Page:Popular Science Monthly Volume 86.djvu/453

Rh of April 18, of the same year (1906). The last and the southernmost of the series occurred on August 12, 1906, and devastated southern Peru and Chile. These various shocks occurred, as stated, along the same line of crustal weakness and parallel to geologically young and lofty mountains, which appear to be still in process of growth. In the same way the Messina, Calabrian, and now this, the most recent of the Italian earthquakes, have all occurred along a line of movement which, starting in southwestern Italy, has experienced movement both further to the southwest and to the north—the last developing along this line of dislocation in the extension of the uplift marked by the Appenines into the division of Abruzzi. Along the same line other shocks will probably be felt from time to time, though there is no telling even approximately as to what the length of this time interval may be. Another series of shocks may occur while this article is being printed, or months, years, or even a century or more may pass before another pronounced movement takes place along the Calabrian-Abruzzi line of fracture; but that it must occur again sooner or later is almost a certainty.

In the regions of old rocks of the earth's crust or in regions of old mountain ranges, a state of equilibrium has been so nearly approached that few earthquake shocks occur. However, in those areas of the earth where young mountain ranges rise or where earth movements are in progress, seismic disturbances are more or less frequent. With the data now available we may map with a very fair degree of accuracy eleven such earthquake areas, all of which are shown on the accompanying map. Also it is worthy of note that the areas chiefly affected by earthquakes are occupied by rocks which are post-Paleozoic in age, and hence, geologically speaking, are relatively young.

The largest earthquake region of the world, which we may term the Alpine-Himalayan area, extends from the Alps to the east of central China. As is shown on the accompanying map, Italy, the eastern Mediterranean, and the warped-down basin of the Caspian, all fall within this area, as do also the Alps, and the young and probably still growing mountain chain of the Himalayas, whose rocks were laid down in the sea at a time, in the geological past, when the Mediterranean formed a connecting link between the Atlantic and Pacific oceans. Within the limits of this region shown on our map, it is estimated that fully 20 per cent, of the more important and widely felt earthquakes occur. The Malaysian earthquake region lies further east. It extends from the Bay of Bengal, across northern Australia, to the region of the Pacific north of New Zealand, and it also embraces all of the East Indian Islands. Its axis probably follows the alignment of the various young and growing mountain regions that traverse the islands of this Archipelago, To the north of the Malaysian belt is the earthquake region which includes the Archipelago of Japan and the Philippine Islands.