Symposium on Microseisms Held at Arden House, Harriman, N.Y. 4-6 September 1952, Sponsored by the Office of Naval Research, and the Geophysical Research Directorate of the U.S. Air Force. (1953) / Chapter Skim
Currently Skimming:
Tripartite Stations and Direction of Approach of Microseisms
Tripartite Stations and Direction of Approach of Microseisms
Pages 9-19
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 9... ...
From January to March 1927, Nasu and Kishinouye temporarily set three horizontal pendulum seismographs near the Seismological Institute of the University of Tokyo to study the phase relation of microseisms at different places. The three stations, with that of the Institute called B, made a set of four, located at the following distances in meters: A B C B 520 920 c 920 430 D 1090 610 520 These investigators concluded, "...
|
From page 10... ...
The accurate and identical timing system, the instrumental homogeneity, and its special design for recording microseisms of periods between 3 and 9 seconds were characteristics of this new tripartite station. The results were very satisfactory in demonstrating beyond doubt that microseismic waves are traveling and not stationary waves, that their direction of propagation can be measured, that the determination of the direction of arrival at Saint Louis of these waves in all observed cases indicated that they came from tropical cyclones over the ocean; and that the bearing followed exactly the movements of the low pressure center and not the location of surf on the rocky coasts.
|
From page 11... ...
Trinidad and Antigua were also discontinued; Roosevelt Roads and Guantanamo Bay are now single stations. Finally in 1949 a special tripartite station was installed on the grounds of the Florissant seismograph station, for the purpose of studying the nature and origin of the 0.2-0.5 second period microseisms by means of special capacity seismographs developed at Saint Louis University under Dr.
|
From page 12... ...
In view of the fact that in the computation of a bearing one is in effect measuring the relative phase differences between the three recordings, it is highly essential that the three seismographs do not introduce any phase shifts into the record; or at least that the three seismographs introduce identical phase shifts. This factor demands special attention if any component of the system, such as the pendelum or galvanometer, has a natural period in the range of the periods of microseisms being recorded.
|
From page 13... ...
Figure 2 shows a graph of the ratio of apparent velocity to the real velocity as a function of the angle between the direction of propagation of two similar wave trains. This indication that the apparent velocity of the recorded microseisms is increased if two or more wave trains simultaneously cross a network has suggested a method of selecting the bearings computed from a tripartite station.
|
From page 14... ...
The interpretation placed on these and similar results for various storms is, that at those instances when the computed velocity is a reasonable value, the recorded microseisms consists of a coherent wave train coming from a single source. The question of refraction and reflection was raised in the preceding paper as a limitation on the usefulness of the tripartite station.
|
From page 15... ...
TRIPARTITE STATIONS AND DIRECTION OF APPROACH OF MICROSEISMS 15 WEATHER MAP FOR inert w««th«r map ahowa the frontal and ptttoia that nxlated 12 hours previous time ,t the targe weather map thown The .irn-i* vwheie precipitation wai fall; v-iterday ate covered with .•: A companion of the two .
|
From page 16... ...
The pounding of large ocean swells from a storm at sea upon a land mass or a continental shelf cannot be the direct cause of large storm microseisms unless an abundance of observational data are disregarded. Microseisms have been repeatedly recorded several days before the energy front from newly formed storm swells could reach a land mass on which the seismograph was located.
|
From page 17... ...
The energy front of ocean swells, racing ahead of similar storms at any average velocity should travel equal distances in water of uniform depth, regardless of the direction from which they come. In other words, if storm swells generate storm microseisms, a seismograph located on an island surrounded by several hundred miles of uniformly deep water should start recording large microseisms as soon as the hurricane is a fixed distance from the station regardless of the direction of approach.
|
From page 18... ...
Microseism bearings from Guam asked for the difference between tropical and extra-tropical storms. Van Straten replied that there are not the sharp fronts in the tropical storms.
|
From page 19... ...
The use of tripartite stations is based on the assumption that the microseisms approximate to a regular, simple wave system, with the wave crests travelling as straight lines. The fact that tripartite stations in some places, and at some times, give excellent results shows that the microseisms sometimes do travel as simple waves, but experience in Great Britain indicates that such behavior is exceptional there.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.