To this end, it develops research on the propagation of waves in elastic, viscous, firmo-elastic (which admit internal friction) and other media, and compares the results of theories with those of observations. In this way, among other things, it has been possible to determine the stratification of the Earth’s crust, the surfaces of discontinuity, and to discover the central core at a depth of approximately 2900 km.
All this presupposes the appropriate recording of the various wave systems that, originating from an earthquake at a given point (at depths varying from 0 to approximately 800 km), propagate in all directions and over all distances. Hence the need for instruments whose recordings allow the real movement of the ground to be reconstructed as faithfully as possible.
I. INSTRUMENTAL SEISMOLOGY
The recording of seismic phenomena is achieved by means of pendular motion, understood in a broad sense. The first seismographs lacked damping, which rendered their recordings unusable for scientific interpretation. The only type of damping whose action does not give rise to theoretical difficulties is that in which the damping force is proportional to the velocity of the movement. Generally, recourse is had to such a type of damping.Seismographs of the Galitzin type, and others with electromagnetic amplification, are used almost exclusively for the recording of distant earthquakes. H. Benioff has succeeded in devising a photoelectromagnetic type of seismograph that permits sufficient amplification for the recording of both distant and nearby earthquakes. The sensitivity of the Benioff transducer is high. When connected to a long-period galvanometer, it is possible to clearly detect displacements of the armature on the order of 5 × 10⁻⁷ cm, equivalent to 1/100 of the wavelength of sodium light.
II. THEORY OF SEISMIC RAYS; INTERNAL CONSTITUTION OF THE EARTH
One of the fundamental subjects, the object of extensive and profound investigations by geophysical seismologists, is that concerning the propagation of seismic energy within the Earth.Generally, reference is made to the seismic ray normal to the wave surface at the point under consideration. The development of the theory of seismic rays (direct propagation, refraction, diffraction, scattering) has made it possible to pursue the currently most reliable knowledge of the Earth’s interior. From this perspective, seismic rays may be considered as the X-rays of the Earth: from the characteristics presented by seismic recordings obtained at various possible distances, it is possible to determine the surfaces of discontinuity, the abrupt jumps in velocity (Table I) or acceleration, and thus the elastic properties and density of the Earth’s interior. It was by this method that Gutenberg determined the depth of the Earth’s core, assigning it a value of 2920 km. Subsequent research has confirmed this value. Table I contains the velocity of seismic waves within the Earth, from the outer surface to the core. There are other methods that allow for the determination of the depth of the core. One of these employs the registration times of longitudinal (or transverse) waves that have reached the observation station after being reflected from the central core. By this means, a value for the depth of the core equal to 2920 km has been obtained, confirming that derived by Gutenberg.
The Japanese Wadati and Masuda resolved in 1934 the question regarding the determination of the travel times of longitudinal waves within the core: certain refinements enabled them to apply the Wiechert-Herglotz method even to the core. Since then, research on the propagation of seismic waves within the core has multiplied.
Here, only the latest results will be mentioned. It has been proven (Lehmann, Gutenberg and Richter, Jeffreys) that even within the core, the propagation of seismic energy does not occur with uniform variation in velocity: within the core there exists a discontinuity (of the first order according to Jeffreys, of the second order according to Gutenberg and Richter) that divides it into an outer part, in which the velocity of the seismic wave is low, and an inner part, characterized by a higher velocity.
Table I.
| 0 | 5.55 | 3.3 | 1202 |
| 400 | 9.1 | 5.0 | 2000 |
In the three figures, the results of research conducted on this subject from 1914 to 1950 are presented graphically.
III. – Generally, the most prominent part of a seismogram is that which follows the recording of transverse waves reflected one or more times against the Earth’s surface: this part, consisting of long-period oscillations with a pronounced sinusoidal character, is made up of waves that, in their propagation, affect only the superficial part of the Earth, particularly the stratification of the Earth’s crust. These are the waves generically indicated by the term surface waves. For distant earthquakes originating at shallow depths, their recording can last several hours.
Surface waves have been and continue to be the subject of a great number of theoretical investigations aimed at explaining their production and propagation mechanisms. Among the most well-known are the theories of Love and Lord Rayleigh. Love’s theory concerns the initial phase of surface waves, consisting of transverse-tangential waves (without a vertical component). Waves oscillating in the principal plane, which includes the hypocenter, the observation station, and the center of the Earth, are instead defined as Rayleigh waves.
The study of surface waves is not only of great theoretical interest; it also serves to obtain the physical characteristics of the Earth’s crust. Table II contains the propagation velocities of seismic waves in the stratification of the Earth’s crust and the thicknesses of these stratifications, obtained by various methods, including those based on the dispersion of surface waves.
| 1000 | 4000 | 5000 | 6000 |
| V, in km/h | | GUTHEBERG E RICHTEN (1938) | |
| | | NUVEA CUBRA (1939) | |
| | | | |
SEISMOLOGY – Variation in the velocity of longitudinal waves from the surface to the center of the Earth.
On average, the Earth’s crust has a thickness of 40 km. In some parts of the world, instead of consisting of two layers, it appears to consist of three superimposed layers. Beneath mountain systems, the stratification presents considerably greater total thicknesses: approximately 60 km beneath the Sierra Nevada (California), 55–60 km beneath the Alps, and 60 km beneath the Apennines. This thickness is, however, significantly reduced in correspondence with the oceans: approximately 20 km for the basins of the Pacific and Atlantic. The stratifications beneath mountains are predominantly sialic, while those beneath oceans are simatic.
IV. EARTHQUAKE
What are the causes of an earthquake? Disregarding earthquakes of volcanic origin and those caused by strictly local, very superficial causes (landslides, slippage of strata, etc.), seismic movements can be divided into two broad categories: those originating in the Earth’s crust (which has a thickness varying between 20 and 60 km.) and those originating at much greater depths.Mountain ranges are the vertical result of tangential forces pressing in from the sides of geosynclinal areas, where, over geological time, thick masses of sediments have accumulated. If orogenic movements, which energetically took place in the geosynclines during the Tertiary period, continue even today—albeit with diminished intensity—it is natural to see earthquakes in the Earth’s crust as one of the ways in which these movements manifest themselves.
The other category of seismic movements concerns earthquakes that occur beneath the Earth’s crust, at depths that can reach very high values (100, 200, 500, 700 km. or more).
At such depths, matter is subjected to temperatures and pressures so extreme that dislocation, landslides, or fractures can be ruled out. Only physico-chemical phenomena involving sudden changes in the state of matter can be invoked to explain their origin; and it is not unreasonable to attribute them to nuclear reactions, analogous to those responsible for the atomic bomb.
The materials that make up the Earth’s crust behave as viscous (in a broad sense) when subjected to forces applied over long periods; as elastic, when forces act suddenly and for a brief time.
The first property is evident in sedimentary rocks, formed over time on the sea floor and later subjected to the slow corrugating action of the Earth’s crust (epeirogenic movements), also revealed by precision leveling; the second is evidenced by the propagation of seismic waves, determined by sudden ruptures of equilibrium (earthquakes). Simple observation of the initial movements recorded by a number of observatories, suitably distributed around the epicenter, now makes it possible to deduce the physical nature of the cause to which the shock is due.
If the first movement (primary longitudinal waves) is recorded, for certain stations, in the direction from the station to the hypocenter (dilation), and for others in the opposite direction (compression), and if these movements are distributed on the surface in such a way as to divide into four quadrants (general case), so that in two pairs of opposite quadrants the ground movements are of the same sign (compressions or dilations), then the earthquake is due to a fracture.
It sometimes happens that in zones of circular shape around the epicenter the initial movement is unidirectional in all azimuths: if this movement is one of dilation, we have a subsidence earthquake; if it is one of compression, the earthquake is one of uplift. In the Alpine region, earthquakes due to fracture predominate; in the upper Adriatic, subsidence earthquakes; in the Apennines, uplift earthquakes.
V. MICROSISMS
The term microsisms refers to a complex of small oscillations, generally appearing as damped sinusoids, which are recorded by seismic instruments, especially during winter and spring. The periods of these oscillations, for properly so-called microsisms, can range from a few tenths of a second to about 12 seconds, depending on the acting cause and the distance from the origin. Among the various types of microsisms, the most frequent and of greatest amplitude are those linked in some way to meteorological phenomena.TABLE II.
| Württemberg | 20 | 5.6 | 3.3 | 25 | 6.3 | 3.7 |
| Northern Alps and surrounding areas | 35 | 5.6 | — | 20–25 | 6.5–7.0 | 3.7 |
| Carnic Alps | 35–40 | 5.7 | 3.4 | 15 | 6.6 | 3.7 |
| Yugoslavia | 15 | 5.5 | — | 25 | 6.3 | 3.65 |
| Central Italy (Apennines) | 25 | 5.3 | 3.0 | 35 | 6.6 | 3.6 |
| New Zealand | 10 | 5.0–5.55 | 3.4 | 20 | 6.5 | 3.8 |
| Central Asia | 35 | 5.5 | 3.20 | 15 | 6.25 | 3.7 |
| Central California | 10 | 5.6 | 3.3 | 25 | 6.7 | — |
| Sierra Nevada (California) | 20 | 5.6 | 3.3 | 40 | 6–7 | — |
* \(d_1\) = thickness of first layer; \(d_2\) = thickness of second layer.
It has been observed that more or less notable microsisms accompany cyclones as they travel across the Atlantic (generally over the sea). While the translational speed of cyclones averages 50–60 km. per hour, microsisms can propagate at speeds on the order of 10,500–11,000 km. per hour—thus 200 or more times faster. This is a particularly noteworthy characteristic, whose importance should not be underestimated in resolving the problem of weather prediction.