110 



Symposium on Microseisms 



Seismic refraction measurements on the 

 submerged continental shelf off the eastern 

 U. S. reveal that a wedge of sediments, thicken- 

 ing seaward, overlies the crystalline basement 

 rock. The details of the variation of sedimen- 

 tary thickness with distance from the shore 

 vary along the coast. An upper sedimentary 

 layer with acoustic properties similar to those 

 of water and a lower layer acoustically inter- 

 mediate between sea water and the crystalline 

 rock below are the major constituents of the 

 sedimentary wedge. 



That these acoustically unique features of 

 oceans are significant has been demonstrated 

 from dispersion studies of earthquake Rayleigh 

 waves in the period range 16-40 seconds. The 

 coherent, sinusoidal oscillations showing a sim- 

 ple, orderly dispersion point up the excellence 

 of the acoustic system for these periods. A 

 simple theory can account for the entire se- 

 quence arrivals of first mode Rayleigh waves 

 having oceanic paths in terms of normal mode 

 propagation over long distances in the water- 

 crystalline rock system. Predictions of the wa- 

 ter and sediment thickness as well as the nature 

 of the crystalline rock underlying ocean basins 

 have been verified by seismic refraction meas- 

 urements (Ewing and Press 1950, Ewing 

 and Press in press, Officer et al. 1952). 



J. E. Oliver has been studying shorter pe- 

 riod surface waves which propagate across the 

 oceans with periods 7-12 seconds. These oscil- 

 lations appear on transverse as well as radial 

 and vertical components, and are best recorded 

 on islands. Preliminary results suggest that 

 they consist of both Love waves and second mode 

 Rayleigh waves which are strongly refracted 

 and almost entirely absorbed at the continental 

 margins. This is in accord with the great dis- 

 continuity known to exist at the continental 

 margin. The dependence of the degree of ab- 

 sorption and refraction on period can be demon- 

 strated from surface wave studies. Investiga- 

 tion of Mantle Rayleigh waves with periods 

 greater than 60 seconds (Ewing and Press 

 1953) indicates that negligible absorption and 

 refraction occurs. Comparison of absorption 

 of first and second mode Rayleigh waves from 

 the same tremor indicates that the shorter 

 waves are much more strongly attenuated by 

 the discontinuity. Evernden (1952) has 

 shown that Rayleigh waves with periods less 

 than about 35 seconds are significantly re- 

 fracted by the continental margin. It seems 

 probable from these results that these effects 

 are even more pronounced for the shorter peri- 

 od microseisms. It is not surprising that re- 

 fraction effects and barriers are among the 

 most significant features noted by those study- 

 ing the data of tripartite stations in view of the 

 length and irregularity of the continental mar- 

 gin. 



It seems significant that the shortest period 

 surface waves from earthquakes in the Atlantic 



Ocean are above the periods generally observed 

 for microseisms. That this is not primarily 

 due to the spectrum of the source is suggested 

 by the fact that body waves occur with micro- 

 seism periods, and the T-phase often present, 

 appears with even shorter periods. 



Atmosphere-Ocean Coupling — ROSCHKE 



(1952) in a recent paper reports that micro- 

 oscillations in the atmosphere of periods less 

 than one minute reach their maximum ampli- 

 tudes in the post-cold front interval and that 

 streams of cold air are more efficient producers 

 of microoscillations than warm air. Data from 

 Columbia microbarographs are in agreement. 

 In view of the previous observation that the 

 type of air mass over the ocean is a significant 

 factor in microseism generation these results 

 strongly suggest that pressure fluctuations in 

 the atmosphere may provide energy for micro- 

 seisms in a manner as yet unknown to us. 

 More data is needed on the areal extent of these 

 oscillations as well as their oceanic amplitudes. 



It has been suggested that vertical os- 

 cillations of the water column analogous to 

 "organ pipe" vibrations may well be a signifi- 

 cant feature of the ocean-rock acoustic sys- 

 tem. Use of this concept to explain micro- 

 seisms is not new (Banerji 1935). On a 

 seismic prospect in shallow water (Burg et al. 

 1951) where the bottom was composed of 

 smooth hard rock, the predominant signal ob- 

 scuring all other waves on short spread seismo- 

 grams consisted of a repetitive pattern of the 

 "organ pipe" modes of vibration of the water 

 layer. In some cases all the modes but one 

 could be filtered revealing a long train of si- 

 nusoidal oscillations with the proper frequency 

 for that mode and water depth. Another as- 

 pect of vertical compressional oscillations of 

 the water column is revealed by a simple cal- 

 culation of the vertical displacements on the 

 ocean floor originating from steady vertical 

 oscillations applied to the surface. The results 

 show, as might be expected from the general 

 theory of transmission through plates, that 

 the ocean is an extremely sharp filter for trans- 

 mission of compressional waves from the sur- 

 face to the bottom — the sharpness originating 

 in the high impedance contrast between the 

 water and mud and the crystalline basement. 

 The peak periods, T, for waves transmitted to 

 the crystalline basement are given by 



T=iv »=!, 2, 3 



Where H is the water-unconsolidated sediment 

 thickness, v is about 5000 ft/sec. Calcula- 

 tions by Dr. Jardetzky have shown (as one 

 might expect from the general theory of fil- 

 ters) that a transient impulse applied to the sea 

 surface appears at the bottom as trains of 

 damped sinusoidal waves having periods cor- 

 responding to the "organ pipe" modes. Al- 

 though these waves can explain microseism pe- 

 riods they cannot be propagated horizontally to 



