1304 
from the stations. Microseisms of Type 5 have been 
studied recently in more detail because of their eco- 
nomic importance. They are probably caused by high 
ocean waves in hurricanes and typhoons. Most of the 
available theory is applicable to these microseisms. 
At most stations more or less regular sinusoidal 
waves are recorded with periods between 4 and 10 sec, 
and such microseisms have been described in several 
hundred papers [4]. Nearly everywhere they have well- 
expressed maxima during winter and frequently are 
scarcely noticeable on typical records during summer. 
At some stations a slight increase during the night has 
been reported. Except for Type 9, it is generally pointed 
out that these microseisms are connected with wind- 
storms over the ocean (Fig. 2), but there is no agree- 
ment concerning the mechanism by which the energy 
is transmitted from the storm to the ocean bottom and 
frequently it is not clear whether Type 6, 7, 8, or still 
another type (¢.g., those attributed to strong winds 
blowing against coastal mountains) is involved. Ac- 
cording to the hypothesis stated in 1903 by E. Wie- 
MICROSEISMS 
seisms with periods of 4 to 10 sec recorded at Uppsala, 
Sweden, belong to at least two types: one due to surf 
driven against the steep Norwegian coast; the other to 
cyclones in the North Atlantic which transmit a small 
fraction of their energy into the water and the ocean 
bottom. In instances like this, when microseisms from 
different sources arrive simultaneously at a station, 
waves with slightly different periods produce interfer- 
ence patterns; aS a consequence, wave groups with 
gradually increasing and decreasing amplitudes are fre- 
quently found at certain stations, but rarely at others. 
Great progress has been made in the study of Types 
5, 6, and 7 by the use of tripartite stations which are 
described in the article in the Compendium by Father 
Macelwane.! For papers giving summaries of the litera- 
ture concerning Types 6 to 9 see, for example, [12, 15, 
18, 19, 32, 36]. 
It is possible that microseisms with longer periods 
include movements produced by strong winds (Type 
10). These differ from Type 4 by their much greater 
periods. Possibly related to them are the compressional 
TaBLE I. Typrs or MICROSEISMS 
Type no. Period (sec) Kind of movement Hypothetical causes Melative distance of Figure 
1 0.2-0.5 Regular ? Local? la 
2 0.2-2 Irregular Surf Local 1b 
3 1-4 Regular Fronts? Rain? Wind? Local le 
4 1-4 Irregular Wind Local 1d 
5 2-6 Regular Ocean waves in hurricanes, typhoons Distant le 
6 4-10 Regular Ocean waves in extratropical disturbances Distant If? 
7 4-10 Regular Surf driven by wind against steep coasts Distant 1f? 
8 4-10 Regular Air-pressure pulsations? Medium? — 
9 4-10 Regular Monsoon and other types of wind Medium? lg 
10 20-100 Irregular Wind? Air currents in instrument vault? Local th 
11 40-200 Trregular Freezing of ground? “Icing” of instruments? Medium li 
chert and developed by Gutenberg [15], ocean waves 
driven by storms toward steep coasts are the source of 
Type 7. On the other hand, Gherzi [11] believes that 
fast changes in air pressure (pumping) are a major 
cause of such microseisms. However, there is some 
doubt whether Type 8 actually exists, since air-pressure 
changes amount to only a small fraction of one atmos- 
phere while the pressure changes produced by storm 
waves in the ocean are of the order of magnitude of 
one atmosphere. 
It is very likely that a mechanism similar to that 
which produces microseisms during hurricanes also op- 
erates in extratropical disturbances. In this case the 
energy of the storms available in a given area is smaller, 
but the region affected by it is much more extended. 
The resulting microseisms are Type 6. On the other 
hand, Banerji [1] believes that the motion of gravity 
waves in water is large enough to reach the ocean bot- 
tom and cause microseisms of Type 6. Finally, similar 
microseisms are caused by monsoon winds [1] (Type 9). 
In one of the most comprehensive of all studies of 
microseismic records and their relationship to meteoro- 
logical phenomena, Bath [2] has shown that the micro- 
movements of more irregular appearance which are re- 
corded by the strain seismographs at Pasadena during 
windstorms as well as during the presence of convection 
currents. These movements seem to be the result of 
compression and dilation of the hill on which the Seis- 
mological Laboratory is built.2 Type 11 includes irregu- 
lar movements, with periods of the order of one minute, 
which seem to be connected with freezing of the ground 
[15, pp. 294-298]. 
Transmission of Energy from Meteorological Distur- 
bances into the Ground 
There is no type of microseism for which a complete 
theory has been worked out. The few theories which 
have been formulated mathematically cover only cer- 
tain phases of the process. These theories may be 
divided into two groups. The first group deals with 
the transmission of energy from the meteorological dis- 
turbance into the ground, the second with the propa- 
1. ‘Practical Application of Microseisms to Forecasting” 
by J. B. Macelwane, S.J., pp. 1812-1315. 
2. See Bull. Amer. meteor. Soc., 20:424, Fig. 3 (1939). 
