Dr. Rudolph attributes earthquake waves to submarine volcanic eruptions alone. It would seem, however, as if each one of the other things above referred to might at times be the direct cause.
[CHAPTER XXXIII]
THE DISTRIBUTION OF EARTHQUAKES
Earthquakes may occur at any part of the earth's surface, at any time of the day, or at any season of the year, yet they are more frequent at certain parts, certain hours, certain seasons.
Since some earthquakes are unquestionably connected with volcanic eruptions, a map or chart of the volcanoes of the earth would also, to a certain extent, show the parts of the earth that are likely to be visited by earthquakes. Since, however, by far the most severe earthquakes are not directly connected with volcanoes, but are due to sudden slips of faulted strata, a volcanic chart would necessarily fail to indicate accurately the principal earthquake regions.
In the preparation of a map showing the distribution of earthquakes over the earth's surface, Mallet adopted the plan of colorings or tintings in such a manner that the depth of the colors would represent not only the parts shaken, but also the relative number of times shaken, as well as the intensity of the shocks. In order to determine the depth of tint to be employed, Mallet divided earthquakes into the following classes according to their intensity:
Great earthquakes, or earthquakes of the first class; or those in which the area affected is of great size, in which many cities have been overthrown, and many people killed, and parts of the surface greatly altered.