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An examination of the different seismograms, Pls. VII and VIII will indicate that the third-phase pulsations of large amplitude reach a maximum, then rapidly decline, and some minutes later are repeated on a diminished scale of amplitude. There may be several repetitions, each succeeding one being less pronounced than its predecessor. Professor Milne continues:

"The main point at issue, and the one to be answered before we enter into further speculations, is whether seismograms showing this musical-like repetition can be interpreted in the manner here suggested. The concluding vibrations of an earthquake have usually been regarded as a disorderly mob of pulsatory movements resulting from spasmodic impulses which gradually grow feebler as the activity at a seismic centre became exhausted. The question before us is whether an earthquake dies by a process analogous to repeated and irregular settlements of disjointed materials, or whether it is a blow or blows which come to an end with musical reverberations inside the world. For the present my opinion inclines to the latter, and I see in the earthquake followers the likeness of their parent."

CHAPTER XIV

SEISMIC DISTRIBUTION AND CHARTING

The Catalogues of De Montessus de Ballore-His Conception of SeismicityFrequency and Average Intensity-The Three Categories of Data: Historic, Seismologic, and Seismographic—Difficulties in Handling Frequency Data-Difficulties of Intensity Averages-De Montessus's Method of Avoiding them-His Numerical Measure of Seismicity— Subdivision of his Catalogue - Projecting Seismicity upon MapsMallet's Method - Locations by Epicentres-Dr. Davison's Method -His Geography of Japanese Quakes, and the Distribution of Milne's Catalogue-Tendency to Great Detail in Charting Seismic Distribution -Seismic Topography

THE

HE widely varying degrees in which different portions of the earth are affected by seismic action have doubtless attracted general attention. All that is called for here is to give a sufficiently detailed statement of the extent to which seismic action varies and its geographic distribution.

This branch of the subject has received far more attention and study from M. De Montessus de Ballore than from any one else—much more, indeed, than from all other seismologists together. His work has been chiefly the compilation and treatment of very large masses of data consisting of catalogues of earthquakes in many parts of the world. They include also all authentic reports, from whatsoever source, which are sufficiently detailed to be of any use. His inquiry into the validity of Perrey's laws relating to the effect

of the moon's attraction upon seismic frequency has already been referred to. In the present chapter an account will be given of his investigations of the subject of "Seismic Geography," as he terms it, which is a name for the geographic distribution of seismic frequency and intensity. The words "frequency" and "intensity" obviously imply some kind of numerical measure, and to this measure he gives the name seismicity.

Every part of the earth's surface which is in any degree, however small, affected by earthquakes may be said to have its own seismicity, just as every region, unless absolutely rainless, has its degree of annual precipitation which is capable of a numerical statement in terms of cubic inches of rainwater falling annually upon each square inch of surface. We may speak of the seismicity of a region just as we speak of its vulcanicity, meaning in either case the degree to which it is affected by seismic or volcanic action. Scientific accuracy, however, demands a full understanding of just what quantity or quantities the term seismicity is intended to include and in what relations.

It is clear that it should include the frequency, i. e., the number per day, week, or year, and the average intensity of the quakes or "seisms" as De Montessus terms them. It is equally plain that there must be a very definite understanding as to the area over which the frequency is to be distributed. With a full knowledge and definite understanding of these three quantities, frequency, intensity, and area, it becomes possible to establish a numerical measure of the seismicity of a district.

But it is not so easy a matter as it might at first seem to

determine satisfactorily either the frequency or the average intensity. It is obvious that the first requirement for determining frequency is a knowledge of the number of quakes occurring in a given period of time. For some districts, the number of which has been steadily increasing from decade to decade during the last thirty years, we have records of varying degrees of fulness, though it is probable that not more than a dozen localities have complete records including all of the quakes. For the greater part of the world the record is more or less fragmentary, and for a very considerable portion there is practically none at all, or next to none.

De Montessus divides the sources of information into three classes: (1) Historic, embracing publications of all kinds other than strictly seismologic. These are histories, narratives, newspapers, etc., whose common feature in this relation is that their accounts are incidental only and not for scientific or statistical purposes. It is quite certain that they mention only a small fraction of the entire number of quakes in any region. (2) Seismologic observations, which are made for the purpose of recording all the sensible quakes of a region, i. e., quakes which are perceptible to the human senses without the help of instruments. They are made by individuals, or by associations, for the purpose of promoting knowledge of seismology and for making it as full and accurate as practicable. (3) Seismographic observations, which are made in observatories, public or private, equipped with delicate instruments, which record all vibrations, great and small.

It might seem as if the seismographic class of records should be much more satisfactory than the others whenever

they are obtainable. De Montessus thinks otherwise. Seismographs record not only true earthquakes but pseudoquakes also, i. e., tremors produced by agencies not of a seismic nature at all, such as the passage of railway trains or carriages or the effect of wind on neighbouring buildings or even upon hills, the beating of the surf, etc., and it is sometimes a difficult matter to distinguish such records. from those of true earthquakes of low intensity. The seismograph also records the dying tremors of quakes originating far outside the district under consideration, and it is equally difficult to separate them from tremors originating within the district. Seismographs are usually installed within, or near, large towns or cities, and for various reasons fail sometimes to pick up the faint tremors caused by small quakes far out in the country, though still within the district, thus giving undue frequency to urban, at the expense of rural, localities. For these and other reasons he regards seismographic records as less desirable than the seismologic as sources for the data upon which frequency estimates should rest. Their errors are always in excess. They give records which should be rejected, but which are difficult to distinguish from those which should be retained.

Seismologic records, he fully recognises, are always defective in the opposite direction, as they fail to note a large number of small quakes. This deficiency, however, he thinks, can sometimes be supplied, and his method of doing so is certainly an ingenious one. In his vast catalogue of earthquakes he finds a considerable number of districts, ninety-four in all, which have at least two of the three kinds of records. These enable him to establish ratios between

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