12 



Symposium on Microseisms 



Imamura, A., Seismic triangulation in Tokyo, Publica- 

 tions of the Earthquake Investigation Committee 

 in Foreign Languages, No. 7, pp. 5-24, 1902. 



Kishinouye, F., Microseisms of four second period ob- 

 served with horizontal seismographs, Bulletm of 

 the Earthquake Research Institute, v. 13, pp. 146- 

 154, and 608-615, 1935. 



Krug, H. D., Ausbreiting der naturlichen Bodenunrule 

 (Mikrosimik) nach Aufzeichnungen mit transpor- 

 tablen Horizontal-Seismographen, Zeitschrift f. 

 Geophys., v. 13, Heft 7-8, pp. 328-348, 1937. 



Macelwane, J. B., Storms and the origin of micro- 

 seisms, Annales de Geophysique, v. 2, fasc. 4, p. 6, 

 1946. 



Shaw, E., Communication de M. J. J. Shaw sur les 

 mouvements microseismiques, Compt.es Rendus des 

 Seances de la Premiere Conference Reunie a Rome 

 du 2 au 10 Mai 1922, Union Geodesique et Geophys- 

 ique Internationale, pp. 52-53, 1922. 



Omori, F., Report on the observation of pulsatory oscil- 

 lations in Japan, Bulletin of the Imperial Earth- 

 quake Investigation Committee of Tokyo, III, No. 

 1, pp. 1-35, v. 5, No. 3, pp. 109-147, 1909, 1913. 



Supplement to U. S. Navy hurricane microseismic re- 

 search. Navaer 50-ir-189, May, 1947a. 



Neiv developments in Naval aerology for reserve aero- 

 logists, v. 1, p. 2, June 1947b. 



Discussion 



J. E. Dinger 

 Naval Research Laboratory 



The tripartite station is a useful tool for 

 studying the direction of approach of micro- 

 seisms. However, in any discussion of a tri- 

 partite station it is essential to point out the 

 limitations which must be taken into considera- 

 tion if one is to obtain the greatest usefulness 

 from the tripartite network. 



First, one must recognize the tolerance im- 

 posed on the accuracy of computed bearings 

 when taking into account the maximum accur- 

 acy of the measurements obtained from a given 

 set of tripartite instruments. The size and 

 shape of the tripartite network, the speed of 

 the paper, and the sharpness of the trace all 

 affect the maximum accuracy that can be 

 achieved, assuming that well-formed micro- 

 seisms are being propagated across the tri- 

 partite network. In fact, the accuracy varies 

 with the direction of approach to a given net- 

 work. A triangle having one large oblique 

 angle will have a considerably greater accur- 

 acy if the microseisms approach along a direc- 

 tion parallel to the long side than will be the 

 case if the approach is at right angles to this 

 long side. An equilateral triangle will come 

 the nearest to giving equal accuracies in all 

 directions. Let us take one typical network 



to illustrate the instrumentation errors one 

 might encounter. Assume an equilateral tri- 

 angle having sides of 4,000 feet, a chart re- 

 cording speed of 150.00 cm/min, and a micro- 

 seismic wave traveling across the network at 

 8,000 ft/sec in a direction which bisects one 

 of the angles. If one can superimpose the 

 traces with an accuracy of ± 1 mm, the maxi- 

 mum errors that can enter the bearing compu- 

 tation will be ± 11° ; a spread of 22°. This 

 example I believe approaches the ultimate in 

 instrumental accuracy ; in practice the errors 

 may be considerably larger. 



In view of the fact that in the computa- 

 tion 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 pende- 

 lum or galvanometer, has a natural period 

 in the range of the periods of microseisms be- 

 ing recorded. Strict attention must be paid 

 to proper damping of all such components. So 

 far as phase shift is concerned a seismometer 

 working into an electronic amplifier which in 

 turn actuates the recording mechanism is to be 

 preferred over a seismometer working directly 

 into a recording galvanometer. In the former 

 case there is no reaction of recording element 

 on the seismometer to complicate phase rela- 

 tions. 



A serious limitation of the tripartite sta- 

 tion arises from the very nature of the micro- 

 seisms. This limitation has been pointed out 

 in the literature by a number of writers, among 

 them being Trommsdorff [1939], Bungers 

 [1939], Leet [1949], Donn & Blaik [1952], 

 and Kammer & Dinger [1951]. This limita- 

 tion arises from the observation that, in gen- 

 eral, microseisms crossing a tripartite station 

 do not consist of a single coherent wave train 

 but rather are the composite of several wave 

 trains which may differ in direction, period, 

 and wave type. One obtains evidence that the 

 microseisms do not consist of a coherent wave 

 when the separation of the three seismometers 

 is large (several miles), for in this case it is 

 often difficult to identify the corresponding por- 

 tions of the three records. The lack of co- 

 herency is also illustrated by Leet [1949] in a 

 five-minute sample record made by a three- 

 component seismograph. This sample record 

 shows a mixture of Love and Rayleigh waves. 

 Leet suggests using a three-component regis- 

 tration at each corner of the triangle so that 

 the type of wave motion in a given interval of 

 the record can be determined. A complete rec- 

 ord of this sort will possibly permit one to se- 

 lect wisely the portion of the record to be used 

 for bearing computation. 



