34 



Symposium on Microseisms 



storm approaches the point from different di- 

 rections, unless it is far at sea, amplitude ra- 

 tios as a means of locating the storm center 

 would hardly be expected to be reliable because 

 in the two cases swells generating the micro- 

 seisms would generally reach the shore at dif- 

 ferent points. Consider for example three 

 hurricanes, A, B, and C, Figure 1, which in 

 the course of their history had passed over 

 point Q. Suppose that microseisms were gen- 

 erated near the shore as a result of the fetch 

 from the hurricanes reaching the shore at R„ 

 Ro, and R 3 , leaving the storm areas at P„ P 2 , 

 and P a , the storm in the interim having moved 

 to Q. In the illustration R, and R 2 are close to- 

 gether and would be expected to yield nearly 

 the same microseismic ratios as stations S, and 

 S S) but in case C, R 3 is closer to S 2 and the am- 

 plitude ratio S./S, would be relatively larger, 

 although other things equal, the microseisms 

 would be much smaller. 



Past and future verification of the am- 

 plitude ratio technique of every circumstance 

 may possibly verify the contention that hurri- 

 cane microseisms are generated under the 

 ocean beneath the storm area. But I have 

 strong evidence that this condition does not 

 generally apply to extra-tropical storms in the 

 western half of the Northern Hemisphere. I 

 will now outline the case history of 5 cyclonic 

 storms of the past three winter seasons and 

 will show that microseisms generated in con- 

 nection with these storms are not by virtue of 

 the position of the storm over the ocean, but 

 more probably by virtue of strong winds on 

 the shore-directed limb of the storm while the 

 storm may have been as far as several hun- 

 dred miles at sea. The microseisms, however, 

 may appear on land as much as 48 hours later 

 after the swells generated by these winds have 

 had time to reach the shore or meet oppositely 

 moving swells generated by an earlier or later 

 storm. The storm in the meantime may have 

 moved inland or far at sea, or it may have be- 

 come dissipated. 



Source of data. In this study the data 

 are from long or intermediate period seismo- 

 graphs operated by the U. S. Coast and Geo- 

 detic Survey and cooperative institutions. Data 

 from cases 1 and 3 are also from the stations 

 of the Canadian network; the Berkeley and 

 Pasadena nets, and from individual stations 

 at St. Louis, Cleveland, and Bermuda. Most 

 of the Canadian and Coast and Geodetic Sur- 

 vey data are from direct-recording seismo- 

 graphs and are therefore non-selective to the 

 periods under study. 



Source location and transmission of micro- 

 seisms. We will first make two basic assump- 

 tions. Figure 2 are shown cross-sections of 

 records from representative stations arranged 

 in order from east to west across North Amer- 

 ica. The solid lines connecting the records 



represent the same hour. The records show a 

 microseismic buildup beginning, peaking, and 

 ending about the same respective times. On 

 most records, the normal, background period 

 was 4 or 5 seconds before and after the build- 

 up. The period on all of the records was 6.0 

 sec. At the beginning and 6.5 to 6.8 sec. dur- 

 ing the peak of the buildup. It will be as- 

 sumed therefore that these microseisms were 

 generated in the same general area; and since 

 the amplitudes from east to west were pro- 

 gressively from relatively large eastward to 

 relatively small westward, it will be assumed 

 that this area was on or off the east coast. 



It follows that microseisms were trans- 

 mitted from the east coast to the west coast 

 with low attenuation and with no material in- 

 crease in period. 



The records shown here are representative 

 of all North American stations, parts of Cali- 

 fornia excepted. The history at Pasadena 

 paralleled that of other North American sta- 

 tions. Berkeley and Ukiah, on the other hand, 

 although they recorded 6 + sec. periods among 

 others as part of the normal background, re- 

 corded no increase in amplitude nor a concen- 

 tration of 6 + sec. periods during the time of 

 the buildup at other stations. Reno and Fres- 

 no have short-period instruments. Reno re- 

 corded definite 6 + sec. periods during the 

 storm, but not before or afterward. Fresno 

 definitely recorded a 6 + sec. period on the ver- 

 tical and indefinitely on the horizontal compo- 

 nents. 



It follows that the Rocky Mountain sys- 

 tem transmits 6 + sec. microseisms from east 

 to west, although not as well as the eastern 

 lowlands perhaps. Furthermore, it follows 

 that the Sierra Nevada, or more likely the Cen- 

 tral Valley of California because of its deep 

 sedimentary rocks, may possibly be an effec- 

 tive barrier to these microseisms. During this 

 time there was only slight evidence of a 6 + 

 sec. period in very faint waves during lulls in 

 local microseismic activity at Bermuda where 

 the dominant periods were 4-5 sec. There was 

 no evidence of a 6 + sec. period at San Juan. 

 Bermuda is closer to any east coast area than 

 Saskatoon, yet 6 + sec. microseisms at Saska- 

 toon were quite strong. The North Atlantic, 

 therefore, apparently absorbs 6 + sec. micro- 

 seisms. 



Figure 5 shows an arrangement similar 

 to that of Figure 2 except that amplitudes are 

 relatively much larger on the west coast, more 

 specifically on the northwest coast, than to- 

 ward the east. The parallelism in amplitude 

 buildup shown here was characteristic of all 

 North American stations except Pasadena and 

 perhaps Ukiah and Tucson. Before and after 

 the amplitude buildup the period was some- 

 what less than 7 sec. at most places. During 



