Fig. 80.—Time-curves of principal epochs of earthquake-waves of distant origin. (Oldham.)[ToList]
Of the smoothed curves drawn between these series of points, those marked A, B, and C represent the time-curves of the beginnings of the first, second, and third phases respectively, while D is the time-curve for the maximum of the third phase.
The concavity of the two lower lines towards the horizontal base-line shows that the surface-velocity of the corresponding waves increases rapidly with the distance, far more so than would be possible with rectilinear motion. The rates at which these waves travel through the earth therefore increase with the depth, and the wave-paths must in consequence be curved lines convex towards the centre of the earth.
If the time-curves A and B were continued backwards to the origin, their inclinations at that point to the horizontal line give the initial velocities of the corresponding waves, which prove to be about 5 and 3 kms. per sec. respectively. Now, according to recent experiments made by Mr. H. Nagaoka on the elastic constants of rocks,[88] the mean velocity of seven archaean rocks is 5.1 kms. per sec. for the longitudinal waves, and 2.8 kms. per sec. for the transversal waves—values which agree so closely with those obtained for the first two series of earthquake-waves as to leave little doubt with regard to their character.
The other time-curves, C and D, corresponding to the initial and maximum epochs of the third phase, are practically straight lines. Some of the records are slightly discordant for the average curve, especially for the initial epoch; but it is often difficult to define the commencement of this phase with precision. At any rate, the observations show no distinct sign of an increase in the surface-velocity of these waves with the distance from the origin. It may therefore be concluded that they travel along the surface with velocities which are practically constant for each individual earthquake, the largest waves at the rate of about 2.9 kms. per sec., and the advance waves with a velocity of about 3.3 kms. per sec., rising occasionally to over 4.0 kms. per sec.
STRUCTURAL CHANGES IN THE EPICENTRAL AREA.
Changes of elevation have long been known as accompaniments of great earthquakes, though many of the earlier observations and measurements left much to be desired in accuracy and completeness. The Japanese earthquake of 1891, however, placed the reality of such movements beyond doubt, and revealed the existence of a fault-scarp, with a height in one place of 18 or 20 feet, and a length of at least 40, if not of 70, miles. In the Indian earthquake of 1897, the fault-scarps were shorter, though more pronounced in character, the largest known (the Chedrang fault) being about 12 miles long, and having a maximum throw at the surface of 35 feet. In some other recent earthquakes, also, remarkable fault-scarps have been developed. After the great shocks felt in Eastern Greece on April 20th and 27th, 1894, a fissure was traced for a distance of about 34 miles, running in an east-south-east and west-north-west direction through the epicentral district, and varying in width from an inch or two to more than three yards. That it was a fault, and not an ordinary fissure, was evident from its great length, its uniform direction, and its independence of geological structure. The throw was generally small, in no place exceeding five feet.[89] Again, in British Baluchistan, after the severe earthquake of December 20th, 1892, a fresh crack was observed in the ground running for several miles in a straight line parallel to the axis of the Khojak range. It coincided almost exactly with a line of springs, and was clearly produced by a fresh slip along an old line of fault, for before the earthquake it had the appearance of an old road, and the natives assert that the ground has always cracked along this line with every severe shock. In 1892, the change in relative height of the two sides of the fault was small, in one place where it was measured being only two inches.[90]
But other changes, besides those in a vertical direction, occasionally take place; though, owing to their recent discovery, comparatively few examples are as yet known. While the throw of the Japanese fault varied greatly in amount, and once even in direction, there was also a constant shift towards the northwest of the ground on the north-east side of the fault, the displacement at one spot being as much as 13 feet. In the fault-scarp formed in 1894 in Eastern Greece, a similar shift took place, though to what extent is unknown. There is, moreover, evidence of actual compression of the earth's crust at right angles to the fault-line. The Neo valley, traversed by the Japanese fault, was apparently narrower after the earthquake than it was before, and plots of ground were reduced from 48 to 30 feet in length—i.e., by nearly 40 per cent. In British Baluchistan, the formation of the fissure referred to above was accompanied both by compression perpendicular, and by shifting parallel, to the fault. The actual displacement in each direction is unknown, but the resultant was not less than 27 inches.