The Wonder Book of Volcanoes and Earthquakes

by Edwin J. Houston

At the present time there is unfortunately much difference in opinion as to the exact cause of earthquakes. By this is not meant the immediate cause, but the ultimate cause. As to the immediate cause, practically all are agreed that quakes of volcanic origin are to be traced to the same forces that produce volcanic eruptions, while quakes of tectonic origin are due directly to the slipping of the strata along the faults. But when inquiry is instituted as to the nature of the forces that cause the volcanic eruptions, or that produce such an alteration of the strata as permits them afterwards to slip and thus jar the earth, there is much difference of opinion.

As can be seen from a few quotations of well-known authorities, only two kinds of earthquakes exist; namely, volcanic earthquakes and tectonic earthquakes.

Dana, for example, while acknowledging that small earthquakes may be caused by the sudden falling of large rock masses into cavities in the crust of the earth, says:

"But true earthquakes come, for the most part at least, from one or the other of the following sources of disturbance.

"1. Vapors suddenly produced, causing ruptures and friction.

"2. Sudden movements or slips along old or new fractures.

"Earthquakes due to the former of these methods are common about volcanoes, and at the Hawaiian islands shakings that are destructive over the island of Hawaii at the moment of some of the more violent eruptions, do not often affect the island of Oahu, a depth of 500 fathoms of water, the least depth between the two islands, being sufficient to stop off the vibrations....

"Earthquakes of the second mode of origin may occur in all regions, volcanic or not. They have their origin mostly in the vicinity of mountain regions, where old fractures most abound. The vibrations may begin in a slip of a few inches, in fact; but where there has been a succession of slips, up and up from 10,000 feet or more, as in the Appalachian, earthquakes of inconceivable volcanic activity must have resulted."

Dana points out that volcanoes stand on lines of fractures in the openings of which their existence began and that, during geological time, slips up or down these fractures have occurred, producing earthquakes and possibly starting eruptions.