 4.2; period | 7 ± ¼ |
| Magnifying factor E.W. motion ... | 3.9; period | 7 ± ¼ |
| Magnifying factor Vertical motion ... | 1.8; period | 4 ± ⅛ |
The damping is relative strong and results from the friction at the stylus and pivots. Starting from a displacement of the stylus of 30 to 40 mm the pen will be brought to rest in but little more than one and one half complete vibrations.
Motion of the pen towards the center of the plate signifies motion of the ground to the West, to the North and to the Zenith.
AFTER-SHOCKS RECORDED AT STUDENT'S OBSERVATORY BERKELEY.
Lat. 37° 52' 24" Long. 122° 15' 41"
Time: 120th meridian West.
April 18. 9h 51m 55s A.M. Personal observation.
East & West motion. Period of one wave near beginning 1.9 sec. amplitude 0.13 mm.
North & South motion. Period of one wave near beginning 0.9 sec. amplitude 0.15 mm.
Vertical motion. Six waves near beginning 0.7 sec. amplitude 0.10 mm.
Duration of vertical motion 37 secs.
April 18. 10h 06m 29s A.M. Personal observation.
East & West Motion, not very regular but about the same as North & South as given below.
North & South motion: period of 7 waves, 35 secs after beginning 1.4 sec. each, amplitude 0.15 mm. Earlier portion of record irregular.
Vertical motion: period of 10 waves hear beginning 0. 7 sec. amplitude 0.06
Total duration 2 minutes.
April 18. 11h 06m 23s. A.M.
East & West motion not measurable.
North & South motion. Period of six waves at beginning 0.3 sec. amplitude 0.03 mm.
Vertical motion - period three waves 0.3 sec. amplitude 0.06 mm.
April 18. 12h 03m 43s. P.M.
East & West motion. Neither period nor amplitude measurable at beginning. About 20 secs thereafter, - Period of three waves 2.5 secs. amplitude very small. North & South motion - period of three waves near beginning 0.4 secs. amplitude 0.03 mm.
Vertical motion, period of 5 waves near beginning 0.7 sec. amplitude 0.06
Total duration of motions - 50 secs.+
April 18. 2h 24m 37s. P.M.
East & West motion. Period of four waves 10 secs after beginning 1.5 secs, amplitude 0.10 mm.
North & South motion. Period one wave at beginning 0.3 sec. then two waves of period of 0.6 sec and amplitude about 0.15 mm. Waves 40 secs later have period of about 1 sec and same amplitude as at the beginning.
Vertical motion. Period 0.6 sec. amplitude 0.12 mm.
Total duration of motion 70 secs or more.
May 17. 8h 21m 37s. P.M. Two observers.
East & West motion: Period of one wave at beginning 1.7 sec. amplitude 0.16 mm.
North & South motion: Period of 5 waves at beginning 3.4 secs. amplitude 0.09 mm 9 minutes later 3 waves of period of 5.4; amplitude 0.12 mm.
Vertical motion: Not measurable.
Total duration twice around plate. 12 minutes.
Duration North & South doubtful, perceptible motion after 6 minutes.
June 4. 11h 51m 07s. P.M.
East & West motion: Period of one wave at beginning 0.6 sec. amplitude 0.03 mm. later two waves of period of 1.5 sec. amplitude 0.07 mm.
North & South motion: Period of two waves at beginning
Vertical motion. Period of two waves at beginning 0.6 sec. period 0.06 mm.
Duration at least 3.5 minutes.
Note: The constants of the Ewing seismograph at the Student's Observatory, Berkeley, are as follows:
| Secs. | ||
| Magnifying factor for N. & S. motion | 3.3; | period 5.3 |
| Magnifying factor for E. & W. motion | 3.0 | period 6.6 |
| Magnifying factor for Vertical motion | 1.7 | Aperiodic |
The damping is relatively much the same as in the case of the instrument at the Lick Observatory, possibly is a little greater, especially for the vertical motion. The orientation of the instrument is slightly different so that Motion of the pen towards the center of the plate signifies motion of the ground to the West, to the South and to the Zenith.
AFTER-SHOCKS RECORDED BY THE OMORI TROMOMETER.
The details of the records from the Omori Tromometer have been fully worked up and tabulated at the Student's Observatory from time to time as the shocks occurred and a list of these is appended hereto.
Courtesy of The Bancroft Library, University of California, Berkeley, CA 94720-6000; http://bancroft.berkeley.edu/
http://content.cdlib.org/view?docId=hb1b69n7t3&brand=oac4
Title: Andrew C. Lawson papers, 1885-1951.: Report of the sub-committee on State Instrumental Records: July 23, 1906
By: Lawson, Andrew C. (Andrew Cowper), 1861-1952
Date: July 23, 1906
Contributing Institution: The Bancroft Library, University of California, Berkeley, CA 94720-6000; http://bancroft.berkeley.edu/
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