OBSERVATIONS AND THEORY OF MICROSEISMS 
the ocean deeps off Kamchatka and Japan, then runs 
to the east of the Marianas and of the Palau Islands 
and then southeastward off New Guinea and the Solo- 
mon Islands. Japanese scientists have pointed out that 
typhoons to the east of Japan are accompanied by large 
microseisms at the stations along the east coast of 
Japan, but only by imsignificant microseisms on the 
west coast of Japan. The station operated at Guam 
under the project of the United States Navy Depart- 
ment is close to the andesite line. It records microseisms 
from typhoons which are centered far away in the ocean 
HURRICANE 
“| 10MM, 
TRACE AMPLITUDE 
% STORM NEAREST 
TO STATION 
RICHMOND,FLA. R 
GUANTANAMO G 
SAN JUAN S 
REYKJAVIK 
1945, seeT, lielishalslieliziie 
ESKDALEMUIR 
CARTUJA 
VICTORIA 
PASADENA 
TUCSON 
SASKATOON 
MILWAUKEE 
ZURICH 
VIENNA 
CHICAGO 
FLORISSANT 
ST. LOUIS 
OTTAWA 
SEVEN FALLS 
CAMBRIDGE 
WASHINGTON 
CHARLOTTESVILLE 
1930, MARCH lzaleslzelezlza 
PULKOVO 
MAKEEVKA 
TIFLIS 
BAKU 
EKATERINENBURG 
TASHKENT 
IRKUTSK 
UPSALA Tas 
1914, van.-Fee. lsilil2is 
1309 
Much smaller effects are to be expected from dis- 
placements along faults inside the major crustal blocks. 
Such faults cover greater parts of the earth’s surface 
than is generally believed by nongeologists. Amplitudes 
of microseisms passing them (e.g., along the Pacific 
coast of the United States) are not much reduced, espe- 
cially in instances where the velocities on both sides of 
the fault zone do not differ much, thus contrasting with 
the deep structural discontinuities of the type repre- 
sented by the andesite line surrounding the Pacific 
Basin. Microseisms are propagated across large parts 
10u, 
GROUND AMPLITUDE 
REYKJAVIK 
OE BILT 
SCORESBY SUND 
PARC ST. MAUR 
IVIGTUT 
KEW 
NEUCHATEL 
HAMBURG 
STRASBOURG 
POTSDAM 
MUNICH 
LEIPZIG 
LUND 
VIENNA 
COPENHAGEN 
PULKOVO 
ABISKO 
1930, van. lalgloliliaelistalistie 
Fr¢. 5.—Amplitudes of microseisms: (a) in the Caribbean during a hurricane (after Gilmore), (b) in North America during a 
storm in eastern Canada (after Gutenberg), (c) in Europe and Asia during a storm approaching northern Norway (after 
Gutenberg), (d) in Europe and Greenland during a storm north of Scotland (afler Lee). (Taken from Gutenberg [16].) 
southeast of Japan; there is some indication that ty- 
phoons moving westward south of Guam produce rela- 
tively irregular and smalJl microseisms at Guam until 
they cross the andesite line in the Caroline Islands. 
The Caribbean loop is usually considered to be the 
boundary of a block with ‘Pacific’ (less andesitic) 
material in the interior of the Caribbean. This explains 
why records of microseisms produced by hurricanes in 
the Caribbean area and recorded there at stations of 
the United States Navy Department show that the 
amplitudes of the microseisms decrease rapidly from 
one island to the other (Fig. 5). For other regions, see 
[5, 20, 21, 29]. 
of the United States without much loss of energy be- 
yond regular absorption (Fig. 5b). 
Effect of the Ground on Microseisms. Amplitudes of 
elastic waves at the surface of the earth depend appre- 
ciably upon the ground on which the instrument is 
located [15, p. 272; 20; 21]. For example, ground satu- 
rated with water is more likely to vibrate with large 
amplitudes than is solid rock, thus providing “‘increased 
sensitivity.”’ However, the greater complexity of the 
records from such stations is a disadvantage and fre- 
quently more than offsets the advantage of the greater 
amplitude. For example, the station installed at Corpus 
Christi, Texas, in 1946 on loose sand recorded such 
