1314 
so that their optimum response range will be in reso- 
nance with the microseisms. Horizontal component 
instruments were chosen in preference to vertical 
component seismographs because they are simpler to 
operate and are much less sensitive to fluctuations in 
temperature and hence require much less elaborate 
vault insulation. 
The seismographs at the outlying corners are 
connected by shielded cable to the respective gal- 
vanometers in the main vault of the tripartite station 
where they record on a triple drum whose speeds of 
rotation and endwise translation are variable. Moderate 
speeds are used as long as there is no evidence of a 
storm. When the amplitudes of the microseisms are 
seen to increase, the drum rates are greatly accelerated 
(Fig. 4). 
I t nantyh, 
a es |e 
"| f ( 
ie Herat ARR a 
SS SOS DE RR NG 
IS NRE Ns eG, eS 
ELON ROIS ON AES OO NO, ONO pe 
NE NS eT 
eS ae oN ene 
WS 
Fie. 4.—Part of a record is at the Miami, Florida, 
tripartite station of the U. S. Navy, August 23, 1949, showing 
large microseisms recorded at the stand-by rate (top) and at 
the hurricane rate (bottom). Time interval between interrup- 
tions in both cases, 15 sec. 
However, the problem still remains as to the mecha- 
nism by which a storm center produces the elastic 
waves on the ocean bottom and why they are so 
conspicuously characterized by groups resembling inter- 
Ue el Hist 
mite onda arya Need 
ANH eo tnd 
qi fat pu " af | ' i we ' i) Nh ANA ni I HWM Mtvalel | AN ' 
Fie. 5—Samples from a record made at the Bermuda 
tripartite station of the U.S. Navy, September 6, 1949, show- 
ing the swell and decay “‘group”’ effect. Time intervals, 15 sec. 
MICROSEISMS 
ference patterns (Fig. 5). An argument in favor of 
swell and decay of activity at the source rather than 
interference from independent sources within the storm 
area is the similar appearance of the records in many 
different directions. A thorough study of the “groups” 
from this point of view remains to be made. 
nen UN 
tA) SN 
my AMY 
AY , 
uy Wf Vas? 
natal! A ae 
o pate av 
ow 
‘ AY, hye. 0 IVa. 8 W cant 
OW A A toe ON © rua d 
Oe 
ae ’ 
Fic. 6.—Record at Corpus Christi for July 9, 1947, showing 
increase in amplitude of 1.3-sec microseisms on lower portion 
of record at the time of a local thunderstorm. Time breaks are 
at 15-sec intervals. 
Suggestions as to the mechanism by which these 
microseisms are produced have been offered by Banerji, 
Gherzi, Macelwane, Longuet-Higgins, and others. But 
the observations necessary to test the validity of any 
one of them are lacking. In most cases the accumulation 
of the necessary critical data would be both difficult 
and costly. For example, observations would be re- 
quired of the existence or nonexistence of standing 
ocean waves in a disturbed area of a hurricane or 
typhoon. The observation of regular “pumping” in the 
air vortex or of vertical oscillation in a water vortex 
97°16'W 
97°20'W 
27°43'N 
ee 
CORPUS CHRISTI BAY 
9 
LAGUNA 
MADRE 
27°39'N 
; 5 
(ARAL thi ae ee ee ee ee 
KILOMETERS 
Fre. 7.—Position of the U. S. Navy tripartite station MNS 
at Corpus Christi, Texas. 
