Seismology

SEISMOLOGY. - The fundamental task of s[eismology] is the study of the physical characteristics of the Earth's interior.

To this end, it develops research on the propagation of waves in elastic, viscous, firmo-elastic (which admit internal friction), etc. media, and compares the results of theories with those of observations. It is in this way that it has been possible to determine, among other things, the stratifications of the Earth's crust, the surfaces of discontinuity, and to discover the central core at a depth of about 2900 km.

All this presupposes the appropriate recording of the various wave systems which, aroused by an earthquake at a given point (at depths varying from 0 to about 800 km), propagate in all directions and to all distances. Hence the need for instruments whose recordings make it possible to trace back, as faithfully as possible, to the actual movement of the ground.

I. INSTRUMENTAL SEISMOLOGY

The recording of seismic phenomena is obtained by resorting to pendulum motion, understood in a broad sense. The first seismographs lacked damping, which made their recordings unusable for the purposes of scientific interpretation. The only kind 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 motion. Generally recourse is had to such a kind of damping.

The Galitzin seismographs, and the others with electromagnetic amplification, serve almost exclusively for the recording of earthquakes of distant origin. H. Benioff succeeded in devising a type of photoelectromagnetic seismograph that allows sufficient amplification for the recording of both distant and nearby earthquakes. The sensitivity of the Benioff transducer is extremely high. Connected with a long-period galvanometer, it is possible to reveal clearly displacements of the armature of the order of 5 x 10⁻⁷ cm, equal to 1/100 of the wavelength of sodium light.

II. THEORY OF SEISMIC RAYS; INTERNAL CONSTITUTION OF THE EARTH

One of the fundamental topics, the object of vast, profound investigations on the part of seismologists and geophysicists, is that concerning the propagation of seismic energy within the Earth.

Generally reference is made to the seismic trajectory normal to the wave surface, at the point under examination.

The development of the theory of seismic rays (direct propagation, refraction, diffraction, scattering) has made it possible to pursue the currently most reliable knowledge about the interior of the Earth. Seismic rays can be considered, from this point of view, as the Roentgen rays of the Earth: from the characteristics presented by the seismic recordings obtained at various distances it is possible to arrive at the determination of the surfaces of discontinuity, at the abrupt jumps in velocity (Table I) or of acceleration and therefore of the elastic and density properties of the Earth's interior. It is by this path 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 the determination of the depth of the core. One of these makes use of the recording times of the longitudinal (or transverse) waves that have reached the observation station after being reflected off the central core. By this path, a value for the depth of the core equal to km. 2920 has been reached, which confirms that obtained by Gutenberg.

The Japanese Wadati and Masuda resolved in 1934 the question relating to the determination of the travel times of longitudinal waves within the core: certain expedients allowed them to apply, also to the core, the Wiechert-Herglotz method. Since then, research on the propagation of seismic waves within the core has followed one another numerously.

Here only the latest results will be mentioned. It has been possible to prove (Lehmann, Gutenberg and Richter, Jeffreys) that even in the core the propagation of seismic energy does not occur with uniform variation of velocity: in 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 greater velocity.

TABLE I.

In the three figures the results of the research conducted on the subject from 1914 to 1950 are shown in graph form.
III. - Generally, the most conspicuous part of a seismogram is the one that follows the recording of the 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, involve only the superficial part of the Earth, in particular the stratification of the Earth's crust. These are the waves generically designated by the term surface waves. For distant earthquakes, originating at slight depth, their recording can last several hours.

Surface waves have been and are the subject of a great number of theoretical investigations, aimed at explaining their mechanism of production and propagation. Among the best known are the theories of Love and of 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, comprising the hypocentre, the observation station and the centre 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 determine 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 those stratifications, obtained by various methods, including those based on the dispersion of surface waves.

On average, the Earth's crust has a thickness of 40 km. In some parts of the world, rather than two, it appears to consist of three superimposed layers. Beneath mountain systems, the stratification presents considerably greater overall thicknesses: about 60 km beneath the Sierra Nevada (California), 55-60 km beneath the Alps, 60 km beneath the Apennines. This thickness is instead notably reduced in correspondence with the Oceans: about 20 km for the basins of the Pacific and the Atlantic. The stratifications beneath mountains are predominantly sialic, whereas they appear simatic beneath the Oceans.

IV. EARTHQUAKE

What are the causes of the earthquake? Setting aside earthquakes of volcanic origin and those determined by strictly local, very superficial causes (landslides, slipping of strata, etc.), seismic movements can be divided into two great categories: those that originate in the Earth's crust (having a thickness varying between 20 and 60 km.) and those that come from greater depths.

Mountain chains are the vertical result of tangential compressive forces that manifest themselves at the sides of geosynclinal areas, in which over geological time great masses of sediments of considerable thickness accumulated. If the orogenic movements, energetically accomplished in the geosynclines in the Tertiary era, continue to our own day, as everything suggests, albeit with diminished intensity, it is natural to see in the earthquakes of the Earth's crust one of the aspects under which those movements manifest themselves.

The other category of seismic movements concerns the earthquakes that occur beneath the Earth's crust, at depths that can assume very high values (100, 200, 500, 700 and more km.).

At those depths, matter is found at temperatures and pressures such as to exclude the possibility of dislocation, landslides or fractures. Only physico-chemical phenomena with abrupt changes of state of matter can be invoked to explain their origin; and it is not rash to attribute them to nuclear reactions, analogous to those, to be clear, to which we owe the atomic bomb.

The materials constituting the Earth's crust behave as viscous (in the broad sense) if subjected to the action of forces applied for a long time; as elastic, if the forces act abruptly and for a short period of time.

The first quality is evident in sedimentary rocks, formed at one time on the bottom of the seas and subsequently subjected to the slow action of folding of the Earth's crust (extremely slow), also revealed by precision levellings; the second is attested by the propagation of seismic waves, determined by abrupt ruptures of equilibrium (earthquakes). The simple observation of the initial movements determined by a shock at a certain number of observatories, suitably distributed around the epicenter, today makes it possible to deduce the physical nature of the cause to which the shock itself is due.

If the first movement (direct longitudinal waves) occurs, for certain stations, in the direction stationhypocenter (dilatation), for others in the opposite direction (compression), and if these movements divide at the surface in such a way as to distribute themselves into four quadrants (the general case), so that in the two pairs of opposite quadrants there are movements of the ground of the same sign (compressions or dilatations), then the earthquake is due to fracture.

It sometimes happens that in circular zones around the epicenter the initial movement is unidirectional in all azimuths: if this movement is one of dilatation, we will have an earthquake of subsidence; if instead it is one of compression, the earthquake is one of uplift. In the Alpine region earthquakes due to fracture predominate, in the upper Adriatic earthquakes of subsidence, while in the Apennines those of uplift predominate.

V. THE MICROSEISMS

By the name of microseisms it is customary to designate that complex of small oscillations, generally having the appearance of damped sinusoids, which are recorded by seismic instruments, especially during winter and spring. The periods of such oscillations, for microseisms properly so called, can vary from a few tenths of a second to about 12 seconds, depending on the causative agent and the distance of the origin. Among the various types of microseisms, the most frequent and of the most notable amplitude are those in any case linked to meteorological phenomena.

TABLE II.

* d_{1} = thickness of first layer; d_{2} = thickness of second layer.

It has been established that microseisms, more or less notable, accompany cyclones along their path over the Atlantic (in general, over the sea). While the velocity of translation of cyclones is on average 50-60 km. per hour, microseisms can propagate with velocities of the order of 10,500-11,000 km. per hour; that is, with a velocity 200 and more times greater. Here is a particularly favorable characteristic, whose importance for the solution of the problem relating to weather forecasting should not be underestimated.

Pietro Caloi