Page:Popular Science Monthly Volume 69.djvu/293



HERE are two sets of disturbances which shake the crust of the earth and therefore go by the name of earthquakes. Eruptive earthquakes are explosions, usually of steam, about a volcano. Tectonic earthquakes are breaks in the overloaded or overstrained crust of the earth, and, for the most part, have nothing to do with the steam vents we call volcanoes. To the last class most earthquakes belong, certainly almost all that have been felt within the United States.

Again, under the name of earthquake we include two very different sets of phenomena, the one the rock-rift or fault, which is the disturbance itself, the other the spreading or interfering waves set in motion by the parting, shearing and grinding of the sundered walls of rocks in the earthquake fault. It is the jarring waves extending in widening and interfering circles which do the mischief to man and his affairs. It is the rift of rock which sends these waves forth on their blind mission of confusion or destruction.

In every tectonic earthquake there is somewhere a fault or rift of rock, with some sort of displacement, permanent or temporary, of the relations of the two sides. In extreme cases, this break extends for miles in a straight line, breaking the surface soil and passing downward to a depth which can be only guessed at, five or ten miles perhaps, perhaps as far down as the crust is rigid. There are undoubtedly destructive earthquakes in which the soil is not broken over the rift of rock, but as a rule, in violent disturbances, the crack comes to the surface, breaking through the overlying soil. In all severe earthquakes there are, moreover, breaks or fissures in the earth having no connection with the fault itself. These are slumps or landslides, and geologically