16 



Symposium on Microseisms 



REFERENCES 



Bungers, R., Die Uberlagerung zweier Wellen ver- 

 schiedener Herkunftsrichtung, Zeitschrift fur Geo- 

 physik, v. 15, pp. 321-332, 1939. 



Donn, W. L., and Blaik, M., A study and Evaluation 

 of the Tripartite Seismic Method of Locating Hur- 

 ricanes. Technical Report on Seismology No. 19 

 of the Lamont Geological Observatory, 1952. 



Leet, L. Don., Discussion of Tripartite Microseismic 

 Measurements. Bull. Seism. Soc, v. 39, pp. 249- 

 255, 1949. 



Kammer, E. W., and Dinger, J. E., Hurricane Well as 

 a Generator of Microseisms, J. of Meteor., v. 8, pp. 

 347-353, 1951. 



Trommsdorff, F., Untersuchungen uber die naturliche 

 Bondenruhe (Mikroseismik) mit transportablen 

 Dreikomponentenstationen. Zeitschrift fur Geo- 

 physik, v. 15, pp. 304-320, 1939. 



Discussion 



Marion H. Gilmore 

 U. S. Naval Air Station at Miami 



Father Ramirez has given an excellent 65- 

 year history on tripartite stations used in the 

 study of earth motions connected with earth- 

 quakes and microseisms. He has described the 

 systems used and mentioned some of the results 

 obtained by investigators in many countries, 

 including a few comments on his own experi- 

 ments at St. Louis University in 1939. His 

 summary of the view point of Naval Aerology 

 in 1947 and again in 1952 is essentially cor- 

 rect. 



Before one is able to discuss adequately 

 the reliability of bearings and cross-bearings 

 from microseismic storms it is first necessary 

 to show where they originate. Father Ramirez 

 dismissed this important point in these words, 

 "The results were very satisfactory in demon- 

 strating 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 ar- 

 rival at St. Louis of these waves in all ob- 

 served cases indicated that they come 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." In spite of these 

 statements there are still a few doubts concern- 

 ing the actual source of regular typhoon-hurri- 

 cane microseisms with periods of 3.5 to 6.0 

 seconds. The pounding of large ocean swells 

 from a storm at sea upon a land mass or a con- 

 tinental shelf cannot be the direct cause of 

 large storm microseisms unless an abundance 



of observational data are disregarded. Micro- 

 seisms 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. There- 

 fore, in order to establish again the fact that 

 this type of storm microseism is generated 

 when the energy from a tropical storm is trans- 

 mitted by some coupling mechanism directly 

 to the ocean floor, around the area of the storm, 

 the following observational data are submitted : 

 1. Microseisms travel approximately 100 miles 

 per minute while the energy front of storm 

 produced swells seldom exceed 18 miles per 

 hour. In other words microseisms could trav- 

 el 740 miles in seven minutes or less hut it 

 would take at least 40 hours for newly formed 

 ocean swells to travel the same distance. The 

 data presented in Figure 7 of the following 

 paper shows three typhoons passing over an 

 area of the Pacific that is almost equal dis- 

 tance from Guam, Okinawa and Manila, or ap- 

 proximately 740 miles from each station. A 

 critical analysis of the data will show that 

 each storm quickly intensified into a typhoon 

 with greater wind force and that the micro- 

 seisms, in all three cases, immediately regis- 

 tered a sharp increase in amplitude at Guam, 

 Okinawa and Manila. It is physically impos- 

 sible for swells to have had anything to do with 

 the simultaneous increase in microseisms be- 

 cause there would have been a delay of at 

 least 40 hours for the intensified swell crest 

 to reach the nearest land mass. Nor could 

 the sudden intensification, which caused the 

 increased microseismic activity, have occurred 

 two or three days earlier for, had such been 

 the case, the swells would have reached Guam 

 several days before reaching the other two sta- 

 tions. The microseismic amplitude curves, 

 Figure 1, show no such delay in either of the 

 three storms. The simultaneous arrival of 

 the larger microseisms at the three stations 

 can be explained only by the theory that energy 

 from severe tropical storms is transmitted to 

 the bottom of the ocean where it immediately 

 generates microseisms that are propagated out- 

 ward in all directions at a speed slightly great- 

 er than 100 miles per minute. It will be noted, 

 also, that the three typhoons were going away 

 from Guam and approaching the other two 

 stations. It is common knowledge that a swell 

 traveling ahead of a storm will attain great 

 height and period, while those traveling in the 

 opposite direction never become prominent. 

 But the per cent of increase in microseisms at 

 Guam was as large or larger than at the other 

 two stations. 



2. Another argument against the surf 

 theory of generation of microseisms is that the 

 amplitude of microseisms recorded at Guam, 

 Swan Island and Bermuda, islands surrounded 

 for great distances by uniformally deep water, 

 show no correlation with the state of the sea 

 surrounding them. Heavy swells may pound 



