Page images
PDF
EPUB

(c) Tremors, Microseismic Disturbances, or Earth Pulsations.

In these tables the instruments or localities are indicated according to the letters of the alphabet. The hours between which tremors were marked are, for example, given thus, Oct. 13, 3 to 24. The times at which they attained a maximum range are given thus, max. 15 to 18. 0 or 24 indicates mid-day. The small figures in brackets give in millimetres the range of motion of the pendulum, as shown upon the photograms.

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

TREMORS, MICROSEISMIC DISTURBANCES, OR EARTH PULSATIONS-continued.

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][ocr errors][ocr errors][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small]

TREMORS, MICROSEISMIC DISTURBANCES, OR EARTH PULSATIONS-continued.

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small]

(d) The Slow Displacement of Pendulums.

All my horizontal pendulums, whether they were situated upon the rock or upon the alluvium, have crept away from their normal position. While some of them have moved towards the east, others have moved towards the west. Irregularities like these have been noticeable upon newly built light foundations. Pendulums like those at A and L, where the foundations were massive and old, during the time that they were observed, had a general tendency to creep towards the west or southwest. These movements, which were often interfered with by permanent displacements caused by earthquakes, closely resembled the gradual displacements observed at stations upon the rock, where, for earthquakes at least, the yielding parallel to the dip of the strata was greater than it was in a direction at right angles to the same.

Although my intention when installing the instruments parallel and at right angles to the dip was to determine in which of these two direc tions yielding was the most pronounced, the observations were not continued sufficiently long to determine whether such changes as were observed had any connection with the secular movements which around Yokohama are apparently proceeding with unusual rapidity. To carry out this investigation, which would not be difficult for a resident at Kamakura, where caves are numerous, at least two installations would be required, and it is not unlikely that within a period of two or three years some measurement of geological changes would be obtained. I also regret to say that the duration of the observations was not sufficiently long to determine whether the wandering of the pendulums had an annual periodicity such as might be expected from the results obtained by observers in Europe.

(e) Periodic movements of several days' duration, and wandering of the pendulums.

In order to determine the existence or non-existence of periodical movements greater than twenty-four hours, the mid-day position of instruments A and J were plotted on squared paper. This was done for dates between October 13 and December 4, 1894. Because the instrument at K during the period wandered so rapidly towards the east,

[blocks in formation]

and had in consequence to be repeatedly re-adjusted, its records have not been considered. Whether the mid-day reading is the best one to represent the mean daily position of the instruments must for the present be left until a second examination of the diagrams has been made, when the analysis may be continued up to the end of the first week in February 1895 (fig. 4).

Bar

FIG. 5.

Oct. 13

17 21 25 29 Nov2 6

10

14 18 22 26 30

1894

[blocks in formation]

By drawing a free curve through the diagrams of mid-day positions of A and J, and reducing this horizontally and vertically one-fourth, the two curves shown in the accompanying figure are obtained (fig. 5).

During the complete period considered neither of the pendulums has changed much in its mean position. Whatever slight changes have

1895.

K

evapora

taken place may be due to the direction in which the instruments or their foundations have warped, or, what is equally probable, they may each have moved away from an exposed area which, in consequence of tion, may have been rising. About this general movement of the pendulums which might be included in the previous section of this report, because it only refers to observations extending over fifty-two days with pendulums, one of which had a light foundation, no definite conclusions can be stated.

One thing, however, which is clear, and which can hardly be attributed to the warping of instruments or their foundations, is that the pendulums wandered at the same time in the same directions. For the first four days the pendulums at A and J moved westwards, for the next twelve days they moved eastwards, after which there was a slight westerly and then an easterly motion up to the fortieth day, when they both turned quickly westwards. This synchronism in direction of motion is evidently due to some general cause, which may act on the surface of the earth or within its interior. A barometric curve determined in a manner similar to that in which the curves for A and J were determined shows that atmospheric pressure was near its maximum when the pendulums were at their western limits, but the relationship between this curve and those of the pendulums is not sufficiently clear to conclude that the movements of the pendulums have been altogether due to fluctuation in atmospheric pressure. Possibly the movements may have been due to evaporation and precipitation lightening or loading some particular area in the vicinity of the pendulums.

The earthquakes of local origin indicated by circles on the curve for A, which occurred during the time that there was a rapid westerly motion, suggest the idea that the movements may have a hypogenic origin.

(f) On the daily change in the position of the pendulums.

The table below gives in millimetres the difference in the mid-day readings of the positions of the pendulums A and J. The sign + indicates that during the past twenty-four hours the pendulum has moved so many millimetres towards the west, while the sign indicates a displacement towards the east. The sign? means that no reading had been taken or that no displacement was observed.

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][ocr errors][ocr errors][merged small][ocr errors][ocr errors][ocr errors][merged small][merged small][ocr errors][ocr errors][merged small][ocr errors][merged small][merged small][ocr errors][merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors]
« PreviousContinue »