THE NEAPOLITAN EARTHQUAKE OF DECEMBER 16TH, 1857.
Half a century ago, seismology was in its infancy. On the Continent,
Alexis Perrey of Dijon was compiling his earthquake catalogues with
unfailing enthusiasm and industry. In 1846, Robert Mallet applied the
laws of wave-motion in solids, as they were then known, to the
phenomena of earthquakes; and his memoir on the Dynamics of
Earthquakes[3] may be regarded as the foundation-stone of the new
science. During the next twelve years he contributed his well-known
Reports to the British Association,[4] and prepared a series of
instructions for the observation and study of earthquake-shocks.[5]
The latter, it is worth noting, contains an outline, but hardly more
than an outline, of the methods of investigation which he developed
and employed eight years afterwards in studying the Neapolitan
earthquake.
The history of Mallet's preparation for his great work is somewhat
strange. No one else at that time possessed so full a knowledge of
earthquake phenomena. It was, however, a knowledge that had little,
if any, foundation in actual experience; for, when he was awakened by
the British earthquake of November 9th, 1852, he failed to recognise
its seismic character. Although this shock disturbed an area of about
75,000 square miles and was felt in all four parts of the kingdom, the
paucity of observations and the absence of durable records combined in
preventing the successful application of his new modes of study.[6]
Nevertheless, with confidence unshaken in their power, he awaited the
occurrence of a more violent shock, but five years had to pass before
his opportunity came towards the close of 1857.
So destructive was the Neapolitan earthquake of this year (Mallet
ranks it third among European earthquakes in extent and severity),
that nearly a week elapsed before any news of it reached the outer
world. Without further loss of time, he applied for and obtained a
grant of money from the Council of the Royal Society, and proceeded
early in the following February to what was then the kingdom of
Naples. Armed with letters of authority to different officials, he
visited the chief towns and villages in the meizoseismal area; and, in
spite of unfavourable weather and the difficulties of travelling in a
country so recently devastated, he completed his examination in little
more than two months. It was a task, surely, that would have baffled
any but the most enthusiastic investigator or one unspurred by the
feeling that he possessed the key to one of the most obscure of
Nature's problems.
Mallet's confidence in the accuracy of his methods was almost
unbounded. His great report was published four years later; but he
seems to have regarded it almost as a text-book of "observational
seismology" and the results of his Neapolitan work as mere
illustrations. His successors, however, have transposed the order of
importance, and rank his two large volumes as the model, if not the
inspirer, of many of our more recent earthquake monographs.
[Illustration: FIG. 2.--Isoseismal Lines of the Neapolitan
Earthquake of 1857. (_Mallet._)]
ISOSEISMAL LINES AND DISTURBED AREA.
The position of the meizoseismal area, to which Mallet devoted most of
his time, is indicated by the small oval area marked 1 in Fig. 2,
represented on a larger scale in Fig. 9. It is 40 miles long and 23
miles wide,[7] and contains 950 square miles. Within this area, the
loss of life was great and most of the towns were absolutely
prostrated.
The next isoseismal, No. 2, which is also shown more clearly in Fig.
9, bounds the area in which the loss of life was still great and many
persons were wounded, while large portions of the towns within it were
thrown down. Its length is 65 miles, width 47 miles, and area 2,240
square miles. The third isoseismal includes a district in which
buildings were only occasionally thrown down, though none escaped some
slight damage, and in which practically no loss of life occurred. This
curve is 103 miles long, 82 miles wide, and includes 6,615 square
miles. Lastly, the fourth isoseismal marks the boundary of the
disturbed area, which is 250 miles long, 210 miles wide, and contains
not more than 39,200 square miles; an amount that must be regarded as
strangely small, and hardly justifying Mallet's estimate of the
Neapolitan earthquake as the third among European earthquakes in
extent as well as in seventy.
DAMAGE CAUSED BY THE EARTHQUAKE.
As regards destruction to life and property, however, the Neapolitan
earthquake owns but few European rivals. Less favourable conditions
for withstanding a great shock are seldom, indeed, to be found than
those possessed by the mediæval towns and villages of the meizoseismal
area. In buildings of every class, the walls are very thick and
consist as a rule of a coarse, short-bedded, ill-laid rubble masonry,
without thorough bonding and connected by mortar of slender cohesion.
The floors are made of planks coated with a layer of concrete from six
to eight inches thick, the whole weighing from sixty to a hundred
pounds per square foot. Only a little less heavy are the roofs, which
are covered with thick tiles secured, except at the ridges, by their
own weight alone. Thus, for the most part, the walls, floors, and
roofs are extremely massive, while the connections of all to
themselves and to each other are loose and imperfect.
Again, the towns, for greater security from attacks in early times,
are generally perched upon the summits and steep flanks of hills,
especially of the lower spurs that skirt the great mountain ranges;
and the rocking of the hill-sites, in Mallet's opinion, greatly
aggravated the natural effects of the shock. The streets, moreover,
are steep and narrow, sometimes only five feet, and not often more
than fifteen feet, in width; and the houses, when shaken down, fell
against one another and upon those beneath them. As Dolomieu said of
the great earthquake in 1783, "the ground was shaken down like ashes
or sand laid upon a table."
Of the total amount of damage, not even the roughest estimate can be
made. The official returns are clearly, and no doubt purposely,
deficient, and obstacles were placed in Mallet's way when he
endeavoured to ascertain the numbers of persons killed and wounded.
Taking only the towns into account, he calculated that, out of a total
population of 207,000, the number of persons killed was 9,589, and of
wounded 1,343.[8] A few towns were marked by an excessively high
death-rate. Thus, at Montemurro, 5000 out of 7002 persons were killed
and 500 wounded; at Saponara, 2000 out of 4010 were killed; and, at
Polla, more than 2000 out of a population of less than 7000.
GENERAL OBJECTS OF INVESTIGATION.
The principal objects of Mallet's investigation were to determine the
position of the epicentre and the depth of the seismic focus. If, in
Fig. 3, F represents the seismic focus (here, for convenience,
supposed to be a point), the vertical line FE will cut the surface of
the earth in the epicentre E.[9] The dotted lines represent circles
drawn on the surface of the earth with E as centre and passing through
the places P and Q.
[Illustration: FIG. 3.--Diagram to illustrate wave-path and angle
of emergence.]
When the impulse causing the earthquake takes place at the focus, two
elastic waves spread outwards from it in all directions through the
earth's crust. The first wave which reaches a point P consists of
longitudinal vibrations, that is, the particle of rock at P moves in a
closed curve with its longer axis in the direction FP. Mallet supposes
this curve to be so elongated that it is practically a straight line
coincident in direction with FP. In the second or transversal wave,
the vibration of the particle at P takes place in a plane at right
angles to FP. These vibrations Mallet, for his main purpose, neglects.
Returning to the longitudinal wave, Mallet calls the line FP the
_wave-path_ at P. The direction EP gives the azimuth of the wave-path,
or its direction along the surface of the earth. The angle LPA, or
EPF, he defines as the _angle of emergence_ at the point P. If Q be
farther from E than P, the angle EQF is less than the angle EPF, or
the angle of emergence diminishes as the distance from the epicentre
increases. At the epicentre, the angle of emergence is a right-angle;
at a great distance from the epicentre, it is nearly zero.
Mallet argued that the direction of the wave-path FPA, or its
equivalents, the horizontal direction EPL and the angle of emergence
EPF, should be discoverable from the effects of the shock at P. The
cracks in damaged buildings, he urged, would be at right angles to the
wave-path FPA; overturned monuments or gate-pillars should fall along
the line EPL, either towards or from the epicentre according to their
conditions of support; loose or slightly attached bodies, such as the
stone balls surmounting gate-pillars, should be projected nearly in
the direction of the wave-path FPA, and their subsequent positions,
supposing the balls not to have rolled, should give the horizontal
direction EPL of the wave-path, and might, in some circumstances,
determine the angle of emergence and the velocity with which they were
projected. I shall return to details later on. For the present, it is
clear that, in the destruction wrought by the earthquake, Mallet
expected to find the materials most valuable for his purpose. Indeed,
so obvious did this mode of examination appear to him, that he could
not conceal his surprise at the blindness of his predecessors. They
seem, he says, "to have been perfectly unconscious that in the
fractured walls and overthrown objects scattered in all directions
beneath their eyes, they had the most precious data for determining
the velocities and directions of the shocks that produced them."
POSITION OF THE EPICENTRE.
_Mallet's Method of Determining the Position of the Epicentre._--In
many cases the examination of a damaged building or of an overthrown
body served more than one purpose, providing materials for
ascertaining the depth of the seismic focus as well as the position of
the epicentre. For the present, however, it will be convenient to
consider alone the method by which the latter object was to be
attained.
[Illustration: FIG. 4.--Diagram to illustrate Mallet's method of
determining position of epicentre.]
Nothing could be simpler than the principle of the method proposed.
The horizontal direction PL of the wave-path at any place P (Fig. 4),
when produced backwards, must pass through the epicentre E; and the
intersection of the directions at two places, P and Q, must therefore
give the position of the epicentre. In practice, it is of course
impossible to determine the direction with very great accuracy, and
Mallet therefore found it necessary to make several measurements in
every place, and to visit all the more important towns within and near
the meizoseismal area.
In a ruined town there are many objects from which the direction may
be ascertained, the most important of all, according to Mallet, being
fissures in walls that are fractured but not overthrown. He regarded
such fissures, indeed, as "the sheet-anchor, as respects direction of
wave-path, to the seismologist in the field," and at least three out
of every four of his determinations of the direction were made by
their means. If the buildings are detached and large, simple and
symmetrical in form, well built and not too much injured, the fissures
in the walls should, he argued, occur along lines at right angles to
the wave-path, whether that path be parallel or inclined to the
principal axis of the building. Cracks in the floors and ceilings
should also be similarly directed, and provide evidence which Mallet
regarded as only second in value to that given by the walls.
[Illustration: FIG. 5.--Plan of Cathedral Church at Potenza.
(_Mallet._)]
No building showed the different kinds of evidence on which Mallet
relied as clearly as the cathedral church at Potenza, the plan of
which is given in Fig. 5, and the vertical section along its axis in
Fig. 12. This is a modern work, nearly 200 feet long, with its axis
directed east and west. The walls are composed of fairly good rubble
masonry and brick; and the arches in the nave and transepts, the
semi-cylindrical roof and the central dome are made of brick. The
fissures represented in both diagrams were drawn to scale by the
cathedral architect before Mallet's arrival, and, as the work of an
unbiassed observer, are of special value. Most of those in the roof,
it will be seen, were transverse to the axial line of the church; but
there were others parallel to this line, one in particular running
right along the soffit of the nave and chancel. There were also
numerous small fissures in the dome, due to local structural causes
and therefore of varying direction, and a large portion of the dome
slipped westward, leaving open fissures of seven to eight inches in
width. The mean direction of the wave-path, as deduced from nine sets
of fissures, none of which differs more than four degrees from the
mean, is W. 2-1/2° S. and E. 2-1/2° N., which corresponds precisely
with the direction of throw on the displaced portion of the dome. The
great east and west fissures in the arch of the nave and chancel
Mallet attributed to a second shock, of the existence of which there
is ample evidence.
[Illustration: FIG. 6.--Fallen gate-pillars near Saponara.
(_Mallet._)]
Next to fissures, Mallet made most use of overthrown objects, such as
the two gate piers near Saponara, represented in Fig. 6. They were
made of rubble ashlar masonry, three feet square and seven feet in
height. Both were fractured clean off at the level of the ground, the
mortar being poor, and fell in directions that were accurately
parallel, indicating a wave-path towards S. 39-1/2°E. A few
observations were also made on projected stones, fissures in nearly
level ground, and the swinging of lamps and chandeliers; but their
value was small, except as corroboration of the more important
evidence afforded by fissures in the walls and roofs of buildings.
_Remarks on Mallets Method._--It would have been more difficult in
Mallet's day than it is now, to offer objections to his method of
determining the position of the epicentre. The focus, as he was well
aware, could not be a point, and, at places near the epicentre (the
very places where most of his observations were made), there must be
rapid changes of direction due to the arrival of vibrations from
different parts of the focus. He records the occurrence of the
so-called vorticose shocks at several places, though he attributes
them to another cause. Perhaps the best known example of such a shock
is that which has been so well illustrated by the late Professor
Sekiya's model of the motion of an earth-particle during the Japanese
earthquake of January 15th, 1887. The motion in this case was so
complicated that the model was, for simplicity, made in three parts,
the first of which alone is represented in Fig. 7.[10] It is clear
that in such an earthquake, Mallet's method would utterly fail in
giving definite results.
While this shock was one of great complexity, another Japanese
earthquake, that of June 20th, 1894, was unusually simple in
character. The movement at Tokio consisted of one very prominent
oscillation with a total range of 73 mm. or 2.9 inches in the
direction S. 70° W.; the vibrations which preceded and followed it
being comparatively small. Most, if not all, of the damage caused by
the earthquake must have been due to this great oscillation; and yet
the cylindrical stone-lamps so common in Japanese gardens were found
by Professor Omori to have fallen in many different directions. Taking
only those which had circular bases, twenty-nine were overthrown in
directions between north and east, sixteen between east and south,
eighty-one between south and west, and fourteen between west and
north.[11] Fig. 8 represents Professor Omori's results graphically,
the line drawn from O to any point being proportional to the number of
lamps which fell in directions between 7-1/2° on either side of the
line.
[Illustration: FIG. 7.--Model to illustrate the motion of an
earth-particle during an earthquake. (_Sekiya._)]
[Illustration: FIG. 8.--Plan of directions of fall of overturned
stone-lamps at Tokio during the earthquake of 1894.]
It will be seen from this figure that most of the stone lamps fell in
directions between west and south-west, and it is remarkable that the
mean direction of fall is S. 70° W.,[12] which is exactly the same as
that of the great oscillation. Somewhat similar results were obtained
by this able seismologist at different places affected by the great
Japanese earthquake of 1891 (Figs. 43 and 44), and the study of the
apparent directions observed during the Hereford earthquake of 1896
leads to the same conclusion.
It thus appears that an isolated observation may give a result very
different from the true direction. Indeed, if we may judge from
Professor Omori's measurements in 1894, the chance that a single
direction may be within five degrees of the mean direction is about 1
in 9. But, on the other hand, it is equally clear from these and other
observations that the mean of a large number of measurements will give
a result that agrees very closely with the true direction.
One other point may be alluded to before leaving Professor Omori's
interesting observations. It would seem, from the list that he gives,
that he exercised no selection in his measurements, but continued
measuring the direction of every fallen lamp indifferently until he
had obtained sufficient records for his purpose. Now, if the number of
fallen lamps at his disposal had been small, say 12 instead of 144,
the mean observed direction would probably have differed from the
direction given from the seismograph.[13] But, on the other hand, a
preliminary survey without any actual measurements would have revealed
at once the predominant direction of overthrow, and a fairly accurate
result might have been obtained by neglecting discordant directions
and taking the mean of those only which appeared to agree with the
mentally determined average.
This, indeed, appears to have been the course followed, more or less
unconsciously, by Mallet in his Neapolitan work. "When the observer,"
he says, "first enters upon one of those earthquake-shaken towns, he
finds himself in the midst of utter confusion. The eye is bewildered
by 'a city become an heap.' He wanders over masses of dislocated stone
and mortar, with timbers half buried, prostrate, or standing stark up
against the light, and is appalled by spectacles of desolation....
Houses seem to have been precipitated to the ground in every direction
of azimuth. There seems no governing law, nor any indication of a
prevailing direction of overturning force. It is only by first gaining
some commanding point, whence a general view over the whole field of
ruin can be had, and observing its places of greatest and least
destruction, and then by patient examination, compass in hand, of many
details of overthrow, house by house and street by street, analysing
each detail and comparing the results, as to the direction of force,
that must have produced each particular fall, with those previously
observed and compared, that we at length perceive, once for all, that
this apparent confusion is but superficial."
[Illustration: FIG. 9.--Meizoseismal area of Neapolitan
earthquake. (_Mallet._)]
_Mallet's Determination of the Epicentre._--Within the third
isoseismal line Mallet made altogether 177 measurements of the
direction of the wave-path at 78 places. These are plotted on his
great map of the earthquake; but, owing to the small scale of Fig. 9,
it is only possible to represent, by means of short lines, the mean or
most trustworthy direction at each place.[14] Producing these
directions backwards, he found that those at sixteen places passed
within five hundred yards of a point which is practically coincident
with the village of Caggiano; those at sixteen other places passed
within one geographical mile (1.153 statute miles) of this point; the
directions at sixteen more places within two and a half geographical
miles; while those at twelve places passed through points not more
than five geographical miles from Caggiano. As the direction of the
shock at places near the epicentre must have been influenced by the
mere size of the focus, this approximate coincidence is certainly
remarkable, and there can be little doubt, I think, that the
epicentre, or, at any rate, _an_ epicentre must have been situated not
far from the position assigned to it by Mallet's laborious
observations.
_Existence of Two Epicentres._--It is difficult, however, to realise
that the impulse at the focus corresponding to Mallet's epicentre was
the origin of all the destruction of life and property that occurred.
The position of the epicentre close to the north-west boundary of the
meizoseismal area, the extraordinary extension of that area towards
the south-east, and especially the great loss of life at Montemurro
and the adjoining towns, can hardly be accounted for in this manner.
Mallet himself recognised that these facts required explanation, and
he suggested that the situation and character of the towns were in
part responsible for their ruin, and the physical structure of the
country for the course of the isoseismal lines. But the comparative
escape of places much nearer Caggiano, and the wide extent of the
meizoseismal area, embracing many towns and villages of varied
character and site and many different surface-features, point
unmistakably to a different explanation.
[Illustration: FIG. 10.--Distribution of death-rate within
meizoseismal area of Neapolitan earthquake.]
One clue to the solution of the problem is afforded by the seismic
death-rate of the damaged towns. From a table given by Mallet (vol.
ii. pp. 162-163), we know the population before the earthquake of the
different communes in the province of Basilicata, and the loss of life
in each due to the shock; and from these figures we can find the
percentage of deaths at nearly every place of importance. As will be
seen from Fig. 10, it varies from seventy-one at Montemurro and fifty
at Saponara down to less than one at all the places marked to which
figures are not attached. There is thus a group of places, with its
centre near Montemurro, where the loss of life far exceeded that in
the surrounding country; and also a slightly less-marked group, with
its centre near Polla, in the north-west of the meizoseismal area;
while in the intermediate region the death-rate was invariably small.
Too much stress should not be laid upon the exact figures, for there
were no doubt local conditions that affected the death-roll. But it
seems clear that one focus was situated not far from Montemurro; while
the north-westerly group of places, combined with Mallet's
observations on the direction, point to a second focus near Polla,
about twenty-four miles to the north-west. It will be seen in a later
section that the observations on the nature of the shock also imply
the existence of a double focus.
DEPTH OF THE SEISMIC FOCUS.
_Mallet's Method of Determining the Depth of the Focus._--In
ascertaining the position of the epicentre, Mallet's work was
remarkable only for the novelty of the method employed by him; but, in
his attempt to calculate the depth of the seismic focus, he was
breaking new ground. That the depth must be comparatively small had
already been recognised, and was indeed obvious from the limited area
disturbed by nearly every earthquake. No one, however, had tried to
estimate the depth in miles; and it is impossible not to sympathise
with Mallet while he accumulated his observations with feverish
activity and subjected them to the first rough examination even if
one cannot share his confidence that he had succeeded in measuring the
depth "in miles and yards with the certainty that belongs to an
ordinary geodetic operation."
The method employed by him for the purpose is no less simple
theoretically than that used for locating the epicentre. If the
position of the latter (E) is known, one accurate measurement of the
angle of emergence EPF, at any other point P would be sufficient to
fix the depth of some point within the focus F (Fig. 11). Here, again,
Mallet relied chiefly on fissures in walls that were fractured but not
overthrown. In detail, these fissures are nearly always jagged or
serrated, for they tend to follow the lines of joints rather than
break through the solid stone, though they sometimes traverse bricks
and mortar alike. But the general course of the fissures, he urged,
would be at right angles to the wave-path, and their inclination to
the vertical should be equal to the angle of emergence.
[Illustration: FIG. 11.--Diagram to illustrate Mallet's method of
determining depth of seismic focus.]
In obtaining measurements of this angle, the buildings to be chosen
are those of large size, with few windows or other apertures, and with
walls made of brick or small short-bedded stones. The cathedral-church
at Potenza perhaps satisfies these conditions more closely than any
other structure examined by Mallet. The plan of the fissures in the
walls and roof has been given in Fig. 5, and Fig. 12 represents the
fissures In the vertical section along the axial line and looking
north, as drawn by the cathedral architect. From these fissures Mallet
calculated the mean angle of emergence at Potenza to be 23° 7'. The
distance of Potenza from Caggiano being seventeen miles, and the
height of the former being 2,580 feet, the depth of the focus
resulting from this observation alone would be 6-3/4 miles below the
level of the sea.
[Illustration: FIG. 12.--Vertical section of Cathedral Church at
Potenza. (_Mallet._)]
_Objection to Mallet's Method._--The weakest point in Mallet's method
is probably his assumption that the wave-paths are straight lines
extending outward from the focus. Even if the depth of the focus is
not more than a few miles, the waves must traverse rocks of varying
density and elasticity, and, at every bounding surface, they must
undergo refraction. If the rocks are so constituted that the velocity
of the earth-waves in them increases with the depth, then the
wave-paths must be bent continually outwards from the vertical, so
that the angle of emergence at the surface may be considerably less
than it would have been with a constant velocity throughout. In this
case, the actual depth will be greater, perhaps much greater, than the
calculated depth. For instance, if the angle of emergence at Potenza
were diminished only 5° by refraction, the calculated depth of the
focus would be too small by 1-3/4 miles.
_Mallet's Estimate of the Depth of the Focus._--Mallet measured the
angle of emergence at twenty-six places, the mean angle (_i.e._ the
mean of the greatest and least observed angles) varying from 72° at
Vietri di Potenza and 70° at Pertosa, which are about two miles from
the calculated epicentre, to 11-1/2° at Salerno, distant about 40
miles. Fig. 13 reproduces part of the diagram on which he plotted the
mean angle of emergence at different places. The horizontal line
represents the level of the sea, and the vertical line one passing
through the epicentre and focus, called by Mallet the "seismic
vertical." The lines on the left-hand side represent the commencing
wave-paths (assumed straight) to the observing stations situated to
the westward of the meridian through the epicentre, those on the
right-hand side corresponding to places to the eastward of the same
meridian. Small horizontal marks are added to indicate the depth in
miles below the level of the sea.
[Illustration: FIG. 13.--Diagram of wave-paths at seismic
vertical of Neopolitan earthquake. (_Mallet._)]
It will be seen, from this diagram, that all the wave-paths start from
the seismic vertical at depths between three and nine miles; but the
points of departure are clustered thickly within a portion, the length
of which is about 3-1/2 miles and the mean depth about 6-1/2 miles. So
great was Mallet's confidence in these calculations that he assigns
the diverging origin of the wave-paths to different points of the
focus, and thus concludes that, while the mean depth of the focus was
about 6-1/2 miles, its dimensions in a vertical direction did not
exceed 3-1/2 miles.
How far Mallet's results should be accepted as correct, it is
difficult to say in our ignorance of the constitution of the earth's
interior. There can be no doubt that the focus was of considerable
size, and that, in consequence, the wave-paths would diverge from
different points of it. But that each wave-path should actually
intersect the focus, and so enable its magnitude to be determined,
would surely involve an approach to some law connecting the direction
of a wave-path with the depth of its own origin, and no such law seems
to be ascertainable. Nor can the limitation of these apparent origins
between certain depths be held to argue that the focus, or any part of
it, was equally confined, for the wave-paths would to a great extent
be similarly refracted. I fear that the only conclusions that we can
with safety draw from Mallet's admirable work are that his figures
indicate the order of magnitude both of the vertical dimensions and of
the mean depth of the focus.
NATURE OF THE SHOCK.
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 pronounced. 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 passes 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 passing 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 quantities.
_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 (illustrated 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 assume 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 assumption 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 Duchessa 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 principal 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, pass 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 passing 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 principal 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 assumes it, for the sake of
illustration, 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.
Relying, however, on knowledge obtained from the study of more recent
shocks, it seems to me probable that the two foci formed parts of one
fault with a general north-west and south-east direction. The slip
causing the first part of the double shock apparently took place
within the south-east focus, and was followed after a few seconds by
one within the north-west focus, greater in amount as well as more
deeply seated. In consequence of these displacements there were local
increases of stress, causing numerous small slips within or near both
principal foci; and, if we may judge from some slight shocks felt at
La Sala, accompanied also by other minor slips in the intermediate
region of the fault.
REFERENCE.
MALLET, R.--_The Great Neapolitan Earthquake of 1857: The First
Principles of Observational Seismology_, etc. 2 vols 1862.
FOOTNOTES:
[3] _Irish Acad. Trans._, vol. xxi., 1848, pp. 51-105 (read Feb. 9,
1846).
[4] _Brit. Assoc. Reports_, 1850, pp. 1-87; 1851, pp. 272-330; 1852,
pp. 1-176; 1853, pp. 117-212; 1854, pp. 1-326; 1858, pp. 1-136.
[5] _A Manual of Scientific Enquiry_, edited by Sir J.F.W. Herschel,
1849, pp. 196-223.
[6] _Irish Acad. Trans._, vol. xxii., 1855, pp. 397-410.
[7] The linear dimensions of the isoseismal lines are obtained by
measurements from Mallet's maps. The areas are given by him in
geographical square miles.
[8] Mallet, by some accident, omitted the losses at Polla and
neighbouring towns from this estimate. Mercalli (_Geologia d'Italia_,
pte. 3, p. 324) gives the number of killed as more than 12,300.
[9] Mallet does not make use of the term _epicentre_; he speaks of the
line FE as the _seismic vertical_. The modern and accepted terms are
used above
[10] _Japan Seismol. Soc. Trans._, vol. xi., 1887, pp. 175-177.
[11] _Ital. Seismol. Soc. Boll._, vol. ii., 1896, pp. 180-188.
[12] Professor Omori gives the mean direction as S. 71° W., but this
was obtained from observation on lamps with square, as well as with
circular bases.
[13] Twelve measurements chosen at random from Professor Omori's list
gave a mean direction of S. 78° W.
[14] When the accuracy of all the observations seemed equally
probable, he adopted the mean of the two extremes as the true
direction.
[15] If _a_ be the amplitude of a simple harmonic vibration, _T_ its
complete period, _v_ its maximum velocity, and _f_ its maximum
acceleration, we have v=2*pi*a/T and f=4*pi^2*a/T^2
[16] _Earthquakes and other Earth Movements_, pp. 81-82.
[17] Obtained from the formula: T=2*pi*a/v=2*pi*x*(1/3)/12
[18] If we take the maximum velocity to be 12 feet per second, and the
period to be one second, the amplitude would be about 11-1/2 inches.
[19] Vol. ii., p. 299. The punctuation of the original is not followed
in the above extract.
[20] _British Association Report_, 1851, pp. 272-320.