TRIPARTITE STATIONS AND DIRECTION OF 

 APPROACH OF MICROSEISMS 



J. E. Ramirez, S.J. 



Institute) Geotisico at Bogota 



Seismographs set at the corners of a tri- 

 angle were used as far back as 1884 by Milne 

 and Japanese seismologists in Tokyo for the 

 determination of the direction of approach of 

 certain earthquake waves with various and 

 generally unfavorable results [Imamura, 1902]. 



This three station distribution, generally 

 consisting of 3 vaults, one at each corner of a 

 triangle, each equipped with seismographs and 

 known today as a tripartite station, began 

 some 45 years ago to be used in an attempt to 

 determine the velocity and direction of micro- 

 seisms. 



A microseismic wave originating from 

 some distant source and traveling along the 

 earth's surface would reach one of the corners 

 of the triangle first and the other two corners 

 subsequently. By measuring the time interval 

 between the arrival of this microseismic wave 

 at the 3 vaults the direction of travel and the 

 velocity of the wave can be found. 



The first trial (at least between two sta- 

 tions) was probably made by Omori in Japan 

 in February, 1908, between the station of Hon- 

 go and Hitotsubashi, the mutual distance being 

 2.29 kms. but he "found it impossible to identi- 

 fy the individual vibrations at the two places" 

 [Omori, 1909, 1913]. 



Hecker [1915] made a trial at Strasbourg 

 in 1915. He used the NS and EW component 

 of the observatory and placed a NS component 

 first 0.58 km. due north of the central station, 

 and later located it 2.4 km. northeast of the 

 observatory. 



In the first case, both stations receiving 

 the time signals from the same clock, the waves 

 at the instrument located 0.58 km. south, ar- 

 rived now earlier and now later than at the 

 other station (in one instance there was a maxi- 

 mum change of 1.0 second, during an interval 

 of 1^4 minutes). In the second case, at a dis- 

 tance of 2.4 km. and with two different clocks 

 marking the time signals, there were greater 

 differences in the arrival times. The experi- 

 ment was discontinued and "only the observa- 

 tions of one day were the ones that could be 



used and even on this particular day micro- 

 seisms were not so strong. 



Shaw [1922] reported at the Rome meet- 

 ing of the first conference of the section of 

 Seismology of the International Union of Geod- 

 esy and Geophysics upon his work in connec- 

 tion with microseisms and stated that at West 

 Bromwich in 1918 during some experiments 

 with Milne-Shaw seismographs situated 20 me- 

 ters apart and in different building he noticed 

 that the recorded microseisms were identical. 

 During the spring 1919, and 1920, he demon- 

 strated that at a distance of 3 km. each micro- 

 seismic wave was still similar on each record. 

 In the early part of 1921 an attempt was made 

 to compare three stations about 16 km. apart. 

 At this distance the waves were quite different 

 and it was impossible to identify them for 

 comparison. In the first months of the year 

 1922 two stations were arranged 4 km. apart 

 and on different directions to the stations used 

 in 1919 and 1920. At this distance the waves 

 were again identified without difficulty. In 

 the 1919 and 1920 experiments the differences 

 in time of reception at the two stations were 

 0.83 seconds. The method gave some evidence 

 that microseisms came from a north-westerly 

 direction to west Bromwich. 



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 : 



These investigators concluded, "... the 

 observations are not sufficient to yield definite 

 results, for they were obtained from records 

 of NS component only." Furthermore, "it 

 was very hard to find corresponding minute 

 marks on records. So the comparison of rec- 

 ords of four stations was drawn only in several 

 cases . . . The variations of amplitude like beats 

 may be due to oscillations of different period 



