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Page:Popular Science Monthly Volume 70.djvu/399

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395
THE JAMAICA EARTHQUAKE

PSM V70 D399 Crushed east and west walls.png

Fig. 8, b. East and West Walls crushed.

decreasing rapidly eastward, less rapidly westward and still less so to the north. Haiti did not feel the shock, neither was it felt at Colon or at Grand Cayman, 175 miles west, but Santiago, 120 miles north, experienced a slight shock.

Cracks in buildings, which at Kingston dip some 50 degrees east, are always perpendicular to the path of the emergence of earthquake waves. Hitherto, the intensity area and epicenter have been regarded as synonymous. But the dip of the angling cracks at Kingston points to a locus of disturbance much to the west of that city, while the lines of isoseismals indicate the intensity area in the eastern half of Kingston. It may readily be imagined, then, that the area of greatest destruction may not be directly above the focus. Suppose a highly elastic rock is there situated, and some distance away is found a plain of loosely-formed material. The destruction in the latter area will far exceed that in the former in spite of its favorable location. Until we register the actual amplitude, wave-length and period and, with the elasticity of the rock underneath, calculate from the more readily-discerned data on adjacent but less elastic media the changes that have occurred in the wave-motion, it will be difficult to determine with accuracy in a region of rocks of widely varying elasticity the location of epicenters. For outliers of rock in plains must deflect, refract and reflect wave-motion and even shadow areas in these plains. The only conclusion then is that the eastern end of the Liguanea plain was the nearest area to the real epicenter that by nature of material would give the greatest amplitude to the destructive epifocal waves. Further, the angle of emergence at Kingston coordinated with the proximity of a probable epicenter, together with the limited area of disturbance, indicates a shallow origin of about three miles.