It is not easy to form any precise image of the earthquake as it appeared to the terrified witnesses within the meizoseismal area. To minds unbalanced by the suddenness of the shock and by the crash of falling houses, actuated too by the intense need of safety, the mere succession of events must have presented but little interest. The interval of two months that elapsed between the occurrence of the earthquake and its investigation was also unfavourable to the collection of accurate accounts from a wonder-loving people. Only one feature, therefore, stands out clearly in the few records given by Mallet--namely, the division of the shock into two distinct parts.
In the central district, this division is perhaps less apparent than elsewhere. At Polla, for instance, which lies close to the north-west epicentre, the first warning was given by a rushing sound; almost instantly, and while it was yet heard, came a strong subsultory or up-and-down movement, succeeded after a few seconds, but without any interval, by an undulatory motion. At Potenza, which is not far from the same epicentre but a few miles outside the meizoseismal area, the separation was more p.r.o.nounced. According to one observer, the first movement was from west to east; and, within a second or two afterwards, there was a less violent shock in a transverse direction, followed immediately by a shaking in all directions, called by the Italians vorticose. Naples lies sixty-nine miles from the north-west epicentre, and here more accurate observations could be made. Dr.
Lardner, well known fifty years ago as a writer of scientific works, describes the first movement felt there as "a short, jarring, horizontal oscillation, that made all doors and windows rattle, and the floors and furniture creak. This ceased, and after an interval that seemed but a few seconds was renewed with greater violence, and, he thought, with a distinctly undulatory movement, "like that in the cabin of a small vessel in a very short chopping sea.""
In five other earthquakes studied in this volume, the separation of the shock into two parts was a well-marked phenomenon. In the Neapolitan earthquake, the separation was so distinct that Mallet took some pains to account for its origin. He regarded it in every case as due to the reflection or refraction of the earth-waves by underlying rocks, though he does not explain why the reflected or refracted wave should be more intense than that transmitted directly. I shall refer to the subject in greater detail when describing the Andalusian, Charleston, Riviera, and Hereford earthquakes. For the present, it may be sufficient to urge that the double shock cannot have been due to the separation of the original waves by underground reflection or refraction, for then the second part should have been generally the weaker; nor to the succession of longitudinal and transverse waves, for, in that case, every earthquake-shock should be duplicated. The only remaining supposition is that there was a second impulse occurring either in the same or in a different focus.
Which alternative should be adopted, the evidence on the nature of the shock is too scanty to determine. The defect is, however, supplemented by Mallet"s observations on the direction of motion; for, at many places within and near the meizoseismal area, he met with the clearest signs of a double direction. Sometimes this was apparent to the senses of the observer; in other cases, damaged buildings presented two sets of fissures. At La Sala and near Padula, the first movement was roughly east and west, the second north and south. At Moliterno, there was evidence of a subordinate shock at right angles to the chief one; in the neighbourhood of Tramutola, its direction was from about E. 30 S.
In these and other cases, Mallet saw the effects of earthquake-echoes; but the underground reflection of earth-waves would give rise to the second part of the shock, not the first as at La Sala and Padula.
Moreover, the secondary directions, though they are seldom recorded accurately, point nearly to an epicentre not far from Montemurro. The observations on the nature and direction of the double shock thus confirm the conclusion, derived from the distribution of the seismic death-rate, that there were two detached foci, one near Polla and the other near Montemurro.
This seems to be the best explanation of the facts recorded by Mallet.
There is, however, a possible difficulty that should not be overlooked--namely, the apparently slight influence of the Montemurro focus on the mean direction of the shock (Fig. 9). At a few places, of course, the mean direction pa.s.ses through both epicentres; at some others, as we have seen, one of the two observed directions points towards the Montemurro epicentre. It is not impossible, also, that Mallet, after the first few days" work, may occasionally have quite unconsciously selected and measured those fissures from the maze presented to him which agreed most closely with his early impressions obtained from the neighbourhood of Polla. But, for places nearer Polla than Montemurro (and these form the majority of those visited by Mallet), the probable explanation of the difficulty is that the Montemurro focus was not so deep as the Polla focus. This, as will appear more fully in the next chapter, would account for the comparatively great intensity in the immediate neighbourhood of Montemurro and for its rapid decline outwards; and it receives some support from an isolated reference by Mallet to two angles of emergence at Padula, one of 25 from the north, and the other of 8 or 10 in the perpendicular walls.
ELEMENTS OF THE WAVE-MOTION.
The elements of the wave-motion, as mentioned in the introductory chapter, are four in number, namely, the period, amplitude, maximum velocity, and maximum acceleration. If any two of these are known for each vibration--and the first two are now given by every accurately constructed seismograph--the others can be determined if the vibrations follow the law of simple harmonic motion.[15]
_Amplitude._--To ascertain the amplitude, Mallet had to rely chiefly on the fissures made in very inelastic walls. If the parts into which such a wall are fractured are free to move, and yet, being inelastic, obliged to remain in the farthest position to which they are carried by the wave, the distance traversed by the centre of gravity of one of the displaced parts should give a "rude approximate measure" of the horizontal amplitude of the earth-wave. At Certosa, near Padula, he thus found the amplitude to be about 4 inches, at Sarconi about 4-3/4 inches, and at Tramutola about 4-1/2 inches. From somewhat similar evidence, the amplitude at Polla appears to have been about 2-1/2 or 3 inches; and, from the oscillation of a suspended clock or watch on a rough wall, about 3-1/2 inches at La Sala and 1-3/4 inches at Barielle. With the exception of Barielle, these places lie nearly on a straight line pa.s.sing through Mallet"s epicentre, and he gives the following table, showing an increase in amplitude with the distance from the epicentre:--
Polla. La Sala. Certosa. Tramutola. Sarconi.
Distance in miles 4.0 13.4 19.0 23.8 30.8 Amplitude in inches 2-1/2 3-1/2 4 4-1/2 4-3/4
The existence of the Montemurro focus must, however, complicate any relation that may connect these two quant.i.ties.
_Maximum Velocity._--The means at Mallet"s disposal for determining the maximum velocity were more numerous than those available for the amplitude. From the dimensions of a fallen column of regular form we should be able, he remarks, to find an inferior limit to the value of the maximum velocity; while a superior limit at the same place may be obtained from some other regular solid which escaped being overthrown.
If a loose body is projected by the shock at a place where the angle of emergence is known, the horizontal and vertical distances traversed by the centre of gravity will give the velocity of projection. Or, if two such bodies are projected at one place, the same measures for each will as a rule give both the angle of emergence and the velocity of projection. A third method depends on the fissuring of walls, supposing that we know the force per unit surface which, when suddenly applied, is just sufficient to produce fracture. Sometimes more than one method must be applied to the same object. The two gate-pillars near Saponara (ill.u.s.trated in Fig. 6) for example required a horizontal velocity of 5.48 feet per second to fracture them, and an additional velocity of 5.14 feet per second to overthrow them.
The well-known seismologist, Professor Milne, urges very forcibly that measurements obtained from the projection or fall of columns are unreliable, for the earlier tremors might cause the columns to rock, and their overthrow need not therefore measure accurately the maximum velocity of the critical vibration.[16] There can be no doubt that Mallet was alive to this difficulty, though he may not have appreciated it at its full value. Thus, at the Certosa de St. Lorenzo, a monastery near Padula, a vase projected from the summit of a slender gate-pier implied a velocity of 21-3/4 feet per second; and the excess of about 8-1/4 feet per second above the velocity determined by other means is attributed by him to the oscillation of the pier itself. How far this source of error enters into other observations it is impossible to say; but it is worth noticing how closely the velocities obtained by different methods agree with one another. Thus, from projection only, we have velocities of 11.5 feet per second at the Certosa, 11.8 at Moliterno and Monticchio, 14.8 at Tramutola, and 9.8 feet per second at Sarconi; from overthrow alone, 11.0 feet per second at Viscolione, near Saponara, and 11.6 at Barielle; from overthrow and projection, 13.2 feet per second at Polla and 12.9 at Padula; from fracture and overthrow, 12.3 feet per second at Potenza and 15.6 at Saponara. The comparatively high values at Tramutola and Saponara, Mallet imagined might be due to the oscillation of the hills on which these towns are built. He therefore omits them in calculating the mean maximum velocity, which he finds to be twelve feet per second, a velocity less than that with which a man reaches the ground when he jumps off a table.
With the same omissions, Mallet gives the following table, showing a general decrease in the maximum velocity as the distance from his epicentre increases:--
Polla. Padula. Certosa. Moliterno. Viscolione. Sarconi.
Distance in miles 4.0 19.0 19.0 29.4 30.0 30.8
Max. vel. in ft. per sec. 13.2 12.9 11.5 11.8 11.0 9.8
On the north side of the epicentre we have:--
Potenza. Monticchio. Barielle.
Distance in miles 17.3 27.1 28.2 Max. vel. in ft. per sec. 12.3 11.8 11.6
It is not impossible that the high calculated velocities at Tramutola and Saponara were partly or entirely due to the impulse from the Montemurro focus.
If we take 4 inches for the amplitude of the largest variation, and 12 feet per second for the maximum velocity, and a.s.sume the motion to have been of a simple harmonic character, the period of a complete vibration would be less than one-fifth of a second.[17] Now, we know from seismographic records that this is roughly the period of the small tremors that form the commencement of an earthquake-shock, while the period of the largest vibrations may amount to as much as one or two seconds. We may therefore conclude either that the a.s.sumption of simple harmonic motion is incorrect, or that the maximum velocity is too great, or more probably perhaps that the amplitude is too small.[18]
SOUND-PHENOMENA.
Mallet was one of the first seismologists to realise the significance of the earthquake-sound; and he attended closely to the subject, though finding the sound even more elusive of precise observation than the shock.
The chief result obtained by him was the comparative smallness of the area over which the sound was heard. He estimates it at little more than 3,300 square miles, or about one-twelfth of that over which the shock was felt. It extends north and south from Melfi to Lagonegro, and east and west from Monte Peloso to d.u.c.h.essa and Senerchia. The sound was thus confined to the region in which the shock attained its most destructive character.
Towards the north and south ends of the sound-area all observers described the sound as a low, grating, heavy, sighing rush, lasting from twenty to sixty seconds, some adding that it was also of a rumbling nature. Near the centre and the east and west boundaries, the sound was distinctly more rumbling; it was shorter in duration, and began and ended more abruptly.
The earthquake, Mallet remarks, "began everywhere with tremors; the sounds generally arrived at the same time; the apparent direction of movement of the tremulous oscillations appeared rapidly to change, and still more rapidly to increase in amplitude; then the great _shove_ of the destructive shock arrived, in some places rather before, in some a little after, the moment of loudest sound, and it died away suddenly (_i.e._, with extreme rapidity) into tremors again, but differing in direction from that of the great shock itself."[19]
The earthquake-sound will be described more fully in the chapter dealing with the Hereford earthquake of 1896, in which it will be found that the phenomena recorded by Mallet are equally characteristic of the slighter shocks felt in this country.
VELOCITY OF THE EARTH-WAVES.
In 1857 little was known about the velocity of earthquake-waves.
Experiments had been made by Mallet himself in 1849 in the neighbourhood of Dublin. These gave 825 feet per second for the velocity in dense wet sand, 1,306 feet per second in discontinuous granite, and 1,665 feet per second in more solid granite.[20] The only earthquake for which the velocity had been calculated was the Rhenish earthquake of 1846, the value ascertained by Schmidt being 1,376 French feet, or 1,466 English feet, per second.
The accurate public measurement of time, which, as Mallet remarks, is one of the surest indications of advancing civilisation, was, however, unknown in the kingdom of Naples; and his attempt was therefore fettered by the rarity of precise estimates of the time of occurrence.
Throughout the whole disturbed area only six good records could be obtained, and three of these (at Vietri di Potenza, Atella, and Naples) were derived from stopped clocks, witnesses of rather doubtful value. At Montefermo and Barielle the time was at once read from a watch, and at Melfi from an accurate pocket chronometer. The times given vary from 9h. 59m. 16s. P.M. (Naples mean time) at Vietri di Potenza to 10h. 7m. 44s. at Naples. Allowing for the supposed change of direction by refraction at the Monte St. Angelo range on the way to Naples, Mallet finds the mean surface velocity to be 787 feet per second. Omitting the Naples record, and taking account of the calculated depth of the focus, the mean velocity becomes 804 feet per second.
MINOR SHOCKS.
A great earthquake rarely, if ever, occurs without some preparation in the form of a marked increase of seismic activity. Perrey records several shocks during the two years 1856-57 that were felt at places as far apart as Naples, Melfi, and Cosenza. On December 7th, 1857, a slight shock, with a report from beneath like the explosion of a mine, was felt at Potenza. Then came the great earthquake on December 16th, at about 10 P.M.
This was followed by numerous after-shocks--how numerous it is impossible to say, for the records are of the scantiest description.
For some hours the ground within the meizoseismal area is said to have trembled almost incessantly. At Potenza many slight shocks, both vertical and horizontal, were felt during the night, and for a month or more they were so frequent as to render enumeration difficult.
Mallet"s last record is dated March 23rd, 1858, when four slight shocks were felt at La Sala and Potenza, but occasional tremors were reported to him until May 1859.
The most important of all these after-shocks was one felt about an hour after the princ.i.p.al earthquake. Everywhere far less powerful, it was yet strong enough to shake down many buildings at Polla that had been shattered by the great shock. Towards the south at Moliterno, and towards the north at Oliveto and Barielle, it evidently attracted very little attention. So far as can be judged from the evidence given by Mallet, the disturbed area seems to have been approximately of the same form and dimensions as the meizoseismal area, and elongated in the same direction, but concentric with the north-west focus.
On the other hand, if we may rely on too brief evidence, several after-shocks recorded only at Montemurro, Saponara, Viggiano, or Lagonegro, were probably connected with the south-east or Montemurro focus.
ORIGIN OF THE EARTHQUAKE.
Mallet"s theories have suffered perhaps more than any other part of his work from the recent growth of our knowledge. From a historical point of view, some reference to his explanation of the origin of the Neapolitan earthquake seems desirable, and his own conscientious work demands it. On the other hand, his conclusions are, for the present at any rate, superseded, and it will therefore be sufficient to describe them briefly.
Most of the wave-paths, as we have seen, pa.s.s within three miles of a point almost coincident with the village of Caggiano. Of the remainder, six traverse a spot about two miles farther to the south-west, and three cross another about two miles farther to the north-east. Neglecting other points of intersection, but taking account of the observed emergences at Vietri di Potenza, Auletta, Polla, etc., Mallet infers that the horizontal section of the focus was a curve (indicated by the dotted line in Fig. 9) not less than ten miles in length, and pa.s.sing from near Balvano on the north, close to Vietri di Potenza, Caggiano, and Pertosa, to a point about two and a half miles west of Polla. Again, he remarks, the observed emergences at places near the epicentre indicate that the vertical section of the seismic focus was either more or less curved, or more probably a surface inclined towards the south-east. He concludes, therefore, that the seismic focus was a curved fissure, 10 miles long and 3-1/2 miles in height, and with its centre at a depth of 6-1/2 miles below the level of the sea.
The production of this great fissure, accompanied, perhaps by the injection into it of steam at high pressure, was regarded by Mallet as the cause of the princ.i.p.al earthquake. He imagines that the rent would start at or near the central point of the focus and then extend rapidly outwards in all directions. In the initial stage, vibrations of very small amplitude would alone be transmitted, and these would give rise to the early sounds and tremors. As the rending proceeded, the vibrations would increase in strength up to a certain point when they produced the shock itself. After this, they would decrease; and, in the final stage, would give place to the small vibrations corresponding to the sounds and tremors that marked the close of the earthquake.
The rush of steam at high pressure into the focus Mallet does not seem to have considered essential, though he evidently regarded it as possible, indeed probable; and he suggests that it may have been in part the cause of the earthquake which occurred an hour later. Though feeling sceptical as to the existence of any general law of increase of underground temperature, he a.s.sumes it, for the sake of ill.u.s.tration, to be 1 F. for every 60 feet of descent. This would give a temperature of 339 F. at the upper limit of the focus, 643 F.
at its central point, and 884 F. at its lower margin. If the focus were filled with steam at each of these temperatures, the corresponding pressures on its walls would be 8, 149, and 684 atmospheres, respectively. As the steam may be supposed to be admitted suddenly and to be unlimited in supply, Mallet infers that it might exist at the tension due to the highest of these temperatures, in which case it would be capable of lifting a column of limestone 8,550 feet in height (or about one-half the depth of the upper margin of the focus), and would exert a pressure on the walls of the focus of 4.58 tons per square inch, or of more than 640,528 millions of tons upon its whole surface.
So many pages have already been given to this interesting earthquake that I must sketch still more briefly my own view as to its origin.
There were, I believe, two distinct foci with their centres about twenty-four miles apart along a north-west and south-east line, and it was to this arrangement that the elongation of the meizoseismal area was chiefly, though not entirely, due. The evidence is insufficient to determine whether the earthquake was caused by fault-slipping; it is in no way opposed to this view, but if the Neapolitan earthquake stood alone, we should hardly be justified in drawing any further inference.