Höfer, when working on the earthquake of Belluno, came to the conclusion that the disturbance originated from two faults meeting each other at an angle of 60°. In this determination he was chiefly influenced by the form of a certain homoseist which was of the form of an elongated ellipse met on one side by a second ellipse, the principal axes of the two ellipses giving the strike of the two faults.
CHAPTER XII.
DISTRIBUTION OF EARTHQUAKES IN SPACE AND TIME.
General distribution of earthquakes—Occurrence along lines—Examples of distribution—Italian earthquake of 1873—In Tokio—Extension of earthquake boundaries—Seismic energy in relation to geological time; to historical time—Relative frequency of earthquakes—Synchronism of earthquakes—Secondary earthquakes.
General distribution of earthquakes.—The records of earthquakes collected by various seismologists lead us to the conclusion that at some time or other every country and every ocean in the world has experienced seismic disturbances. In some countries earthquakes are felt daily, and from what will be said in the chapter on earth pulsations it is not unlikely that every large earthquake might with proper instrumental appliances be recorded at any point on the land surfaces of our globe. The area over which any given earthquake extends is indeterminate. The area over which an earthquake is sensible is sometimes very great. The Lisbon shock of 1755 is estimated as having been sensible over an area of 3,300 miles long and 2,700 miles wide, but in the form of tremors and pulsations it may have shaken the whole globe.
The regions in which earthquakes are frequent are indicated in the accompanying map, which, to a great extent, is a reproduction of a map drawn by Mallet. The regions coloured with the darkest tint are those where great earthquakes are the most frequent. The actual number of earthquakes which have been felt in the differently coloured areas are given, when speaking of the relation of seismic energy to season.
When looking at this chart it must be remembered that if we were to make a detailed map of any one of the different countries where earthquakes are frequent, we should find in it all the differences that we observe in the general chart. For instance, one portion of Japan, where perhaps sixty shocks are felt per year, would be coloured with a dark tint, whilst other portions of the same country, where there is only one slight shaking felt every few years, would be left almost uncoloured. The black dots indicating the position of volcanic vents are even more general in their signification than the tinted areas. Professor Haughton gives for the world a list of 407 volcanoes, 225 of which are active. These numbers are the same as those given by A. von Humboldt. Of the active volcanoes 172 are on the margin of the Pacific, and of the total number eight are in Japan. From my own observations in Japan independently of the Kurile Islands, I have enumerated fifty-three volcanoes which are either active or have been active within a recent period. In a few years’ time this list will probably be increased. I mention this fact to show how very imperfect our knowledge is respecting the number of volcanic vents existing on our globe. If we were in a position to indicate the volcanoes which had been in eruption during the last 4,000 years, the probability is that they would number several thousands rather than four or five hundred.
An inspection of the map shows that earthquakes chiefly occur in volcanic and mountainous regions. The most earthquake-shaken regions of the world form the boundaries of the Pacific ocean. It may be remarked that these boundaries slope beneath the neighbouring ocean at a much steeper angle than the boundaries of countries where earthquakes occur but seldom. The coasts of South America, Kamschatka, the Kuriles, Japan, and the Sandwich Islands, for example, have slopes beneath the Pacific from one in twenty to one in thirty. The coasts of Australia, Scandinavia, and the eastern parts of South America, where earthquakes are practically unknown, have slopes from one in fifty to one in two hundred and fifty. Many earthquakes have taken place in mid-ocean. In the Atlantic Ocean M. Perrey has given about 140 instances of such occurrences.
The majority of the earthquakes which shake Japan appear to have their origin in the neighbouring ocean. If we could draw a map of earthquake origins, it is probable that the greater number of the marks indicating these origins would be found to be suboceanic and along lines parallel to the shores of continents and islands which rise steeply from the bed of deep oceans. In countries like Switzerland and India, our marks would hold a relationship to the mountains of these countries.[90] Looking at the broad features of the globe, we see on its surface many vast depressions. Some of these saucer-like hollows form land surfaces, as in central Asia. The majority of these, however, are occupied by the oceans. Active volcanoes chiefly occur near the rim of the hollows which have the steepest slopes. The majority of earthquakes probably have their origin on or near the bottom of these slopes. To these, however, there are exceptions, as for instance the earthquakes in the Alps, in the hills of Scotland, and the shakings which are occasionally felt in countries like Egypt. The earthquakes which shake the borders of the Pacific have their origins in, and their effects are almost exclusively felt on, the sides of the bounding ridge facing this ocean. In Japan it is the eastern sides of the islands which suffer, the western side being almost as free from these convulsions as England.
Similar remarks may be made about the eastern side of South America, especially the southern portion of the continent. At Buenos Ayres, for example, there has been no disturbance since Mendoza was destroyed, some twenty years ago. In British Guiana slight shocks are occasionally felt in the low delta which forms the settled portion of the colony, but they are extremely rare.