Symposium on Microseisms 



5. The direction of motion changes con- 

 stantly, and each horizontal component shows 

 a series of alternations of maximum and mini- 

 mum groups ; . . . . the average period being 

 6.8 s " 



Subsequently Omori [1903] wrote: "It 

 thus seems probable that the pulsatory oscilla- 

 tions are essentially composed of two series of 

 vibrations, whose periods are respectively 

 about 4 sec. and 8 sec. ; large pulsatory move- 

 ments which are caused by very deep cyclones 

 having generally the 8 seconds period." 



Ten years later Omori [1913] reported a 

 comparison of the microseisms recorded at 

 Tokyo with those recorded at Hitotsubashi 

 about two kilometers distant in which he found 

 it impossible to identify individual vibrations 

 on the two sets of records. This negative re- 

 sult, which may have been due to instrumental 

 deficiencies probably led Honda [ ] and 

 other later Japanese investigators to adopt the 

 view that microseisms are stationary waves 

 set up at the locality of the observing station. 



At the meetings of the International Seis- 

 mological conferences and later of the Interna- 

 tional Seismological Association, microseisms 

 were of interest from the first and a standing 

 committee for the study of microseisms was 

 soon appointed. It was at the second Inter- 

 national conference held at Strasbourg in 1903 

 that Wiechert [1905] proposed his well-known 

 surf theory. Laska [1902] on the other hand 

 correlated maximum amplitudes of micro- 

 seisms with steepest barometric gradients on 

 the basis of the records at Lemberg. Klotz 

 [1909] presented a report at the Zermatt meet- 

 ing of the International Seismological Associa- 

 tion in 1908 in which he said among other 

 things : "5. A well-marked Low sweeping up 

 the Atlantic Coast from Florida to Newfound- 

 land is almost always accompanied by marked 

 microseisms. 6. Microseisms are but slight- 

 ly, if at all, influenced by the movements of 

 Lows across the continent." 



It must be remembered that in nearly all of 

 these early investigations attention was fo- 

 cussed on the band of microseismic frequencies 

 which lies between one-fourth and one-eighth 

 herz or those of still lower frequency because 

 these microseisms were so prominent on the 

 records of the seismographs then in use. The 

 study of microseisms of higher frequency be- 

 came possible much later with the introduc- 

 tion of more suitable types of instrumentation. 



Gutenberg [1910] published his doctoral 

 dissertation at Goettingen in which he pre- 

 sented extensive data which he interpreted as 

 supporting Wiechert's surf theory and he cor- 

 related the microseisms of four to ten seconds 

 in Germany with surf on the southern part of 

 the west coast of Norway. This conclusion he 

 supported vigorously through the succeeding 



years [Gutenberg 1912, 1921, 1924, 1927, 1928, 

 1931, 1936]. More recently he has modified 

 and broadened his views in accord with his ex- 

 cellent later researches. 



In addition to the surf theory, three other 

 main theories or groups of theories have been 

 advanced to explain microseisms. These are 

 first, theories of local origin, meteorological 

 or geological, at or near the recording station, 

 secondly, theories of thermometric or baro- 

 metric gradients travelling over continental 

 areas; and thirdly, theories connected with 

 storms or storm waves at sea. 



The last named theory casually proposed 

 by Bertelli [1878] and by Omori [1903] and 

 formally reported by Klotz [1909] as a result 

 of his observations at Ottawa, and specifically 

 formulated and defended by Gherzi [1923, 

 1924, 1926a, 1926b, 1928, 1930, 1937], by Ban- 

 erji [1929, 1930, 1935] and by Zanon [1936, 

 1938] in the nineteen twenties and thirties, has 

 come to occupy the center of the modern micro- 

 seismic stage. 



Most of these studies were made by tem- 

 poral correlation, as were a number of more 

 recent investigations. At the second meeting 

 of the Eastern Section of the Seismological So- 

 ciety of America held in Ottawa, Canada, a 

 committee was appointed to correlate and map 

 microseismic amplitudes recorded at all seismo- 

 graphic stations in the United States and Can- 

 ada. A more ambitious program of correla- 

 tion has been inaugurated by the Association 

 of Seismology of the International Geodetic 

 and Geophysical Union in 1952 involving simul- 

 taneous observations of microseisms in the 

 whole world. 



However, many seismologists have felt 

 that temporal correlation of amplitudes and 

 periods from place to place is not sufficient to 

 distinguish between possible sources and have 

 sought to determine the bearing of the origin 

 through measurements of the direction of prop- 

 agation of the microseismic waves. Some have 

 attempted to do this by vector methods, using 

 the separately recorded components on the as- 

 sumption that microseisms are Rayleigh waves. 

 Others beginning with Omori [1913] and 

 Hecker [1915] attempted to find the direction 

 independently of any assumption by means of 

 the time interval between arrivals at closely 

 spaced recording stations but failed because of 

 inadequate instrumentation. Shaw [1922] in 

 the years 1918 to 1922 obtained some evidence 

 by this method that microseisms arrived at 

 West Bromwich, England, from the northwest. 

 Krug [1937] at Goettingen in 1936-1937 used 

 three individually timed portable seismographs 

 at the corners of an isoceles triangle with lim- 

 ited success. But the following year 1937 

 Trommsdorff [1939] and Ramirez [1940], in- 

 dependently using the simultaneously timed tri- 

 partite station method, found that the bear- 



