Sketch of the History of Microseismology 



suggested the division of microseisms into two 

 major groups : a) microseisms caused by local 

 effects and b) by distant sources of energy. 

 Group a) included microseisms from traffic 

 and industry with periods of less than 2 sec- 

 onds, from local wind storms and from local 

 surf. Group b) consisted of microseisms from 

 ocean waves (surf) with periods from 3-10 

 seconds, of microseisms with periods of about 

 l /% minute (from distant wind storms? — this 

 type may be spurious and these "microseisms" 

 may have been caused by air currents in the 

 instrument vault), and of microseisms with 

 periods of 1 or more minutes during periods 

 of frost near the station. Later, additional 

 types were reported [Gutenberg and Andrews 

 1951, Gutenberg 1951]. It is very important 

 to recognize the type which is recorded in a 

 given instance. Several types from apparent- 

 ly different causes have about the same periods, 

 but differ in their appearance. 



REFERENCES 



Hecker, 0., Seismometrische Beobachtungen in Pots- 

 dam, Veroff. Preuss. Geodat. Inst, v. 29, 1906. 



GUTENBURG, B., Ueber seismische Bodenunruhe, Phy. 

 Zeit., v. 11, pp. 414-450, 1910. 



GUTENBURG, B., Die seismische Bodenunruhe, Gerlands 

 Beitr., Geophys., v. 11, pp. 314-361, 1912. 



GUTENBURG, B., and Andrews, F., Bibliography on Mi- 

 croseisms, Mim. Rpt., Seismol. Lab., Pasadena, 

 1951. 



GUTENBURG, B., Compendium of Meteorology, pp. 1303- 

 1311, 1951. 



Discussion 



James Tinley Wilson 

 University of Michigan 



Father Macelwane has given an excellent 

 summary of the early history of the study of 

 microseisms and of the steps which have led 

 to our present state of knowledge. I do not 

 feel it necessary to comment in detail on Father 

 Macelwane's remarks, rather I would prefer 

 to present some further comments. Most, if 

 not all, of this information is already known 

 to Father Macelwane and to the rest of you 

 but it constitutes more of the data that we 

 must keep in mind when considering the sub- 

 ject. 



The fact that the study of microseisms is 

 not a closed book is evidenced by this confer- 



ence. If further evidence be needed, we have 

 but to remember that there is not complete 

 agreement among seismologists as to either 

 the nature or the origin of microseisms, al- 

 though they have been subject to study for 

 some seventy-five years. Certain aspects of 

 the subject are, of course, fairly clear. For 

 example, Omori's observations in 1901 quoted 

 by Father Macelwane would not look too out 

 of place in a modern publication. A seismolo- 

 gist of the present day might study his seismo- 

 grams (and the literature) for some time and 

 not do much better. 



The problem is such that we cannot afford 

 to neglect any possibilities and must as far as 

 possible consider all of the observations. Mi- 

 croseisms being almost always with us, we usu- 

 ally have more of the latter than we know what 

 to do with. 



The items I am going to mention come for 

 the most part under the headings of "nature" 

 or "origin." As Father Macelwane has done, I 

 will limit myself primarily to those micro- 

 seisms which have periods in the range of 

 three to twelve seconds. 



Although a case has been made on both 

 observational and theoretical grounds for mi- 

 croseisms being stationary waves of some sort 

 of "free vibration," the present concensus of 

 opinion seems to be that they are traveling 

 waves. The usual assumption has been that 

 they are surface waves and more specifically of 

 the Rayleigh type. I cannot help but feel that 

 the latter assumption is based almost solely on 

 the fact that they have a vertical component. 

 The suggestion has been made, of course, that 

 they are a mixture of Love and Rayleigh waves. 

 In this connection it might be pointed out that 

 attempts to obtain the direction of approach by 

 comparing the phase of horizontal and vertical 

 components on the assumption that the micro- 

 seisms are Rayleigh waves has not led to as 

 good results as the well known tripartite meth- 

 od. Various attempts have been made to com- 

 pare the observed periods and amplitudes with 

 those expected from Rayleigh waves in certain 

 types of crustal structures. The results have 

 usually been tantalizing but not conclusive. 



While on the subject of the nature of mi- 

 croseisms, mention might be made of factors 

 also related to origin. As noted before, it has 

 been a common observation that microseisms 

 are larger in the winter than in the summer 

 and, in at least a general way, I think it can 

 be said that they are larger in coastal regions 

 than in the continental interiors. Further, all 

 seismogram borrowing seismologists know that 

 there are certain stations, for example Perth, 

 where the microseisms are a constant nuisance. 

 This variation with season and geography 

 seems to have fathered some of the theories of 

 microseismic origin. Wiechert's surf theory 

 and the oceanic storm theories of Gherzi and 



