Microseisms in North America

Our most sensitive seismographs are never at rest. They show that there are always small movements of the earth's crust which are called "microseisms." Investigations (1) have shown that there are different kinds of such movements, caused by traffic, industry, wind, rain, waterfalls, waves beating against the coasts, freezing of the soil, and probably some other causes. In the following we shall deal only with that kind of microseisms which is characterized by a regular, nearly sinusoidal motion with periods in general between four and ten seconds. These have been found throughout the world.