Great Neapolitan Earthquake of 1857/Part I. Ch. I
PART I.
CHAPTER I.
OF THE QUESTIONS FOR INQUIRY, AND METHODS OF OBSERVATION, OF EARTHQUAKE PHENOMENA.
Were Seismology an older and more mature branch of science than it is, it would be impertinent to enter at any length into the means and methods by which it is to be pursued, in the observation of earthquake phenomena. Dating, however, for anything approaching to scientific guidance or precision, not more than twelve years back,[1] it is the more necessary to make generally intelligible the methods of observation which can be pursued in a seismic region after the occurrence of the shock, in order that the evidence upon which conclusions may be drawn as to the direction, velocity, amount of movement, &c., of the latter may be accepted with their just weight; and the rather, because as yet it is not to all persons quite self-evident how any information whatever can be had or conclusions drawn as to a phenomenon so perfectly transient and momentary as an earthquake shock, by examination, at a considerable time after its occurrence, of the region over which it has passed.[2]
An earthquake, like every other operation of natural forces, must be investigated by means of its phenomena or effects. Some of these are transient and momentary, and leave no trace after the shock, and such must either be observed at the time, or had from testimony. But others are more or less permanent, and, from the terrible handwriting of overturned towns and buildings, may be deciphered, more or less clearly, the conditions under which the forces that overthrew them acted, the velocity with which the ground beneath was moved, the extent of its oscillations, and ultimately the point, can be found in position and depth beneath the earth's surface, from which the original blow was delivered, which, propagated through the elastic materials of the mass above and around, constituted the shock.
Again, certain effects, such as landslips, fissures, alterations of water-courses, &c., are produced of greater or less permanency affecting the natural features of the shaken country.
The observation of each of these classes of effects bears reference to two distinct orders of seismic inquiry.
By the first, we seek to obtain information as to the depth beneath the surface of our earth at which those forces (whether volcanic or otherwise) are in action, whose throbbings are made known to us by the earthquake, and thus to make one great and reliable step towards a knowledge of the nature of these forces themselves; and this is the great and hopeful aspect in which seismology must be viewed and chiefly valued. It affords, if not the only, certainly, in the existing state of knowledge, the best means by which we can entertain a well-founded expectation of ultimately obtaining clear and certain ideas as to the material and state of the internal mass of our planet, and comprehending the true nature and relations of volcanic energy.
By the second order of inquiry we seek to determine the modifying and moulding power of earthquake upon the surface of our world as we now find it; to trace its effects and estimate their power and extent upon man's habitation and upon himself. The first order of inquiry must be pursued by methods, chiefly mechanical, physical, or mathematical. The second by these, combined with the observational tact and largeness of a disciplined imagination and eye that are amongst the accomplishments of the physical field-geologist. Thus finally uniting our knowledge derived from both directions, ultimately to form a clear conception of what is the function of the earthquake in the Cosmos, and to recognize the connection, fitness, order, and beauty, even of the volcano and the earthquake, as parts of the machinery of a wondrous and perfect creation. Like every aspect of nature, that we obtain with the more enlarged and undimmed eye of truth, it will prove to us that even here the great Author of all, is a God of order, not of confusion.
These, then, were the questions that I set before me. To endeavour to find the position, superficially and in depth of the centre of impulse of the shocks of the 16th December 1857, and to observe and discuss the effects of the earthquake, actual and prospective, upon the face of the country, in relation to all its physical conditions.
The method of investigation which I purposed to adopt is based upon the very obvious truth, that the disturbances and dislocations of various solid objects by the shock of earthquake, if carefully observed with reference to their directions and extent of disturbance, and to the mechanical conditions in play, must afford the means of tracing back from these effects, the directions, velocities, and other circumstances of the movements or forces that caused them. This mode of examination, strange to say, appears to be perfectly new, and to have escaped the attention of all previous examiners of earthquake-shaken districts, as well as of all writers upon the subject. Thus the government reporters (for example) upon the great Calabrian earthquake of 1783, or those more recency (Palmieri and Scacchi) upon that of Basilicata in 1851, seem 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.
The idea of applying number and measure to these never seems to have occurred to them. They merely describe the particulars in a loose and general way, and only occasionally as curious, or remarkable, or inexplicable exemplars of the power of the disturbance. Hence they failed to draw a single conclusion of certainty or scientific value as to the place whence the shock emanated, how deep this was under the earth, or in what direction it emerged from beneath it.
As this method of seismic observation then, is novel, as I trust to show that it has proved fruitful in result, in this its very first application to nature; and as, when fully developed, it will be found a real "Organon," a powerful machine for future discovery, I make no scruple in treating at considerable length of its methods, in the hope that they may become understood, diffused, and applied by others. This method, I believe, will be hereafter recognized as one of the most fruitful applications yet made of mathematics to physical geology.
Before leaving England for Naples, I communicated my views as to this method of investigation to my friend the Rev. Samuel Haughton, F.R.S., Professor of Geology, Trinity College, Dublin, and requested him to arrange for me a series of workable equations that should embrace most of the conditions as to direction and velocity of fractured or overthrown bodies that I expected to meet. With the utmost readiness, he applied his adroit and eminently practical mathematical powers to the task, and from him I received the equations given at pp.&160;125 et seq.—I. to XLV.—which formed some of the most valued working tools of my deductions.
- ↑ This was written in 1858.
- ↑ With the exception of the author's 'Instructions' in the 'Admiralty Manual,' nothing whatever has been written bearing upon such methods of observation. Mr. Hopkins, in his mathematical résume of the laws of elasticity, as bearing upon seismology, communicated to the British Association (17th Report, Oxford, 1847), incidentally points out the geometric conditions by which, if the emergence of a shock were known, the depth of the origin might be ascertained. The procedure generally had been previously suggested by the author in his original memoir on 'Dymamics of Earthquakes.' Trans. Roy. Irish Acad., vol. xxi., Part 1. The methods employed in this work are altogether distinct from that noticed by Mr. Hopkins.