Tripartite Stations and Direction of Approach of Microseisms 



17 



against a steep coast within one hundred feet 

 of a seismograph or approach a continental 

 shelf without causing the least increase in 

 hurricane type microseisms. At other times 

 each of the above stations have recorded micro- 

 seisms more than ten times normal amplitude 

 while the state of the sea around the islands 

 ranged from a dead calm to swells of only one 

 to three feet. Many other investigators also 

 have reported a complete lack of correlation 

 between surf and microseismic amplitude. 



3. There is still another obvious reason 

 why storm microseisms used in tracking severe 

 tropical disturbances could not be caused by 

 ocean swells striking a coast. The energy 

 front of ocean swells, racing ahead of similar 

 storms at any average velocity should travel 

 equal distances in water of uniform depth, re- 

 gardless of the direction from which they 

 come. In other words, if storm swells gener- 

 ate storm microseisms, a seismograph located 

 on an island surrounded by several hundred 

 miles of uniformly deep water should start re- 

 cording large microseisms as soon as the hurri- 

 cane is a fixed distance from the station re- 

 gardless of the direction of approach. If this 

 were true lines drawn around such a station, 

 for example Bermuda, showing the location of a 

 90 knot hurricane when microseisms are first 

 recorded should be nearly circular. But this is 

 far from true. The Bermuda seismograph has 

 recorded many large microseismic storms when 

 hurricanes of similar intensity moved directly 

 toward the station from the east, the south, or 

 the west and these data were used to prepare 

 the chart shown in Figure 6 of the following 

 paper. The microseismic range for hurricanes 

 located south or west of the station is double 

 the range to the east. It is therefore obvious 

 that swell activity reaching the Bermuda coast 

 cannot account for the generation of such storm 

 microseisms because the limiting distance is 

 not the same in all directions. The only theory 

 that agrees with all the available data is that 

 the microseisms are generated in the ocean 

 bed in the vicinity of the disturbance. 



The microseisms generated in the vicinity 

 of a storm are transmitted outward in all direc- 

 tions through the earth's crust according to 

 established laws of physics pertaining to wave 

 motion in an elastic medium. Wave motion 

 through a perfectly homogeneous substance is 

 transmitted in straight lines from the source 

 and decreases in amplitude, in direct propor- 

 tion to the square of the area covered. How- 

 ever, such conditions of complete homogenity 

 exist over very limited areas of the earth. Thus, 

 it is only natural to expect that most micro- 

 seisms generated by storms do not arrive at a 

 station from the exact direction of the storms. 

 "Microseismic Barriers," major discontinuities 

 in the earth's crust or gradual changes in the 

 density and elasticity of a portion of the earth's 

 crust, are very logical phenomena that appear 

 to reflect, to refract, and to absorb microseisms. 



The following conclusions are therefore 

 apparent: (a) The true origin of storm micro- 

 seisms appears to be in the area of strong 

 winds of a hurricane or typhoon, (b) It is 

 possible to calculate accurately the direction 

 microseisms are traveling when they pass over 

 a tripartite station. If such microseisms from 

 tropical storms were always propagated out- 

 ward in concentric circles it would be very 

 simple to determine their exact origin and the 

 location of the storm by means of cross bearings 

 from two or more tripartite stations. How- 

 ever, microseisms, as explained above, do not 

 always travel in straight lines, and this results 

 in large errors unless maps of the characteristic 

 refraction patterns are constructed around 

 each tripartite station. Until such charts are 

 made and the proper corrections applied micro- 

 seismic bearings will continue to look like 

 those in Figure 1. 



Discussion from the Floor 



Melton. I would like to comment at this point 

 that many of our observations constitute a sta- 

 tistical problem, and it should be valuable to 

 examine some of our observations in that light. 

 This term "coherence" which we have been 

 using is much discussed in the literature, and, 

 in particular, the subject of cross correlation 

 as we move these two seismographs apart. It 

 is obvious that if we place them on the same 

 pier their correlation should be unity or 100 

 per cent, provided the instruments are operat- 

 ing properly. We should like to see the curve 

 of this correlation factor as the separation is 

 increased to the order of distances we have 

 been discussing. 



Byerly. (Commenting on tripartite measure- 

 ments) It makes a difference how you set 

 these seismographs down, too. You can set 

 them down side by side and they won't show 

 the same thing at all if you don't set them down 

 properly. 



Melton. Yes. But this business of properly 

 planting a seismograph is a controllable thing. 

 Working in the marshes of Louisiana which 

 literally float, I have observed that when re- 

 flection instruments were simply set down in 

 the mud, some of the reflections came out ly- 

 ing on their backs. However, the proper way 

 to plant such instruments is on about 20 feet 

 of pipe pushed firmly into the marsh, and if 

 one plants them this way the reflections come 

 out properly. 



(Byerly asked if there were any cases of mi- 

 croseisms and no storms. Gilmore replied that 

 there are a few cases. Peoples asked if the 

 reverse was true in any case. Gilmore's reply 

 was yes, but then it turns out the storm has 

 been over-rated in intensity. Melton asked 

 about the reliability of the intensity of the 

 storms. Van Straten replied that as the storms 

 slow down they usually intensify. Macehvane 



