SECT. 5] 



MICROSEISMS 



709 



Iyer et al. (1958) assumed such a point-source to be acting at London and 

 drew rays outwards in all directions. By the reciprocity principle, this diagram 

 applies to the same rays approaching London and the change in direction can 

 be estimated. Their diagram is shown in Fig. 7. 



40° 35°30°25°20°I5° 10° 5° 0° 5' 



Fig. 7. Refraction of microseisms approaching the British Isles. (After Iyer, Lambeth and 

 Hinde, 1958, Fig. 2.) 



4. Storm Tracking and Estimation of the Direction of Approach of 



Microseisms 



It has been seen that microseisms should be a very useful tool in tracking 

 and estimating the strength of storms. The microseism period is half the wave 

 period in the storm area from which the wind speed can be estimated. If the 

 microseisms were Rayleigh waves, then the ratio of the r.m.s. amplitude of 

 the horizontal components should give the tangent of the bearing angle, and 

 the quadrant of approach could be determined from the phase relationships 

 between the three components. The use of microseisms for this purpose received 

 a great impetus during World War II and they were used to track the 

 hurricanes which wreak such havoc on the eastern American coast. Their use 



