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NATURE 



[July 28, 1887 



Charleston earthquake was relatively great, and we find 

 reason for believing that, among those great earthquakes 

 of the last 150 years of whose effects we possess any 

 considerable knowledge, none have originated from a 

 much greater depth, and few from a depth so great. Our 

 reasoning is this : — Very few earthquakes have been felt 

 at a distance from the origin so great as 1000 miles. 

 But the greitest distance at which the tremors are felt is 

 the best measure of the total energy of the shock. On 

 the other hand, the intensity of the Charleston earthquake 

 in the epicentral tract was relatively low in comparison 

 with other great earthquakes. If, then, any shock is 

 more intense at the epicentre, without extending to a 

 greater distance than that of the Charleston earthquake, 

 it is certain that its focus was nearer the surface. This 

 is true of the vast majority of recent earthquakes which 

 have been sufficiently investigated. It is suggested that 

 all estimates of the depth of foci much exceeding that of 

 the Charleston earthquake are in need of re-examination. 

 The city of Charleston is situated from eight to ten 

 miles outside of the area of maximum intensity, and did 

 not experience its most destructive power. Following 

 the law which we have laid down, the intensity of the 

 shock at Charleston was only three-tenths what it must 

 have been at the epicentrum and about one-third the 

 intensity at Summerville. -The diagram showing the 

 long intensity curve stretching from Charleston to a point 



tO:U)1817JGJSMJ3JlItlO 3876 S^SSl 

 N.W. 



Fig. 5. — Intensity curve of maximum shock twenty miles each side of 

 epicentrum. 



forty miles north-west of it will illustrate the position of 

 the city with reference to the varying force of the shock. 



Had the seismic centre been ten miles nearer to 

 Charleston, the calamity would have been incomparably 

 greater than it was, and the loss of life would probably 

 have been appalling. Another circumstance greatly broke 

 the force of the shocks. All the coastal region of the 

 Carolinas consists of a series of clays and quicksands, 

 which have been penetrated by artesian borings to a 

 depth of 2000 feet, and which are believed to have a 

 much greater thickness. These beds of loose material no 

 doubt absorbed and extinguished a considerable portion 

 of the energy of the shocks. We have already remarked 

 that a wave passing from firmer and more elastic material 

 into material less firm and elastic pfoduces at first an 

 increased amplitude of wave-motion, which is liable to be 

 more destructive or injurious to buildings. But if the 

 mass of less consistent strata be very great, the reverse 

 result is produced by reason of the rapid extinction of 

 the energy in passing through a considerable length or 

 thickness of very imperfectly elastic material. We cannot 

 but think that Charleston owes in some measure its 

 escape from a still greater calamity to the quicksands 

 beneath the city. 



Another aspect of the same fact, if such it be, is found 

 100 miles west and north-west of Charleston. Here the 

 loosely-aggregated sediments of Tertiary and Cretaceous 

 age which cover the Carolina coastal plain have thinned 



out, and the crystalline rocks appear at the surface, thinly- 

 covered with soil and alluvium. All along the junction 

 of these loose strata and superficial material with the 

 metamorphics the intensity of the shocks was con- 

 spicuously greater than to the eastward and southward. 

 The loose covering of these firm rocks is just thick enough 

 to give full effect to the increased amplitude of vibration 

 which occurs when the wave passes from very solid and 

 elastic rocks to those which are less so. 



We have also endeavoured to reach some trustworthy 

 estimate of the amplitude of movement at the surface, but 

 the results are meagre and far from satisfactory. The 

 " amplitude of the earth particle " in any earthquake is a 

 question of great practical importance, and it is much to 

 be regretted that no better facilities for determining it can 

 be obtained. There were, however, many occurrences at 

 Charleston bearing upon this question, which are ex- 

 tremely difficult to explain upon any valuation of the 

 amplitude less than lo inches to a foot. Such amplitudes, 

 however, were most probably limited to spots here and 

 there, while in other spots it was probably much less. 

 That within a small area the amplitude of movement in 

 the surface soil varies between very wide limits seems to 

 be a practically certain conclusion from the observations. 

 In Charleston it appears to have been greatest in the 

 '• made ground," where ravines and sloughs were filled 

 up in the early years of the city's history. The structures 

 on higher ground, though severely shaken, did not suffer 

 so much injury. 



With regard to the time data from which the speed of 

 propagation must be computed, we are not yet in a posi- 

 tion to give final results, but can only state how the prob- 

 lem stands at present. The time reports have been 

 placed in the hands of Profs. Rockwood and Newcomb, 

 with the request that they would scrutinize and discuss 

 them. But neither has been able to finish as yet the task 

 he has so courteously undertaken. Probably the greatest 

 difficulty in the way of determining the speed of propaga- 

 tion arises from the ill-defined character of the disturb- 

 ance at considerable distances from the origin and from 

 the very considerable duration of it. Wherever a time 

 observation seems to be well authenticated, there still 

 remains in most cases the difficulty of deciding to what 

 particular phase of the earthquake the record refers. 

 And this difficulty is a very serious one. At Summerville 

 the first shock came almost like an explosion. Before 

 people had time to think, they were pitched about like 

 ten-pins. At Charleston there was a perceptible interval 

 estimated at from five to eight seconds from the first note 

 of warning to the maximum of the great shock. At 

 Savannah (ninety miles distant), the interval from the be- 

 ginning to the first maximum was considerably longer — 

 probably ten to twelve seconds; at Augusta (i 15 miles), 

 the interval was still greater. And, generally speaking, 

 the greater the distance the more the phenomena were 

 " long drawn out." The duration of the earthquake at 

 Charleston will probably never be known with accuracy. 

 But the general testimony ranges between fifty and ninety 

 seconds. At Washington (450 miles). Prof. Newcomb 

 with his watch in his hand observed a duration of per- 

 ceptible tremors, with two maxima, lasting about five and 

 a half minutes. Prof Carpmael's magnetographs re- 

 corded the disturbance, and he interprets their photo- 

 graphic traces as showing a duration of about four minutes. 

 Mr. G. W. Holstein, of Belvidere, New Jersey, gives five 

 minutes very nearly as the observed duration. From 

 other localities come well-attested observations showing 

 durations of several minutes, though few of these pretend 

 to give the whole time with any accuracy. This pro- 

 gressive lengthening of the shocks is a well-marked feature 

 of the testimony. The explanation suggests itself at once. 

 The elastic modulus of compression being greater than 

 that of distortion, the speed of the normal waves is the 

 greater while the waves of distortion lag behind. 



