Though most materially assisted by the previous labours and partial
catalogues of Von Hoff, Cotte, Hoffman, Merrian, and, above all, of
Perrey, the preparation of this catalogue--which demanded visits to the
chief libraries of Europe, and the collating of some thousands of
authors in various languages and of all time--was a work of great and
sustained labour, which, except for my dear son's help, I should never
have found time and power to complete. Professor Perrey, formerly of the
Faculté des Sciences of Dijon, now _en retrait_, who has devoted a long
and useful life to assiduous labours in connection with Seismology, was
our great ally; and his catalogues are so large and complete for most
known parts of the world after 1842, that we were able to arrest our own
catalogue at that date, and take M. Perrey's as their continuation up to
1850.
The whole British Association Catalogue thus embraces the long historic
period of from 1606 B.C. of vulgar chronology, when the first known
Earthquake is recorded, to A.D. 1850; and the base of induction which it
presents as to the facts recorded extends to between 6,000 and 7,000
separate Earthquakes. My Fourth Report ("Reports, British Association,
1858,") is occupied principally with the discussion of this great
catalogue, and with that of several special catalogues produced by other
authors with limited areas or objects.
The discussion of M. Perrey's local catalogues with those of others, in
reference to a supposed prevalent apparent horizontal direction of shock
in certain regions--as to distribution, as to season, months, time of
day or night, relation to state of tide--the bearings of the views of
Zantedeschi and others as to the probable existence of a terrane
tide--the supposed relations of the occurrence of Earthquakes upon the
age of the moon, as deduced by Perrey, viz.: that 1st, Earthquakes occur
most frequently at the syzygies; 2nd, that their frequency increases at
the perigee and diminishes at the apogee; 3rd, that they are more
frequent when the moon is on the meridian than when she is 90° away from
it--and the views of several authorities as to the distribution of
Earthquakes in time and in space--occupy the first 46 pages of this
Report.
It then proceeds to discuss the distribution in time and in space as
deduced from the full base of the great catalogue.
The results as to time are reduced to curves, and those as to space (or
distribution over our globe's surface) to the great seismic map
(Mercator's projection), upon which and in accordance with certain
principles and conventional laws, which admit of the indication of both
intensity and frequency, all recorded Earthquakes have been so laid down
as to present a real indication of the distribution of seismic energy
for the whole historic period and all over the world.
The original of this map, which also shows the Volcano (size, about 7
feet by 5 feet), remains for reference in the custody of the Royal
Society. A reduced copy was published with the Report, and to a still
more reduced scale has been reproduced in other places. It is impossible
here to do more than refer to a few of the more salient points.
As regards distribution in time, durational seismic energy may be
considered as probably constant during historic time, though it is
probably a decaying energy viewed in reference to much longer periods.
It does not appear of the nature of a distinctly periodic force.
1. Whilst the minimum paroxysmal interval may be a year or two,
the average interval is from five to ten years of comparative
repose.
2. The shorter intervals are in connection with periods of
fewer Earthquakes, not always with those of least intensity,
but usually so.
3. The alternations of paroxysm and of repose appear to follow
no absolute law deducible from these causes.
4. Two marked periods of extreme paroxysm are observable in
each century (for the last three centuries), one greater than
the other--that of greatest number and intensity occurring
about the middle of each century, and the other towards the end
of each.
As respects season, there appear distinct indications of a maximum about
the winter solstice, and equally so of a minimum rather before the
autumnal equinox. It is not improbable that there is a remote relation
between Earthquakes and the annual march of barometric pressure.
We may expect, at present, one great Earthquake about every eight
months, and were we possessed of a sufficient report from all parts of
our globe, we should probably find scarcely a day pass without a very
sensible Earthquake occurring somewhere, whilst, as regards still
smaller tremors, it might almost be said that our globe, as a whole, is
scarcely ever free from them.
As respects the distribution of seismic energy in space of our earth's
surface, it is that of bands of variable and of great breadth, with
sensible seismic influence extending to from 5° to 15° transversely,
which very generally follow:
1. The lines of elevated tracts which mark and divide the great
oceanic or terra-oceanic basins (or _saucers_, as I have called
them, from their shallowness in relation to surface, in this
discussion) of the earth's surface.
2. And in so far as these are frequently the lines of mountain
chains, and these latter those of volcanic vents, so the
seismic bands are found to follow these likewise. Isolated
Volcanoes are found in these bands also.
3. While sensible seismic influence is generally limited to
the average width of the band, paroxysmal efforts are
occasionally propagated to great distances transversely beyond
that.
4. The sensible width of the band depends upon the energy
developed at each point of the length, and upon the accidental
geologic and topographic conditions along the same.
5. Seismic energy _may_ become sensible at any point of the
earth's surface, its efforts being, however, greater and more
frequent as the great lines of elevation and of volcanic
activity are approached; yet not in the inverse ratio of
distance, for many of the most frequently and terribly shaken
regions of the earth, as the east shore of the Adriatic, Syria,
Asia Minor, Northern India, etc., are at great distances from
active Volcanoes.
6. The surfaces of minimum or of no known disturbance are the
central areas of great oceanic or of terra-oceanic basins or
saucers, and the greater islands existing in shallow seas.
Space obliges me to pass unnoticed here many minor but not unimportant
deductions. The discussions as to distribution in time and space occupy
seventy-two pages of this fourth and last Report, the remainder of which
(thirty-one pages) embraces the description and mathematical discussion
as to seismometers, to which I may refer, as comprising the most
complete account of these instruments that has, I believe, been anywhere
given.
The appendix to the Report comprises the entire bibliography of
Earthquakes collected during those researches, and a concluding chapter
on desiderata, and inquiries as to ill-understood phenomena supposed to
be connected with Earthquakes.
* * * * *
In 1849-50, I was honoured by the request to draw up the article
"Earthquake Phenomena," which has appeared in the first and subsequent
editions of the "Admiralty Manual of Scientific Inquiry." Originally the
subject was intended to have formed part of the article on Geology,
entrusted to Mr. Darwin, who consulted me upon the subject; and upon my
representing how much Earthquakes had, within a short time, become
matter for the mathematician and physicist, he, with a singleness of eye
to science which it is but just to place on record, took the necessary
steps with the Admiralty authorities that Earthquakes should form a
separate article, and advised its being placed, as it was, in my hands.
To record this will, I believe, be sufficient justification for my
reference to this article, in which a good deal of information as to
Seismometry is to be found.
* * * * *
By recurring to Mr. Hopkins's Report on Earthquake Theory, before
remarked upon ("Report of British Association, 1847"), it will be seen
that the solutions of the problems which he there gives for finding the
depth of focus of shock are founded upon the _velocity of propagation_
of the wave in the interior of the mass, the _apparent horizontal
velocity_ and the _horizontal direction of propagation_ at any proposed
point being known (p. 82).
By this it appears plainly that at that time Mr. Hopkins supposed that
it was the _velocity of translation_ of the wave of shock that did the
mischief, and not the _velocity of the wave particle_, or wave itself.
And, further, that the former might be obtained by reference simply to
the modulus of elasticity of the rock of any given formation, as,
indeed, was my own earliest view when I produced my "Dynamics of
Earthquake" in 1846. From the remarks already made as to the vast
difference between the actual transit velocity in more or less
discontinuous rocks--such as they occur in Nature--it will be equally
obvious that Mr. Hopkins's methods, as above mentioned, are
impracticable, even were there no confusion between the velocity of
translation of the wave and that of the wave particle or wave itself.
This applies also to the demonstration and diagram (taken from Hopkins)
given by Professor Phillips ("Vesuvius," pp. 258-259).
In December, 1857, occurred the great Neapolitan Earthquake, which
desolated a large portion of that kingdom; and an opportunity then arose
for practically applying to the problems of finding the directions of
earthquake shock at a given point through which it has passed, and
ultimately the position and depth of focus, other methods, which I had
seen, from soon after the date of publication of my original Paper
(1846), were easily practicable, and the details of which I had
gradually matured.
Bearing in mind that, in the case of the normal vibration in any elastic
solid of indefinite dimensions, the direction of motion in space of the
_wave particle_ coincides in the first semiphase of the wave, and at the
instant of its _maximum velocity_ with the right line joining the
particle and the focus or centre of disturbance, it follows that, in the
case of earthquakes, the normal vibration of the wave of shock is always
in a vertical plane passing through the focus and any point on the
earth's surface through which the shock passes (assuming for the present
no disturbing causes after the impulse has been given), and that at such
a point the movement of the wave particle in the first semiphase of the
wave is in the same direction or sense as that of translation; and at
the moment of maximum velocity the direction in space of the motion of
the wave particle is that of the right line joining the point through
which the wave has passed with the focus or centre of impulse.
If, therefore, we can determine the direction of motion of the wave
particle in the first semiphase, and its maximum velocity, we can
obtain, from any selected point, a line (that of emergence of the shock)
_somewhere in which_, if prolonged beneath the earth, the focus must
have been; and if we can obtain like results for two or more selected
points, we decide the position and the depth of the focus, which must be
in the intersection of the several lines of direction of the wave
particle motion at each point, when prolonged downwards.
Now, as I have said, it is the _vibration of the wave itself_, _i.e._,
the motion of the wave particle that does the mischief--_not_ the
transit of the wave from place to place on the surface; just as in the
analogous (but _not_ similar) case of a tidal wave of translation
running up an estuary and passing a ship anchored there, it is not the
transit up the channel, but the wave form itself--_i.e._, the motion of
the wave particles--that lifts the ship, sends her a little way higher
up channel, drops her to her former level, and sends her down channel
again to the spot she lay in just before the arrival of the wave.
Everything, therefore, that has been permanently disturbed by an
earthquake shock has been thus moved in the direction and with the
maximum velocity impressed upon it by the wave particle in the first
semiphase of the wave; and thus almost everything that has been so
disturbed may, by the application of established dynamical principles,
be made to give us more or less information as to the velocity of the
wave particle (or as we, for shortness, say, the velocity of shock), the
direction of its normal vibration, and the position and depth beneath
the earth's surface, from which came the generating impulse. We thus
arrive at these as simply and as surely as we can infer from the
position taken by a billiard ball, on which certain forces are known to
have acted, the forces themselves and their direction; or, from a broken
beam, the pressure or the blow which fractured it.
It is obvious, then, that nearly every object disturbed, dislocated,
fractured or overthrown by an earthquake shock is a sort of natural
seismometer, and the best and surest of all seismometers, if we only
make a judicious choice of the objects which being found after such a
shock, we shall employ for our purpose. This was the principle which I
proposed to the Royal Society at once to apply to the effects of the
then quite recent great Neapolitan Earthquake of 1857, and which,
through the liberality and aid of that body, I was enabled to employ
with the result I had pretty confidently anticipated, namely, the
ascertainment of the approximate depth of the focus.
_Every_ shock-disturbed object in an earthquake-shaken country is
capable of giving _some_ information as to the shock that acted upon it;
but it needs a careful choice, and some mechanical [Greek: nous], to
select _proper_ and the best objects, so as to avoid the needless
perplexity of disturbing forces _not_ proper to the shock, or other
complications.
When properly chosen, these natural seismometers, or evidences fitted
for observation after the shock, are of two great classes, by which the
conditions of the earthquake motion are discoverable:
1. Fractures or dislocations (chiefly in the masonry of
buildings), which afford two principal sources and sorts of
information, namely:
_a._ From the observed _directions of fractures or fissures_,
by which the _wave path_, and frequently the _angle of
emergence_, may be immediately inferred.
_b._ Information from the preceding, united with known
conditions as to the strength of materials to resist
_fracture_, by which the _velocity_ of the fracturing impulse
may be calculated.
2. The overthrow or the projection, or both, of bodies large or
small, simple or complex. From these we are enabled to infer:
_c._ By direct observation, the _direction in azimuth_ of the
wave path.
_d._ By measurements of the horizontal and vertical distances
of overthrow or of projection, to infer either the _velocity_
of projection, or _angle of emergence_.
Fractures by shock present their planes always nearly in directions
transverse to the wave path. Projections or overthrow take place (unless
secondarily disturbed) in the line of the wave path, or in the vertical
plane passing through it: but the direction of fall or overthrow may be
either in the same direction as the wave transit (_i.e._, as the motion
of the wave particle in the first semiphase), or contrary to it.
It is thus obvious that the principal phenomena presented by the effects
of earthquake shock upon the objects usually occurring upon the surface
of the inhabited parts of the earth, resolve themselves into problems of
three orders, and are all amenable to mechanical treatment, viz.:
1. Problems relating to the direction and amount of velocity
producing fracture or fissures.
2. Problems relating to the single or multiplied oscillations
of bodies, considered as compound pendulums.