Influence of Instrumental Observation on the Theory of Types of Vibrations and the Path of Earthquake 
Waves
In the beginning, instrumental observation of earthquakes was used to prove the types of movements, as they were postulated in the theories of wave propagation of earthquakes by MALLET, HOPKINS and others, and to provide exact data on the direction and speed of earthquake waves. While MALLET assumed in his construction and application of his earthquake instruments that the biggest destruction is caused by a vertical stroke of the longitudinal compression wave that transforms into wave movements on the surface, and he did not attribute any importance to transversal waves, this opinion changed when it became possible to register the complete course of an earthquake by appropriate instruments. And it was discovered that the recordings are already triggered by small and rapid vibrations and not by the following stronger vibrations with longer periods. EWING assumed these preliminary tremors to be longitudinal vibrations and the subsequent major tremors to be elastic transversal vibrations:

"In all probability the quick-period tremors were normal vibrations, while the larger motions were transverse vibrations".[1]

EWING describes an elastic transversal vibration with horizontal direction of vibration of the mass particles. Th. GRAY was of the same opinion, and he claimed to have discovered this some time before EWING.[2] This claim led to a dispute over priority, and EWING dismissed GRAY's observation as inadequate later. C.G. KNOTT dealt with types of movements in earthquake waves in a more elaborated way than EWING and GRAY.[3] He interpreted preliminary tremors also as longitudinal waves (normal motion) with a very short duration of vibration, but he also distinguished purely elastic and quasi-elastic motions of the surface. These quasi-elastic movements occur in epicentral regions where parts of the crust are stressed beyond their limit of elasticity. During propagation, the elastic surface waves lose energy and will be observed by instruments in a large distance as horizontal movements.

Around 1880, John MILNE, in agreement with EWING and GRAY, interpreted the first diagrams of local earthquakes in Japan[4] as elastic, longitudinal and transversal vibrations. In the case of longitudinal waves, - in his opinion caused the preliminary tremors -, he followed KNOTT's position in attributing their vertical position to the fact that these vibrations originate from a quasi-elastic surface, accelerated by gravitation. The exceptionally high propagation speed over large distances led him to the assumption that the preliminary vibrations are caused by longitudinal waves that travel through the interior of the earth.[5] Originally he interpreted the long waves of the main disturbance near the epicentre as transversal waves that propagate on the surface. Later, during the strong Mino-Owari earthquake in 1891, MILNE observed the movement of his instruments and of a basin filled with water.[6] He had the impression that the long earthquake waves would be periodically changing their inclination and are not elastic transversal vibrations, and thus attributed them to motions of the crust that floats on the fluid basis that occur due to gravitation. According to MILNE, wave motions in the ground were observed by many eye-witnesses during a major earthquake on Jedo on October 28, 1891.[7] It also corresponds to eye-witness reports during a major historical earthquakes in Calabria (1783) and Kachar (1869) where the emergence of fissures coincided with an up-bending of the surface beyond the limit of elasticity. Comparative experiments carried by MILNE[8], SEKIYA and OMORI [9] at the surface and at a depth of not more than 10 feet or 18 feet, also showed that the free surface has its own conditions of motion, manifesting itself in visible undulations that are very similar to water waves.

A new era of instrumental earthquake observation was initiated by recording of remote earthquakes with the horizontal pendulum built by REBEUR-PASCHWITZ. He noticed that the speed of the preliminary tremors increases strongly with the distance and concluded that these longitudinal vibrations propagate through the interior of the earth, while he considered, as MILNE did, the main waves as gravitational waves propagate along the surface of the crust which floats on magma.[10] Later he also observed an increase in speed with the distance in these major waves and concluded that they are also movements that propagate through the earth as transversal elastic vibrations.[11] But during the recording of nearby earthquakes, AGAMENNONE[12] saw that the continuous refraction and splitting of wave forms at the borders of different materials, the crust consists of, continuously provokes the origin of both wave forms so that it is impossible to observe them separately and successively. But G. VICENTINI in Padua was able to prove that in some cases it is possible to separate the two phases in seismograms.[13]

In 1893, A. CANCANI published a treatise on earthquakes[14], that received much attention in Italy, where he followes WERTHEIM's theory and managed to prove with his instrumental observations that the preliminary tremors propagate almost twice as fast as subsequent main tremors and he concluded that the preliminary tremors are longitudinal vibrations and the main tremors are transversal vibrations, in line with the theory of elastic wave movements. While EWING and GRAY considered the main tremors as elastic transversal motions with a horizontal direction of vibration of the mass particles, CANCANI assumed a vertical direction of vibration that is supposed to lead to undulations on the surface, similar to water waves and MILNE's gravitational waves.

MILNE's considerations about the nature of surface waves were continued by Franz Eduard SUESS on the occasion of the Ljublijana earthquake on April 14, 1895. In his voluminous monograph about this earthquake he dedicated a whole chapter on the 'theoretical considerations on the nature of movements'[15]. There, he confirmed MILNE's position that vibrations with small amplitude and brief duration of vibration that were recorded in the seismogram, and that were called ripples, are doubtless to be attributed to a longitudinal wave that travels through the interior of the earth, while the slow large waves show a very different way of movement already in the pleistoseist zone. The complete separation of both forms of motion in larger distances and the fact that their speeds are in relationship 1:2, led him to the conclusion that they represent the longitudinal and the transversal waves of WERTHEIM's theory who had postulated them as motion forms of waves that travel through solid bodies. So SUESS assumes two kinds of waves that propagate inside the earth, the longitudinal and transversal waves that both transform into another form at the surface. In this connext, he makes the important remark that partially confirmed by the existence of RAYLEIGH- and LOVE-waves is at least today:

'It is not unthinkable that in analogy to the longitudinal and transversal waves we can also assume two types of surface waves that propagate at different speeds.'[16]

But he adds:

'But the facts do not seem to support this assumption.'

As early as 1896, Eduard SUESS anticipates for seismology the importance of RAYLEIGH's purely theoretical considerations on the surface wave by citing from his treatise[17] from 1885-1886:

'In a mathematical treatise Lord Rayleigh has dealt with the problem of a possible surface wave on an elastic, firm medium from a theoretical point of view without discussing earthquake phenomena in detail. According to his studies the particles vibrate in such a wave in elliptic trajectories and the propagation speed of such a wave is always lower than that of the transversal wave.'

"Lord Rayleigh hat in einer mathematischen Abhandlung das Problem der möglichen Oberflächenwelle auf einem elastischen, festen Medium vom rein theoretischen Standpunkte behandelt, ohne auf die Erdbebenerscheinungen näher einzugehen. Seinen Studien zu Folge schwingen in einer solchen Welle die Partikelchen in elliptischen Bahnen und die Fortpflanzungsgeschwindigkeit einer solchen Welle ist immer kleiner als die der transversalen."[18]

These waves, that are very similar to water waves, are the ones with the lowest propagation speed but with the longest duration.

A decade before A.E.H. LOVE discovered the second surface wave that was named after him there were anticipating approaches to describe this wave as corresponds to a transversal wave with purely horizontal direction of vibration and a purely horizontal deformation of rock without vertical displacement. F. OMORI[19] assumed that the surface of the earth can be supressed due to its elasticity and that it swings back and forth in the sense of translatory motions without the emergence of inclinations. He arrived at this assumption on the basis of seismograms that were recorded by two identical horizontal pendula during a local earthquake on November 7, 1878. These pendula were tuned to record different durations of vibrations and to be sensitive to inclinations. The result was that the support of the horizontal pendulum can carry out motions of different periods and different distances without inclining towards any direction but that it displaces into a certain direction inside the frame.

Independently of OMORI, W. SCHLÜTER in Göttingen also arrived at a similar result. He tried to resolve the big dispute of whether 'inclinations or translatory displacements of the surface' involving by buildings, a special instrument that he called 'clinograph'. The final result of his series of experiments in 1899, when 20 earthquakes of different kinds and intensities were registered, was, that the widely accepted assumption among seismologists that the instruments are triggered by inclinations is wrong. He carried on stating, that motions so far observed in seismographs can only be caused by 'translatory vibrations' of earth particles, which he understood as the vibrations with straight or elliptic trajectories, in contrast to 'inclinatory vibrations'. SCHLÜTER explicitly adds that he cannot rule out the existence of inclinatory vibrations, but that they are unnoticeable.[20] And he also thought that he would be able to record an inclinatory wave by using a clinograph with an arm of several meters of length. Due to his early death, this instrument was never built, although he had already drawn the design. Clinometric investigations were also carried out by John MILNE[21] in Japan, and in his earthquake observatory in Shide on the Isle of Wright. Initially, he built his own clinometer with photographic recording and extended it by a vertical spring seismograph. The diagrams produced in this way confirmed SCHLÜTER's position that the horizontal pendula vibrating rather due to horizontal displacements of the ground than by inclinations of the ground. MILNE did not exlude the possibility of inclinations of the ground but he assumed that at least they would have to be very very small. But MILNE maintained the assumption of the undulatory character of long waves, both body waves and surface waves.

The followers of the inclination theory, such as P.G. ALFANI[22], were more radical. At the geodynamic observatory in Ximeniano he thought to have proven during observations with an improved instrument by VICENTINI that the lateral acceleration is unacceptable and in reality it is nothing else than a true system of transversal waves.

The solution of this dispute was delivered ten years later by the proof of the existence of transversal surface waves with purely horizontal direction of vibration, that was described for the first time by A.E.H. LOVE in 1912.

As we know today, horizontal lateral displacements and vertical inclinations in the form of waves that are similar to water waves are not mutually exclusive alternatives of surface waves, but can both be observed by modern instruments separately from each other as so-called Love- and Rayleigh waves.

End

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