APPLICATION OF MICROSEISMS TO FORECASTING 
this type of microseisms to vibrations caused by the 
beating of surf on a rocky coast. The fourth theory 
held that group microseisms are produced in some 
manner by storm centers at sea and are propagated 
through the earth’s crust to the continents. This theory 
was espoused by Klotz, Shaw, Lee, Banerji, and 
especially by Gherzi who suggested as the mechanism 
a “pumping” or vertical oscillation of the air vortex 
in a typhoon, hurricane, or other closed circulation 
over deep water, thus explaining why microseisms cease 
to be produced when the storm center reaches land. 
Many investigators have attempted to decide be- 
tween these various theories by correlating large 
amplitudes chronologically with other phenomena, but 
without any decisive results. Several, for example, 
Linke, Lee, Archer, Neumann, and Leet, have 
attempted to determine the direction of arrival of the 
waves, on the assumption that they are true Rayleigh 
waves, by determining the relative amplitudes of the 
three components on the records of a single station. 
This method also proved inadequate. 
A direct attack on the problem of direction of the 
propagation of the microseisms without any assumption 
was made independently by Ramirez at Saint Louis 
and by Trommsdorf at Géttmgen in 1938 by means 
of the observations of the time intervals between the 
successive arrivals of the same wave at the three corners 
of a tripartite station, using these time intervals be- 
tween successive arrivals to calculate both the direction 
of propagation and the speed of travel. Suitable 
equations for this purpose were developed by Krug, 
Gilmore, Macelwane, and Schuyler who also devised a 
nomogram. 
The following equations are valid for any tripartite 
station. Let A, B, and C (Fig. 2) be the known angles 
of the triangle named counterclockwise; and let a, b, 
and c represent the respective opposite sides of known 
length. Assume that a given microseismic wave front 
has passed over vertices B and C and is now at A. 
Drop perpendicular BP from vertex B to the wave 
front and another perpendicular CQ from vertex C to 
the wave front. Let X be the angle QAC between the 
wave front and the side b, and let Y be the angle PAB 
between the wave front and side c. Let ctn A = k and 
let the constants (b/c) see A = mand (c/b) ese A = n. 
Now let the interval of time (Fig. 3) between the 
instants at which the chosen wave crest passed over B 
and A be tsa, and that between the times of its passage 
over C and A be teu. Then, 
cone a nea = /P, 
BA 
and 
Cine — 71 tae k. 
toa 
Ramirez found that the direction from which the 
group microseisms arrived at Saint Louis always 
pointed toward a storm at sea. The U. S. Navy adopted 
the Ramirez method in 1943. While the method as 
applied by Ramirez at a single tripartite station gave 
1313 
only the direction, it showed as a matiter of fact, in the 
case of a hurricane off the Atlantic Coast, that this 
direction continuously pointed toward the center of 
the storm as it moved up the coast and did not point 
P 
Beenie —" 
— 
—S 
— 
— 
= 
— 
Zz 
20 
an 
42 
Xx ZB 
7a 
—s 
— 
— 
—_ 
Q 
Fie. 2.—A wave front of microseisms passing any tripartite 
station with a seismograph at A, B, and C, respectively. 
toward the area where heavy surf existed. The location 
of the storm center in latitude and longitude could 
only be found by cross bearings from more than one 
tripartite station. By 1945 the U.S. Navy installations 
in the Caribbean area under Orville and Gilmore had 
Nal 
pve\/ rv dev 
VEYA WA MWY 
30 SECONDS 
Fig. 3.—Simultaneous time marks showing succession of 
arrivals at stations A, B, and C. (Courtesy M. H. Gilmore.) 
obtamed satisfactory evidence that these microseisms 
actually do originate in the storm area at sea. Accord- 
ingly the method has been carried to the Pacifie Ocean 
with equal or even greater success. There is no longer 
any doubt of the origin of this type of microseisms and 
of its direct applicability to the tracking of hurricanes, 
typhoons, and other similar storms from the time of 
their formation until they reach the land. 
The seismographs in use by the U. 8. Navy are of 
the horizontal component, electromagnetic, Spreng- 
nether type. Their periods and those of the Leeds and 
Northrup galvanometers are set at about six seconds 
