Fig. 79.—Seismographic Record of Tokio Earthquake of 1894. (Omori.)[ToList]
The vibrations that produce the sensible shock are by no means all that are present during an earthquake. The Indian earthquake, for instance, seemed to last about three or four minutes at Midnapur; but the movements of the bubble of a level showed that the ground continued to oscillate for at least five minutes longer (p. 280). Many of these unfelt waves are rendered manifest by seismographs, although there are still others that elude registration either from the extreme shortness or the great length of their periods.
In Fig. 79 is shown the principal part of a diagram obtained at Tokio during the Japanese earthquake of June 20th, 1894 (p. 18), the curve representing the N.E.-S.W. component of the horizontal motion during the first 25 seconds of the record. The instrument employed is one specially designed for registering strong earthquakes, and is unaffected by very minute tremors. Those which formed the commencement of this earthquake lasted for about 10 seconds, as shown by ordinary seismographs, and the vibrations had attained a range of a few millimetres before they affected the instrument in question. For the first 2½ seconds, they occurred at the rate of four or five a second. The motion then suddenly became violent, and the ground was displaced 37 mm. in one direction, followed by a return movement of 73 mm., and this again by one of 42 mm., the complete period of the oscillation being 1.8 seconds. The succeeding vibrations were of smaller amplitude and generally of shorter period for a minute and a half, then dying out during the last three minutes as almost imperceptible waves with a period of two or more seconds.[81]
Though incomplete in some respects, this diagram illustrates clearly the division of the earthquake-motion into three stages—namely, the preliminary tremors, the principal portion or most active part of an earthquake, and the end-portion or gradually evanescent slow undulations. In all three stages, however, both tremors and slow undulations may be present; and, as the latter, owing to their long period, are more or less insensible to human beings, the ripples of the final stage give the impression of a tremulous termination as described above. The duration of each stage varies considerably in different earthquakes. Thus, in a valuable study of 27 earthquakes recorded at Miyako, in Japan, during the years 1896-98, Messrs. Omori and Hirata show[82] that the duration of the preliminary stage varies from 0 to 26 seconds, with an average of about 10 seconds; that of the principal portion from 0.7 to 26 seconds, also with an average of about 10 seconds; and that of the end portion from 28 and 105 seconds, with an average of about one minute. The total apparent duration, however, depends on the instrument employed; one of the earthquakes, that of April 23rd, 1898, disturbing the seismograph at Miyako for two minutes; while, at Tokio, a horizontal pendulum designed by Professor Omori oscillated for at least two hours. The periods of both ripples and slow undulations, again, vary from one earthquake to another; but it is worthy of notice that the average period of the undulations is almost constant in all three stages of the motion, being 1.1, 1.3, and 1.3 seconds, respectively, for the east-west component of the horizontal motion, and 1.0 second throughout for the north-south component. For the ripples, the average period is .08 second in the preliminary stage, .10 second in the principal portion, and .08 second again in the end portion; those of the principal portion being slightly larger in amplitude, as well as longer in period, than the ripples of the first and third stages.
SOUND-PHENOMENA.
Besides the ripples already mentioned, there are others of still smaller amplitude and shorter period that are sensible, but as a rule only just sensible, to us as sounds. All the known evidence points to the extraordinary lowness of the earthquake-sound. According to some observers, it seems as if close to their lower limit of audibility; while others, however intently they may listen, are unable to hear the slightest noise. In other words, the most rapid vibrations present in an earthquake do not recur at a rate of much more than about 30 to 50 per second; or, if they do, they are not strong enough to impress the human ear.
To most observers, the sound seems to increase and decrease in intensity with the shock, and so gradually and smoothly does this change take place that the sound is frequently mistaken for that of an underground train approaching the observer's house, passing beneath it, and receding in the opposite direction. Some persons, especially if situated within the meizoseismal area, hear also loud crashes in the midst of the rumbling sound and simultaneously with the strongest vibrations. At a moderate distance, say from 30 to 40 miles, the sound becomes more harsh and grating while the shock is felt; and, at a greater distance, even this change disappears, and nothing is heard but an almost monotonous sound like the low roll of distant thunder. The explanation of this is that the sound-vibrations are of different periods and varying amplitude, and the limiting vibrations tend to become inaudible with increasing distance, the lower on account of their long period, the higher owing to their small amplitude.
The magnitude of the sound-area depends, even more than that of the disturbed area, on the personal equation of the observers. The lower limit of audibility varies not only in different individuals, but also in different races. In Great Britain, it is doubtful whether an earthquake ever occurs unaccompanied by sound; and in the meizoseismal area the noise is heard by nearly all observers. With Italians, the average lower limit of audibility is higher than with the Anglo-Saxon race; slight shocks frequently occur without noticeable sound, but with strong ones, the larger number of observers is sure to include one or more capable of hearing the rumbling noise. The Japanese are, however, seldom affected by the most rapid earthquake-vibrations, and the strongest shocks may be unattended by any recorded sound. The result is manifest in the size of the sound-area in different countries. In the Hereford earthquake, the sound-area contained 70,000 square miles; in the Neapolitan earthquake, about 3,300 square miles; while, in Japanese earthquakes, the sound is rarely heard more than a few miles from the epicentre.