May 20, 1887.] 



SCIENCE. 



497 



for believing, that among those great earthquakes 

 of the last hundred and fifty 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 a thousand 

 miles ; but the greatest distance at which the 

 tremors are felt is the best measure of the total 

 energy of the shock. On the other hand, the in- 

 tensity of the Charleston earthquake in the epi- 

 central tract was relatively low in comparison 

 with other great earthquakes. If, then, any shock 

 is more intense at the epicentre without extend- 

 ing to a greater distance than that of the Charles- 

 ton earthquake, it is certain that its focus was 

 nearer the surface. This is true of the vast ma- 

 jority of recent earthquakes which have been 

 sufficiently investigated. It is suggested that all 

 estimates of the depth of earthquake foci much 

 exceeding twelve miles are in need of re-examina- 

 tion. 



The city of Charleston is situated from eight 

 to ten miles outside of the area of maximum in- 

 tensity, and dill not experience its most destruc- 

 tive power. Following the law which we have 

 laid down, the intensity of the shock at Charles-' 

 ton was only three-tenths what it must have 

 been at the epicentrum, and about one-third the 

 intensity at Summerville. The diagram (fig. 5) 

 showing the long intensity curve stretching from 

 Charleston to a point forty miles north-west of it, 

 will illustrate the position of the city with refer- 

 ence to the varying force of the shock. 



Had the seismic centre been ten miles nearer to 

 Charleston, the calamity would have been incom- 

 parably greater than it was, and the loss of life 

 would probably have been appalling. Another 

 circumstance greatly broke the force of the shocks. 

 All of 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 

 two thousand 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 ma- 

 terial less firm and elastic, produces at first an in- 

 creased 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 be- 

 neath the city. 



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

 found a hundred miles west and north-west of 

 Charleston. Here the loosely aggregated sedi- 

 ments 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 ma- 

 terial with the metamorphics, the intensity of the 

 shocks was conspicuously greater than to the east- 

 ward and southward. The loose covering of these 

 firm rocks is just thick enough to give full effect 

 to the increased amplitude of vibration which oc- 

 curs when the wave passes from very solid and 

 elastic rocks to those which are less so. 



We have also endeavored to reach some trust- 

 worthy 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 regret- 

 ted that no better facilities for determining it can 

 be obtained. There were, however, many occur- 

 rences at Charleston bearing upon this question, 

 which are extremely difficult to explain upon any 

 valuation of the amplitude less than ten 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 position to give final results, but can 

 only state how the problem stands at present. 

 The time reports have been placed in the hands of 

 Professors Rockwood and Newcomb, with the re- 

 quest 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 propagation arises from the ill-de- 

 fined character of the disturbance at considerable 

 distances from the origin, and from the very con- 

 siderable duration of it. Wherever a time obser- 

 vation seems to be well authenticated, there still 

 remains, in most cases, the difficulty of deciding 

 to what particular phase of the earthquake the 



