A Manual of Seismology

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1921 Excerpt: ...to converge towards the village of Fig. 49. Mallet's method of determining the epicentre of the Neapolitan earthquake of 1857. Caggiano. They intersect, indeed, at various points within an area indicated by the curve on the map--a line which Mallet regarded as the surface-trace of a fissure about 10 miles long, within which the earthquake probably originated. The method of directions, which Mallet used with such skill in this disastrous earthquake, has lately fallen into undeserved neglect. The extreme variability of the direction in any place cannot be overlooked, and solitary observations are not only valueless, but misleading. But, if the method be modified by the substitution of the mean of a large number of measurements for a few scattered observations, it may still be found a useful instrument in the investigation of a destructive earthquake (sects. 58, 59). 128. Method depending on the Intensity. The first method depending on observations of the time, even if it could be applied with accuracy, requires a large disturbed area for its use. For. _ Fig. 50. Map of the Inverness earthquake of 1901. the method of directions, a strong shock is also essential. The chief advantage of the third method is that it can be applied in earthquakes of every degree of strength. In violent shocks, the area of ruined towns and villages usually encloses, if it does not mark out, the epicentral area. In the weakest shocks, the disturbed area is itself so small that any place at which it is felt may be regarded without much error as coincident with the epicentre. R. Mallet, The Great Neapolitan Earthquake of 1857, vol. 2, 1862, pp. 235-247. In these cases, the determination of the epicentre is only approximate. Greater accuracy is attained by the construction of a se...