OBSERVATIONS AND THEORY OF MICROSEISMS 
By B. GUTENBERG 
California Institute of Technology 
Observations and Causes of Microseisms 
As soon as fairly sensitive seismographs were avail- 
able, it was found that the ground is never at rest. 
Various terms such as pulsations, pulsatory oscillations, 
microseismic movements, and microseisms were used to 
describe these continuous small movements which are 
not caused by earthquakes. Hecker, in 1906, divided 
the microseisms, as they are now usually called, into 
four groups: (1) those with periods of less than 4 sec, 
(2) those with periods of about 7 sec, (8) those with 
periods of about 30 sec, and (4) those with periods of 
about one minute and more. Later, additional types of 
microseisms were described. For a bibliography, see [4]. 
There is little agreement about the causes of micro- 
seisms. This is partly due to the fact that at one sta- 
tion certain types of microseisms prevail and are de- 
seribed; at another station, different types. In addition, 
the seismographs in use have widely different charac- 
teristics so that some emphasize waves with periods of 
a fraction of a second, others waves with periods of 10 
see or even more. (Compare Figs. 1c and 1f.) The fact 
that there are several types of microseisms which differ 
rather little in their appearance has added to the con- 
fusion. Many authors have claimed that hypotheses 
concerning causes of microseisms given by other seis- 
mologists are mcorrect while, actually, completely dif- 
ferent types of microseisms were involved in the discus- 
sions. In Table I a summary of some of the more 
common types of microseisms is presented, and Fig. 1 
shows some typical records. 
No movements of the ground due to traffic and in- 
dustry (which are usually also called microseisms) are 
included here. In addition, “microseisms” of Type 10 
(Table I) are frequently caused by the direct effect of 
air currents in the vault on the instruments, and Type 
11 probably contains instances where subfreezing tem- 
peratures caused freezing of moisture on the instru- 
ment and spurious ‘‘microseisms.”” Movements with 
periods of more than 1 minute (for example from chang- 
ing loads at coasts during high seas or high tides) are 
now usually classified as ‘‘tilt.” In general, irregular 
and short-period microseisms are connected with rather 
local causes. On the other hand, Types 6 and 7 have 
been recorded thousands of miles from the source, for 
example in Irkutsk, Central Asia, during storms near 
the Norwegian coast. 
Very little is known about Type 1 (Table I). These 
microseisms are under investigation by Father Macel- 
wane and his collaborators [25]. Microseisms of Type 2 
are observed at stations near coasts and are caused by 
local surf [15, p. 269]. 
Types 3 and 4 are probably of local origin. They have 
been attributed to effects of windstorms and cold fronts 
near the station, but no detailed theory has been pub- 
lished on the mechanism by which they are produced. 
They are being investigated at Fordham [24] and at 
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Fie. 1—Typical records of microseisms. 
a. Short-period microseisms (Type 1, Table I), Wood- 
Anderson torsion seismograph, Florissant, Missouri. (After 
Macelwane.) 
b. Microseisms from local surf at Helgoland, January 18, 
1918; Wiechert 200-ke horizontal seismograph; successive lines 
are about 1 hr apart. (After Gutenberg.) 
c. Microseisms of Type 3 (Table I), recorded at Pasadena on 
December 26, 1948; short-period Benioff vertical seismograph. 
d. Microseisms during windstorm at Gottingen, November 
Aes ee 1200-kg vertical Wiechert seismograph. (After Guten- 
erg. 
e. Microseisms recorded at Zi-ka-wei near Shanghai from 
typhoon over the ocean, October 3, 1923; Galitzin seismograph; 
lines are about 14 hr apart. (After Gherzi.) Records of Type 6 
or 7 are frequently similar to this record. 
f. Microseisms of Type 6 or 7 (Table I), recorded simul- 
taneously with Fig. le at Pasadena by long-period Benioff 
vertical seismograph. t 
g. Microseisms during monsoon recorded at Zi-ka-wei on 
November 22, 1922; Galitzin seismograph. (After Gherzt.) 
h. Microseisms of Type 10 (Table I). (After Whipple.) ‘ 
1. Microseisms of Type 11 (Table I), recorded at Zi-ka-wei 
on January 4, 1917; Galitzin seismograph. (After Gherzi.) 
Pasadena. Caloi [5] has found that microseisms with 
periods of 2-3 sec at Trieste are correlated with seiches 
in the Gulf of Trieste which in turn are caused by low 
pressure centers. 
Types 5 to 8 usually originate at greater distance 
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