Page:Popular Science Monthly Volume 74.djvu/142

138 with the fracture system of the more firmly consolidated rocks.

Either the same or a separate triangulation section of the party should have charge of the re-occupation of primary triangulation stations in order to see what changes in position and elevation of these stations are properly ascribable to the earthquake. It may well be doubted if more ideal conditions could anywhere be found for such a study. If continued changes should be found to occur during the progress of the surveys, as is by no means improbable, the opportunity thus offered to compare mass movements of the ground with the time of prominent aftershocks should be regarded as of the first importance.

In every great earthquake which is studied, perhaps the most important line of attack is found in the distribution of the surface intensity of the shocks. It is now everywhere acknowledged that this intensity or amplitude of movement (and it is on this that damage to structures depends), is in a large measure determined by the elastic or non-elastic nature of the underlying material. Amplitude of movement is least on so-called "solid rock," it is greater on non-coherent deposits such as alluvial material, and it is probably greatest over so called "made ground," with its tin-can and crockery ingredients.

With the passing of the centrum theory it is inevitable that the study of the immediate basement of each locality should enter upon the quantitative stage of development. The local quantitative effect of the surface layers is a factor which to an approximation may be known and for this reason should be eliminated, if the seats of movement are to be determined. Local thickness and relative elasticity of the unconsolidated materials in the basement must therefore, be determined, and the value thus obtained be deducted from the total local intensity, if we are to arrive at the genesis of the disturbance. Accurate geologic maps and earlier detailed seismological studies in Calabria and Sicily are favorable to an extended study of this subject.

There are few, if any, places where within a circumscribed area more elaborate magnetic observations have been carried out than about the straits of Messina. Before the earthquake of 1905 a detailed magnetic survey for this district had been completed. It is almost certain that large changes would be revealed by a new survey since it has been shown in Japan that important changes in the isomagnetics resulted from the great earthquake of 1891. The importance of magnetic records to earthquake study is each year being made more apparent.