pendulum clocks stopped; a few factory chimneys of very bad construction damaged.

"No. 2. Maximum acceleration = 900 mm. per sec. per sec. Walls in Japanese houses are cracked; old wooden houses thrown slightly out of the vertical; tombstones and stone-lanterns of bad construction overturned, etc. In a few cases changes are produced in hot springs and mineral waters. Ordinary factory chimneys are not damaged.

"No. 3. Maximum acceleration = 1200 mm. per sec. per sec. About one factory chimney in every four is damaged; brick houses of bad construction partially or totally destroyed; a few old wooden dwelling-houses and warehouses totally destroyed; wooden bridges slightly damaged; some tombstones and stonelanterns overturned; shoji (Japanese paper-covered sliding doors) broken; roof tiles of wooden houses disturbed; some rock fragments thrown down from mountain sides.

"No. 4. Maximum acceleration = 2000 mm. per sec. per sec. All factory chimneys are broken; most of the ordinary brick buildings partially or totally destroyed; some wooden houses. totally destroyed; wooden sliding doors and shoji mostly thrown out of the grooves; cracks two or three inches in width produced in low and soft grounds; embankments slightly damaged here and there; wooden bridges partially destroyed; ordinary stone lanterns overturned.

"No. 5. Maximum acceleration = 2500 mm. per sec. per sec. All ordinary brick houses are very severely damaged; about three per cent. of the wooden houses totally destroyed; a few tera, or Buddhist temples, thrown down; embankments severely damaged; railway lines slightly curved or contorted; ordinary tombstones overturned; ishigaki, or masonry walls, damaged here and there; cracks one or two feet in width produced along river-banks; waters in rivers and ditches thrown over the banks; wells mostly affected with changes in their waters; landslips produced.

"No. 6. Maximum acceleration =4000 mm. per sec. per sec. Most of the tera, or Buddhist temples, are thrown down; fifty to eighty per cent. of the wooden houses totally destroyed; embankments shattered almost to pieces; roads made through paddy

fields so much cracked and depressed as to stop the passage of wagons and horses; railway lines very much contorted; large iron bridges destroyed; wooden bridges partially or totally destroyed; tombstones of stable construction overturned; cracks a few feet in width formed in the ground, accompanied sometimes by the ejection of water and sand; earthenware buried in the ground mostly broken; low grounds, such as paddy-fields, very greatly convulsed, both horizontally and vertically; sometimes causing trees and vegetables to die; numerous landslips produced. "No. 7. Maximum acceleration much above 4000 mm. per sec. per sec. All buildings except a very few wooden houses are totally destroyed; some houses, gates, etc., projected one to three feet; remarkable landslips produced, accompanied by faults and shears of the ground."

Professor Omori then compares the foregoing absolute scale with the Rossi-Forel as follows:

CHAPTER X

VARIATION OF SURFACE INTENSITY

General Law of Decrease of Intensity away from the Epicentrum-Methods of Locating the Epicentrum-Seebach's Proposition-Determination of Points of Maximum Decline of Intensity-Method of Computing Depth of Focus-Intensity Curves as Dependent upon Focal Depth-Earthquakes with Deep Foci Compared with those of Shallow Foci-Maximum Focal Depths-Probably Never Greater than Fifteen Miles

HE grouping of intensity observations and their assemblage in the form of isoseismals furnishes us with the means of making important comparisons of earthquakes with each other. It furnishes us a scale of proportions by which we can judge, even though roughly, still with a fair and useful degree of approximation, the relative values of several important factors. The most striking of these are the depth of the origin, or ideal centrum, and the total energy of the earthquake.

The equation of the intensity curve already given contains the means of computing the depth of origin in absolute terms, provided certain facts in the distribution of intensity can be identified on the ground after an earthquake. Two points are required for this computation. The first is the epicentrum. The position of this point is obviously the centre of the isoseismal curves, unless local accidents of the ground lead to manifestations of intensity which are not

proportional to the inverse square of the distance from the origin. In case of such accidents our inferences as to the epicentrum would be more or less vitiated, and we should have no means of correcting our error. That this anomaly sometimes occurs is unquestionable. But ordinarily it is not of sufficient magnitude to lead to any large error in determining the point. The cases where it is liable to be most misleading are those quakes where the intensity at the epicentrum is small and the tremors extend over a large region. Where the intensity is high, and the disturbed district is not of extraordinarily great extent, there is seldom any difficulty in determining the epicentrum with a good degree of precision.

In those quakes which are generated by long dislocations or faults, or in those which result, like the New Madrid quakes, from the sudden sinking of a large tract of ground, the idea of an epicentrum is hardly applicable. There is in such cases no centrum, properly speaking. The causal action is as much superficial as subterranean, and is so widely diffused, and its distribution is so irregular and formless that the notion of a central, and, therefore, epicentral point, becomes incongruous. On the other hand, this diffused and irregularly distributed origin may approximate more and more to the relation of a centrum and point of origin as the distance increases. But at present we are concerned with the near neighbourhood of the origin and must exclude from our analysis those quakes whose origins are not of the compact and well-centralised kind.

Several methods have been employed by various investigators for locating the epicentrum. Among them is that of

Seebach, who uses the coseismals, or lines drawn through the point at which the first impulse, or perhaps the chief impulse, is felt at the same instant of time. According to the theory these should, under perfect conditions of propagation, form circles around the epicentrum, as in the case of the isoseismals. By determining three points of a co

a

FIG. 53.

Seebach's Method of Estimating Depth of Centrum by Means of
Coseismals.

seismal curve the epicentrum follows at once. It is found at the intersection of the perpendiculars to two chords given by the three coseismal points, Fig. 53. The merits of this method are proportional to the accuracy with which the coseismal points are determined. These are time observations, liable not only to errors in noting the time, but also to much uncertainty as to what particular shock or oscilla