Seismology

"Seismology (see 8.817 and 24.589). - Strictly speaking, seismology is that department of knowledge which is concerned with the study of earthquakes, and such was its meaning up to the end of the 19th century, the older seismology being exclusively concerned with the earthquake which could be felt. In the early nineties it was discovered that suitably designed and sufficiently sensitive seismometers recorded disturbances which were evidently connected with great earthquakes, and, as it was known that the intensity of disturbance decreased with an increase of distance from the central region of greatest violence, it was natural to conclude that the very small disturbances, registered at great distances, were due to the same cause that gave rise to the destructive earthquake. With the accumulation of observations difficulties began to arise; it was found that neither the magnitude of the disturbance, nor the distance at which record could be obtained, bore any constant relation to the magnitude of the earthquake. Some shocks of great violence gave small records, not very extensively distributed, while others of much less severity at the place of origin gave much larger records and were registered all over the earth. The distant records, however, continued to be regarded as records of the earthquake itself, and are still generally described as such. In 1909 R. D. Oldham, when examining the circumstances of the Californian earthquake of 1906, arrived at the conclusion that the fractures and dislocations of the surface rocks, which gave rise to the destructive earthquakes, were but a secondary consequence of a deep-seated disturbance, to which he gave the name of bathyseism, and suggested that this, and not the earthquake, was the origin of the disturbance which, propagated through the interior of the earth, gave rise to the long-distance records, commonly known as seismograms. Subsequent consideration of other earthquakes confirmed his belief in the correctness of the conclusion, and from this it results that the word " seismology " is at present being used to cover two distinct and independent departments of study, which may be distinguished as teleseismology 61)Xe, distant) or the study of the long-distance records, and engysseismology (Ey-yin, near) or the study of the earthquake proper, each being distinct and independent offshoots of the bathyseism, or deep-seated disturbance. It is the first of these which, at the present day, is more especially meant by seismology, and it is an instance of the way in which words gradually depart from their original meaning, that the term should have come to imply something which has no direct connexion with earthquakes.

Nothing is known at present of the origin of the bathyseism, and very little of the depth at which it originates. The latter probably varies considerably, those disturbances which give rise to well-marked teleseisms and moderate surface earthquakes taking place at greater depths than - others which are accompanied by violent and destructive earthquakes. The only suggestion which has yet appeared of the depth of origin is by Dr. G. Walker, who has followed up certain investigations, started by Prince Boris Galitzin, of the angle of emergencies of the wave-paths, and finds that in many cases they indicate an origin at a considerable depth (it may be as much as 1,200 km.) below the surface. These investigations require following up before they can be accepted as conclusive, but the suggestion is of interest; there is no inherent impossibility, and it seems to offer a possible explanation of some difficulties which have arisen in the interpretation of the long-distance seismograms.

Since 1910 many improvements in detail have been made in the instruments used for obtaining the long-distance records of the newer, or tele-, seismology, and an entirely new principle was introduced by the late Prince Boris Galitzin for a direct measurement of the acceleration of the true motion of the ground. This instrument is based on the fact that, if a plate of quartz is subjected to pressure between two sheets of metal, a free electric charge appears in those sheets, the amount of which is proportioned to the pressure. An instrument was actually constructed on this principle and subjected to experimental tests, but has not been applied to the recording of natural disturbances, owing to the death of the inventor and the effects of the political revolution in Russia.

The rate of propagation of wave-motion through the earth, as registered by long-distance seismographs, has been investigated by Dr. C. G. Knott, who has succeeded in solving the mathematical difficulties of the problem. He finds that the rate of transmission of both the first phase, condensational, and of the second phase, distortional, waves increases continuously till the wavepath attains a depth of about three-tenths of the earth's radius, the wave-paths reaching lesser depths than this having a continuously curved form, convex towards the centre of the earth. Beyond this the rate of propagation is nearly constant, even decreasing at certain depths so that some of the wave paths are concave towards the centre in part of their course. Below sixtenths of the radius the distortional wave is killed out, and is not registered at distances greater than 1 20 ° from the epicentre. The rate of propagation of the two forms of wave-motion is about 7.2 and 4.0 km. per second respectively, near the surface of the earth, and about 12.8 and 6.8 at depths of over 1, Soo kilometres.

For the older seismology or study of earthquakes proper, see Geology (section Dynamical Geology). /n==Authorities==

 - The best general introduction to the newer seismology is Dr. G. W. Walker, Modern Seismology (1913). The most complete is by Prince B. Galitzin, original in Russian; a German translation, Vorlesungen in Seismometrie, appeared in 1912. G. W.

Walker, " Focal depth and the Time Curve," Brit. Assn. Rep. (1917, p. 13). C. G. Knott, " The Propagation of Earthquake Waves Through the Earth, and Connected Problems," Proc. Roy. Soc. Edinburgh (1919, p. 1 57). (R. D. 0.)