SECT. 5] MICROSEISMS 707 



Investigations have been carried out to determine whether microseisms are 

 Rayleigh waves alone or a mixture of Love and Rayleigh waves. If they are 

 pure Rayleigh waves, some very simple properties would follow. Because of 

 the elliptical motion, the vertical component as recorded by a three-component 

 seismograph should differ in phase from the two horizontals by 90°. The sign 

 of the phase difference depends on the way the seismographs are connected 

 and the direction from which the microseisms come, and, according to the same 

 conditions, the two horizontal (E-W and N-S) component waves will be either 

 in or out of phase. For instance, at Kew Observatory, for Rayleigh waves 

 coming from NW-quadrant, E-W and N-S components are out of phase, 

 vertical being 90° ahead of N-S. For Love waves, however, for the NW- 

 quadrant, the two horizontals are in phase and they are out of phase for waves 

 coming from the SW- quadrant. 



Lee (1935) carried out an extensive investigation of the Kew microseisms to 

 determine their nature. He found the ratio of the vertical to the greater of the 

 two horizontal components after recording for a long period of time and it was 

 greater than the value of 0.68 for the simple case, being about 0.9. He ac- 

 cordingly concluded that the microseisms were Rayleigh waves travelling 

 through a thick layer of granite with a thinner layer of limestone of 1 km 

 thickness on top. Lee also found that the phase difference between waves 

 recorded on the three components were consistent with Rayleigh waves coming 

 from the quadrant in which the storm appearing to produce the microseisms 

 was situated. He thus concluded that the contribution due to Love waves was 

 negligible. Leet (1934), working on New England microseisms, had, however, 

 concluded that both types of wave were present. Later work on direction 

 finding by microseisms (section 5, page 711) has also shown that the contribu- 

 tion due to Love waves is considerable. 



3. Refraction of Microseisms 



If it is assumed that when microseisms traverse the sea, the Rayleigh waves 

 are of the type formed between a semi-infinite compressible solid with a layer 

 of water on top, as described by Stoneley (1926), then the phase velocities can 

 be calculated. The waves become dispersive and the wave velocity also depends 

 on the depth of water: the shallower the depth the greater the velocity. Micro- 

 seisms should then be refracted as they traverse different depths of water. As 

 far as is known, the depth does not affect the speed of Love waves. Estimates 

 of Rayleigh wave velocity based on this theory have been published by Scholte 

 (1943), Press and Ewing (1948) and Longuet-Higgins (1950). These estimates 

 are only of a qualitative character but they give some indication of the effect 

 of refraction. More recently, Hochstrasser and Stoneley (1961) have calculated 

 wave velocities for a two-layered medium covered by a layer of water, but values 

 for shallow depths are not yet available. Darbyshire has constructed refraction 

 diagrams for the island of Bermuda and for the British Isles. Those for 



