1310 
large microseisms of irregular types that it had to be 
discontinued. Solid rock is most suitable for the instal- 
lation of seismographs for practically all purposes. 
Desirable Lines for Future Research 
The empirical and theoretical results concerning mi- 
croseisms can be considered to be first approximations 
at best. While some progress can be expected from 
various extensive projects now under way, many im- 
portant studies remain to be undertaken. Among those 
in which progress would be most helpful are the follow- 
ing, many of which offer no practical difficulties but 
have not been investigated because of the high cost: 
1. Empirical 
a. More detailed studies (similar to those described 
in [2]) on types of microseisms and their causes (lead- 
ing to improvement of Table I). 
b. Study of wave types and characteristics of ob- 
served microseismic waves (for all types in Table I), 
and determination of the “spectrum” of microseisms 
at a variety of locations. 
c. Determination of the direction of approach of 
microseismic waves of various types at different sta- 
tions by use of tripartite stations and of different types 
of instruments at a given station. 
d. Additional determinations of velocities of micro- 
seismic waves and their correlation with velocities of 
seismic waves found from earthquakes and from blasts 
in the same region. 
e. More data on the relationship between the periods 
of the cause of the microseisms in specific instances 
and the periods of the recorded microseismic waves in 
various parts of their “spectrum.” 
f. Study of the relationship between microbarometric 
waves and microseisms (only a few data exist). 
g. Measurements of pressure variations at greater 
depths in the ocean and their correlation with ocean 
waves at the surface (amplitudes and periods) as well | 
as with microseisms recorded nearby on land. 
2. Theoretical 
a. Study of the effect of free vibrations of the ground 
on microseismic waves, especially resonance effects. 
(This is a minor problem for earthquake waves where 
there are rarely ‘‘continuous” waves within the range 
of periods prevailing in microseisms.) 
b. Quantitative data on effects of maximum or mini- 
mum group velocity on amplitudes (Press and Ewing’s 
theory [31]). 
c. Extension of the theory of Press and Ewing to 
other models, especially under assumption of a gradual 
or sudden increase of velocity with depth in the mate- 
rials which form the ocean bottom. 
d. Study of the transition of elastic waves from one 
structure to another, for example, gradually or sud- 
denly from a model of the type considered by Press 
and Ewing to a typical continental structure. 
e. Study of the transition of surface waves from one 
structure to another with different elastic constants 
down to a depth of (1) a fraction of the wave length, 
(2) a multiple of the wave length. 
10. 
11. 
12. 
13. 
14. 
15. 
16. 
17. 
18. 
il). 
20. 
MICROSEISMS 
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