Intermediate earthquakes, or those in which, although the area affected is great, yet the destruction of buildings, or loss of life, has been comparatively small.
Minor earthquakes, or those which, although capable of producing small fissures in the crust, generally leave but few or no traces of their occurrence.
The greatest distance to which earthquake waves of the first class extend is taken by Mallet as being over a diameter of 1,080 miles; those of the second class over a diameter of about 360 miles, and those of the third class over a diameter of about 120 miles.
According to the Rossi Forel scale already given, earthquake shocks are divided according to their relative intensity into ten separate classes, viz.: I. The micro-seismic; II. The extremely feeble; III. The very feeble; IV. The feeble; V. The moderately intense; VI. The fairly strong; VII. The strong; VIII. The very strong; IX. The extremely strong; X. Shocks of extreme intensity.
An earthquake map prepared according to Mallet's scale would show a greater depth of color or tint in the neighborhood of the volcanic districts of the earth and especially in the neighborhood of the mountain regions, where tectonic quakes are most frequent. Oceanic areas would be left almost untinted, not because earthquakes do not occur on the bed of the ocean, but because of the difficulty of observing such earthquakes at great distances from the land. So far from earthquakes being absent on the bed of the ocean it is most probable that they are more frequent there than elsewhere.
Prepared in this way, Mallet's map would show a preponderance of earthquakes along the borders of the continents, especially along the "Great Circle of Fire" on the borders of the Pacific Ocean.
Dutton as well as some others assert that the "Great Circle of Fire" on the shores of the Pacific has in reality no existence; that, instead of there being a continuous region of volcanoes, there is in reality nothing more than a considerable number of volcanoes arranged in groups along the borders of this ocean, but separated by spaces containing no marked volcanic activity. We do not think this a tenable position, since it is well known that volcanoes lie along great lines of fissures at different points or openings which are kept open by subsequent volcanic activity, while the remaining portions are closed soon afterwards; and, moreover, in parts of these so-called non-volcanic regions, there are probably extended regions of extinct volcanoes.
Since the time of Mallet many maps have been made to show the distribution of earthquakes. Among the best of such is that by M. de Montessus de Ballore.
Some idea of the great amount of work required for the preparation of Montessus' map may be formed when one learns that the catalogue of earthquakes collected by him for this purpose included for the years 1880 to 1900, 131,292 quakes.