Popular Science Monthly/Volume 86/May 1915/The Earthquake Areas of the Earth with Special Reference to the Recent Italian Earthquake
|THE EARTHQUAKE AREAS OF THE EARTH WITH SPECIAL REFERENCE TO THE RECENT ITALIAN EARTHQUAKE|
UNIVERSITY OF VIRGINIA
THE recent Italian earthquake, which occurred on January 13 at a cost of 30,000 lives and $60,000,000 in property, calls attention to the fact that the most disastrous shocks of this sort are, in general, not connected at all with volcanoes or volcanic areas. This is contrary, of course, to the view widely held that all earthquakes are caused by, or in some way connected with, volcanic activity. In disproof of this view may be mentioned the great Calabrian earthquake of 1783. This great earthquake occurred in southwestern Italy, but in no wise affected the volcanic areas, and this is also true of the terrible Messina earthquake of 1908, which, though it devastated various towns and cities in that part of Sicily not far distant from Mount Etna, arose from earth movements in the Strait of Messina. It was not at all connected with Etna, which at that time exhibited no unusual volcanic activity, and, too, there were no other evidences that could possibly suggest its connection with the earthquake just alluded to.
The recent earthquake which first occurred east of Rome in the Abruzzi district was followed by severe shocks in the province of Calabria. Neither Vesuvius nor Etna is reported as exhibiting any special activity. Indeed it is evident that they are in no way connected with these shocks which have been so appallingly destructive of life and property.
Broadly speaking, there are two types of earthquakes, namely, the volcanic, which is due to shocks accompanying volcanic explosions, and the more disturbing and disastrous tectonic type, caused by faulting and Assuring accompanying warping of the earth's so-called crust. In Italy, these tectonic earthquakes, such as the Calabrian earthquake of 1783, the Messina earthquake of 1908, and the recent earthquake that wrought such destruction in the Abruzzi district are all of the tectonic type, and all are the result of earth fractures brought about by mountain-making movements. In this connection it will be of interest to consider just what areas of the earth are affected in this way, and why, in certain regions, earthquakes both occur and recur, while, on the other hand, similar earth movements are not experienced nor to be anticipated in regions where mountain-making movements are no longer actively in progress.
The greatest of the Italian earthquakes have been the result of earth fracture accompanied by a succession of destructive shocks. They have been due to the fact that Italy lies within an area characterized by a state of unstable equilibrium. Its mountains and the most lofty mountains of the world, the Himalayas, were uplifted during the last great period of mountain-building—for it must be remembered that not all mountains are of the same age—and that the rocks of the earth had been folded into great arches, upon which were superinduced other folds, long before the uplift of the Himalayas. In other words, most of the highest mountains of the earth are much younger geologically than the older and less lofty ranges, just as the Rockies are vastly younger than the mountains of the Appalachian region, while they, in turn, are much younger than the Blue Ridge—itself younger than the remnants of other mountains which, however, in the attribute of elevation, are mountains no longer—since now only their upturned eroded roots remain marking sites where their masses formerly stood in the long distant past, as great, towering topographic features. Old mountains thus become off and their sediments may again, in the evolution of the continents, be uplifted elsewhere after eons of ages to form new and even much higher mountains than before.The cause of the movements of the earth which produce mountains is a long story in itself—and so its discussion here must needs be omitted. For our present purpose it will suffice to say that such movements do occur. Furthermore, they proceed very slowly, so slowly that while such deformations are in progress they may be at such a rate as to be wholly imperceptible and yet, acting through a vast period of time, be sufficient to lift rock formations from beneath the sea to thousands of feet above sea level. In the course of the uplift, accompanied as it is by folding, obviously enormous stresses are developed and from time to time rocks give away and Assuring and faulting ensue; and when this happens the shocks that are felt are said to be the result of what is termed a tectonic earthquake. Later on other movements may occur along a line of weakness produced in this way, and at different points along this line and at different times, just as the California earthquake was the next to the last of a series of eight movements along a well defined line of crustal fracture. The movements which culminated in the California earthquake began in the north off the coast of British Columbia. The first and third of the series occurred in this region in September, 1899, and October, 1900. They were then felt off Central America in January, 1900, and April and September, 1902. After a lapse of over three years, sufficient stresses had accumulated to inaugurate another movement, upon the southern continuation of the same line. This shock was noted off southern South America on January 31, 1906; and then several months later came the California earthquake
Map Showing the Areas of the Earth Chiefly Affected by Seismic Disturbances. It will be noted that within certain of these areas are the highest mountains of the world—which were uplifted during the most recent of the world's mountain making periods. Certain other areas are also marked by crustal warping and such lack of equilibrium as to produce more or less frequent earthquakes.
1. Alaska-British Columbia belt. 2. California-Ecuador belt. 3. Peru-Patagonia belt. 4. West Indian area. 5. Bermuda area. 6 Iceland area. 7. North African-Portugal-Ireland area. 8. Alpine-Himalayan area. 9. South India-Madagascar region. 10. Malaysian area. 12. Japan-Phillipine area.
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. As is well known, the islands of the former are almost constantly shaken by earthquakes. This region and the East Indian region, as will be observed from the map, are not sharply marked off from one another.
Along the western coast of North America, there are three earthquake regions. These are shown separately, but as a matter of fact are connected by narrow strips, marking areas of more restricted seismic disturbance. The central belt is thus to be connected, on the one hand, with the southern belt, and, on the other, with the northern belt. The most northern of these three belts lies along the coast of Alaska and British Columbia; the central or California-Ecuador belt, begins in California, includes all of Central America, and ends in Ecuador; the southern, or Peru-Patagonia belt, follows the coast of South America from Peru to Patagonia.
Both the West Indian and the South Indian-Madagascar earthquake regions extend over areas which have experienced more or less profound downward crustal warping. The West Indian region, possessing definite features of marked structural instability, suggest the Malaysian area, which latter, however, is characterized by young and growing mountains, but also exhibits in certain of its parts warping of the kind previously alluded to. The West Indian area is also a region of unusually active earthquakes which are due doubtless to a continuance of the earth's movements by which the old continent of Antilla has been broken up into the islands that go to make the present Archipelago. The South Indian-Madagascar region is supposed to mark the site of a former land mass now vanished under the Indian Ocean. This earthquake region extends from South India in a southwesterly direction to the island of Mauritius and to the east of Madagascar.
The North Atlantic may be divided into three earthquake regions—of these three, one lies northeast of Iceland and parallels the coast of Scandinavia. The second and largest of these three regions extends from North Africa in a northward direction along and past the coasts of Spain and Portugal to the west of the Bay of Biscay, and thence to the west of Ireland. It was along the line of movement here described that there developed the terrible Lisbon earthquake. The third of these areas, which is about the size of the first and the least active of the three, parallels the eastern coast of the United States and includes the islands of the Bermudas.
Some of these earthquake regions, such as the last of the eleven mentioned, are notably free from violent volcanic activity; but, as we have seen, even in a region containing an active volcano the most powerful earthquakes often affect the non-volcanic districts. Thus, to recapitulate, the appalling Calabrian earthquake of 1783, though near the volcanic areas of Sicily and not a great distance from Vesuvius, affected a part of southern Italy where there are no volcanic rocks. The disastrous Lisbon earthquake was also in a non-volcanic area. Likewise, the region of occurrence of the recent Italian earthquake shows that it also is situated in a non-volcanic area. Furthermore, as a general rule—though earthquakes often happen in volcanic regions due to explosive shocks—the most disastrous and most powerful earthquakes occur in non-volcanic areas and are tectonic in their nature. They—the latter—are due to a series of rapid shocks which accompany movements along lines of weakness, such as faults or previously made fissures due to mountain-making movements. The throw of these faults need not be very great in order to produce disastrous results.
The frequent coincidence in the distribution of earthquakes and volcanoes may be said to be due to their dependence upon a common cause, in the sense that volcanoes may be regarded as incidental to and the result of great earth movements. Such movements evidence either the collapse of large areas of the earth's crust, or crustal uplift—each a warping, and both due to movements of an orogenic or mountain-making character. These crustal crinklings, moreover, which occur on a vast scale, are not confined to one period of the earth's history, but have been experienced by the earth at different times since the very beginning and are likely to continue to the end—which, considering the physics of the earth, and various well-known geological facts, appears to be fully as remote as the genesis of the earth itself.