Part 2 (1/2)
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 rus.h.i.+ng 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]