The microseisms in North America show in general the same peculiarities that they show in other regions, especially in Europe. When there is a storm near the coast we find short waves which are more or less irregular and which have periods between one half and several seconds. At more distant stations the periods are in general between four and nine seconds.
The amplitudes are influenced by the subsoil. The stations at Milwaukee and Chicago (United States Coast and Geodetic Survey) especially show larger amplitudes than other stations. These larger movements may be caused by the moist nature of the soil near Lake Michigan. The smallest microseisms were found at Tucson. The subsoil there is sand or loose gravel.
To eliminate the effect of the subsoil and of errors in the magnification of the instruments, the mean maximum of four time intervals was taken as the unit at every station. Amplitudes measured in these units are called “relative amplitudes.” If one draws maps with these relative amplitudes at the different stations, one finds that the lowest values cover one region and the highest values another. Figure 4 shows the values and lines of equal relative amplitudes on four different dates. The lines could be drawn only very roughly as the values are influenced by various errors and as only a very few points could be used. Such maps were drawn for all cases and hours discussed in this paper.
The distribution of the areas with large microseisms depends upon the position of the low-pressure area in the following manner: Low-pressure areas approaching the west coast of North America cause an increase in microseisms near the region where they approach the coast. These movements are not propagated very far along the coast nor perpendicular to it. At the stations in the center of the continent there is sometimes a slight increase of motion. Storms and low-pressure areas over the central parts of the continent do not cause microseisms at all. Also when they reach the coast of the Atlantic in the southern or middle east no microseisms or only local motions arise. But as soon as they approach this coast in the north, microseisms increase over the whole continent, especially if the depressions have steep gradients and cause storms against the coasts of Newfoundland and Canada.
So we find that, just as in Europe, surf due to storm against steep, rocky coasts is the cause of the microseisms with periods of four to nine seconds. Also, just as in Eurasia, where the movements are propagated best if the Scandinavian Shield is shaken, we find the largest area with large microseisms if the Canadian Shield is shaken by the storm-surf. In Europe the young mountains in the south cause rather rapid decrease in the motion. In western America the higher folded mountains nearly prevent the propagation of the movements by reflecting and refracting a large part of the energy.
Observations show that neither the air-pressure, nor its change, nor storm can be the cause of the microseisms. The result of the calculations is that no possible disturbance near the surface of the ocean can be propagated through the water to the bottom, but that the energy of the waves transferred by the surf to the coast is large enough to cause the movements. Therefore, Wiechert's hypothesis that the surf produced by storm blowing against a steep rocky coast is the cause, will hold over the whole earth. Special conditions, the areas permitting good propagation and mountain ranges hindering the movements, must be investigated in special studies. In addition to its general views, this paper constitutes the first study of this kind in the case of North America.