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ABSTRACT

 Microseismic observation was carried out at Nagoya University during the International Geophysical Year 1957-58 and later periods.The seismograph was deviced for this observation.The general nature of microseisms as well as microseismic storms accompanying typhoons and cyclones was studied.The comparative observation of microseisms was also performed at various stations of the Nobi Plain,centering around Nagoya.
 The microseisms dominant all the year round at Nagoya University have about 3 sec period and 3 to 8 μ amplitude.The amplitude in the winter season is nearly twice as much as that in the summer normal period.When microseismic storms culminate,the period comes to present the second peak at 5-7 sec in the case of cyclone,or 8-9 sec in the case of typhoon,in addition to the first peak at about 3 sec.As to amplitude,its distribution becomes so uniform that any dominant amplitudes are hardly pointed out.The result of data of several typhoons shows that many of the observed variations of microseismic amplitude and period may be explained simultaneously by regarding the shore line near the observation point as a source area of microseisms.The results of comparative observations of microseisms are summarized below.
 The predominant period of microseisms in Nagoya and its vicinity is 3 to 4 sec which is understood as the foundation characteristics representing a generalized geologic structure of the area.Amplitude varies considerably with location of observation point,ranging in value from 0.2 to 8 when the value at Nagoya University is taken as unity,value of amplitude is related to the thickness of the soft foundation represented by Quaternary beds.

INTRODUCTION

 Microseismic observations have been carried out as a study in the International Geophysical Year program which was commenced in July 1957.Part of the results has been already reported [8],revealing value of such observations.Pertaining to the mechanism of generation of microseisms under abnormal meteorological conditions,a tentative conclusion has been reached.However,microseismic motions are largely influenced by regional characteristics of the land,and this fact adds importance to local observations.Ever since the IGY program was started,geophysicists of Nagoya University have been performing continuous observation of microseisms and the result has been partly reported [3].
 The writers of the present paper studied on(i)general nature of microseisms as observed in the Nagoya district,and(ii)characteristics of microseismic storm caused by meteorological abnormality on the basis of the one-point observation data obtained during the IGY period.The result of the study disclosed that the data were not sufficient for understanding the general nature of microseisms in this district.Hence,the writers performed comparative observations of microseisms in Nagoya City and the surrounding areas.The result is reported herein.PartI deals chiefly with the result of one-point observations by Nagoya University,and PartII with the result of comparative observations.In this paper the writers attempted to clarify the mechanism of generation and propagation of microseisms.
 The seismograph used in this study was specially devised by the senior writer for observation of microseisms.Constants are listed in Table 1.Characteristic curves are shown in Fig.1.Fig.2 is a photograph of the general view of the seismograph.

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FIG.1.Frequency response curve of the seismograph.Ordinate:Magnification.Abscissa:Period.
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写真 FIG.2.Photograph of the seismograph.
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TABLE1.CONSTANTS OF INSTRUMENTS

PART I.CONTINUOUS OBSERVATION OF MICROSEISMS IN NAGOYA

Array1.General nature of microseisms in the district
 Except for the observations performed by us during the IGY period,no record of systematic observation of microseisms in the region around Nagoya is known.Thereupon,in the first place we had to acquire general knowledge of amplitude and period of microseisms,as well as their seasonal variations,in the area to be studied.We examined the records(two horizontal components 1 of 1958 for the most part,and read the amplitudes following the IGY microseismic observation regulations.One point of amplitude for each day was picked up and a monthly running average of vicissitude was calculated.The result is given in Fig.3 which may be regarded as representing seasonal changes in the amplitude of microseisms.It is known from the figure that the amplitude variation with direction is not conspicuous,and seasonal or any regular changes are almost unrecognizable.A similar fact is found in the records of observations in other districts.However,it can be pointed out,if necessary,that the N-S component is slightly larger than the E-W component in August and September.
 The average amplitude for the year is about 10 μ which tells that Nagoya is one of the areas of most remarkable microseisms,like Tokyo,Sakata and Tori shima.Considering the fact that Tokyo and Sakata(Yamagata Prefecture)are situated in the geologically young sedimentary plains,the large amplitudes of microseisms in Nagoya are not contradictory to the geology of the neighborhood of our observation station,consisting of Pleistocene sediments.
 The amplitude of microseisms is believed to vary greatly with season,attaining its maximum generally in winter and becoming smaller in summer.Our case is not exceptional.The amplitude as small as 7 p in June to July becomes twice as much,15 μ,in December to January.The abrupt rise in the amplitude during the period from August to September may be explained by the fact that meteorological abnormality(especially typhoon)is most frequent during this period,giving rise to microseismic storms.For comparison,the data related to the E-W components in the records of amplitude in Tokyo during the same period are plotted in Fig.3.The figure reveals the same tendency as mentioned above.According to the annual records,the maximum amplitude is about 100 μ,seldom exceeding this value even in a fairly strong microseismic storm.
 The foregoing paragraphs have dealt with the amplitude.Now,as to the period,annual changes are very small,being about 3 sec throughout the year,as was already reported partially.This tendency is one of the marked differences between Nagoya and other districts.
 Therefore,we made somewhat detailed researches on the distribution of amplitude and period.For the sake of convenience,we divided one year into two seasons,summer(April-September)and winter(October-March),referring to the past results.For each season we obtained the frequency distribution by the following method:In the summer season,we selected seven periods of two days each during which no microseismic storm or similar abnormal phenomenon took place(such period will be called here a normal time of microseisms),and microseismic waves recorded in about 20minutes of each period were picked up.Prior to this,we had ascertained that an almost stationary frequency distribution would be obtained by reading the microseismic waves for every five minutes separately and then reading continuously for about 20minutes.Thus,about 3000 waves in the normal times of the summer season were examined and the frequency distributions of microseismic amplitude and period were obtained,as illustrated in Figs.4(a)and 5(a).As to the winter season,on the other hand,there was no calm period that lasted as long as two days,so we had to set the duration of normal time at one day each.Reading of about 3000 waves was performed likewise.The result is illustrated in Figs.4(b)and 5(b).The figures can be regarded as representing the general characteristics of amplitude and period in normal times.
 In these figures,it is noticed that the amplitude and the period are symmetrically distributed on both sides of the dominant value in each period.In other words,both amplitude and period have only one dominant value,without the second and the third dominant values as found in the observation data of Tokyo and Kyoto [8].
 Comparison between Figs.4(a)and 4(b)reveals that the dominant amplitude in the summer normal time is 2.5 to 3 μ,I,whereas that in the winter season attains to 5 μ nearly twice as much the former,as in the case of Fig.3.The upper limit of the amplitude is 7 to 8 μ in the former,while in the latter the value becomes as large as 13 μ Thus,the amplitude distribution is clearly different between the summer and the winter seasons.As to the period,however,the distribution shows good concordance between the two seasons even in detailed points,including the values of the dominant period(2.9 sec in summer,2.9 sec in winter),the upper limit(4.9 sec in summer,5.1 sec in winter),and the range(1.3 to 4.9 sec in summer,1.3 to 5.1 sec in winter).The values larger than 4 sec in winter are exceptional,but these may be ascribed to the duration of a calm time which was about one day shorter than that of the summer normal period.These facts will lead to a conclusion that the distribution of the period during calm times of microseisms is generally constant throughout the year.
 What would,then,be the cause of such marked difference in amplitude between times as mentioned above? Here let us take a look at Figs.6(a)and 6(b)which were prepared from the data of one day(June 18,1958)when a weak cyclone was situated near Japan and a cold front had just passed through the country.With regard to the period,Fig.6 bears a close resemblance to Figs.4(b)and 5(b),rather than to Figs.4(a)and 5(a).As to the amplitude,the resemblance is closer to the winter tendency than to summer,although Fig.6(b)has somewhat larger values.This relationship is applicable to other less abnormal meteorological conditions.(For example,refer to Section 2 which discusses the relationship between cyclones and microseisms.)
 Thus,it may be summarized that the influence of meteorological abnormality upon microseisms is manifested more conspicuously by the increase of amplitude than by the elongation of period.This result agrees well with those obtained by other investigators [7,10,11].The markedly large values of dominant and maximum amplitudes in the winter normal times are explained by that the variation of meteorological abnormality is far greater in winter than in summer,hence the influence appears in the amplitude of microseisms during the periods which we regarded as the winter normal times.
 On the basis of the above-mentioned results,we can conclude that the microseisms dominant all the year round at the observation point in the compounds of Nagoya University have about 3 sec period and 3 to 8 it amplitude.
2.Microseismic storms accompanying abnormal meteorological conditions
 The foregoing section has discussed the results of observations of the days when meteorological changes were relatively small,that is,calm periods of microseisms.It is known,however,that amplitude and period of microseisms present distinct changes−microseismic storms—with approaching meteorological abnormality such as typhoon and cyclone.Accordingly,if the mode of changes of microseisms is studied,in relation to the kind,scope,character,and course of progress of the meteorological abnormality,we may find a clue useful for understanding the mechanism of generation of microseisms.Many of the past research works on microseisms are supposed to have been performed from this point of view.We shall proceed with our discussion from the same view-point.The data herein dealt with were selected from among the readable records of the times of abnormal meteorological conditions during the years of 1958 through 1960.For convenience of explanation,the data were divided into two portions,one is for the microseismic storms that are most conspicuously manifested in typhoons,and the other is for those accompanying cyclones.
 I.Microseisms accompanying typhoons.—Typhoons used in our study are four(one in 1958;three in 1960),and their scope and path are illustrated in Fig.7.Record reading was made at intervals of 12hours,as represented in Fig.7 by the black circles on the path of each typhoon.A few examples of the frequency distribution thus obtained are given in Figs.8 and 9.These are the results pertaining to T.5811 and T.6014.For comparison,the data of Fig.4(a)and Fig.5(a)—distributions in the summer normal times—are plotted in the distribution diagram of T.5811.
 Examining the frequency distribution of amplitude and period,in reference to the path of typhoons in Fig.7,we find that the distribution bears a close resemblance to that in the summer normal times,if the location of the typhoon is far away from Japan(the distance between the center of the typhoon and the observation point is greater than 1600km).That is to say,any influence of the typhoon is not recognized yet.As the typhoon approaches,however,the amplitude shows a gradual increase,and two maxima that are barely noticeable at 900km distance expand markedly at 400km distance.Especially,the rise of the upper limit of amplitude is much greater than the increase of dominant amplitudes.When microseismic storms culminate,the period comes to present the second peak at 5-7 sec,in addition to the first peak at about 3 sec.As to the amplitude,its distribution becomes so uniform that any dominant amplitudes are hardly pointed out.For easier understanding of this status,we prepared Figs.10(a)and(b)showing the maximum and the mean maximum values of amplitude and period which were picked out of 12 hour each distributions,and their relations to the time.The maximum amplitude or period represents the maximum values in the read records,and the mean maximum is the average of several values before and after the largest amplitude or period.
 Ever since the theory of 「stationary waves」 was advocated by Longuet-Higgins [5].a most prevalent idea has been that the relationship between meteorological abnormality and generation of microseisms should be ascribed directly to a change in pressure exerted on the sea bottom by the waves caused by abnormal meteorological conditions.At present,discussion of the generation of microseisms seems to be focused rather on the source of stationary waves.With regard to this subject,there are the following three views:
(1)The central area of a typhoon is where the stationary waves will be generated.
(2)Waves generated in the central area of a typhoon approach the coast,and there the stationary waves will be generated.
(3)Both(1)and(2)are the cause of stationary waves.
 No matter which of the above three views is accepted,the main point of argument is laid on the relationship between the location of a typhoon center and the direction of propagation of microseisms,and on the time of changes in the microseismic amplitude and period correlated with the time of the records made by the sea wave meters on the coast.Santo [11,12,13],Okano [9,10] and others think,on the basis of the IGY data,that the view of(2)is most appropriate as far as the microseisms observed in Japan are concerned.Keeping this view in mind,let us examine Figs.10(a)and(b).In both amplitude and period the maximum value(or the mean maximum value)increased roughly in proportion to the approach of a typhoon.Increase in amplitude is more conspicuous and rapid than increase in period,so that even after the amplitude reached its maximum(20th,15h)and began to decrease the period is still increasing.Similar relations were reported by Dinger [1] and others.However,Okano [9] states that a reverse relation is possible.That is,depending on the typhoon’s path,the degree of its development,and the location of the observation point,the time when the maximum amplitude can be attained is later than the time when the period reaches its maximum.Comparison of Fig.10 with Fig.7 reveals that the time when the microseismic storm culminated is roughly coincident with the time when the typhoon neared the observation point.This fact may be explained as follows:Although the extension of the central area of a typhoon is difficult to estimate as it is intricately related to the central pressure,the barometric gradient,the travel time of wind,etc.,it is believed to be 250km in radius when the central pressure is over 985mb and about 50km if the central pressure exceeds 1000mb.Using these values,the size of the disturbance source of typhoon T 6014 is assumed to be about 250km,and that of T.6012 is about 100km.This indicates that when these typhoons were nearest to Japan,the Pacific coast(T.6014)or the Japan Sea coast(T.6011,T.6012)adjacent to the observation point was already within the source of disturbance.As a result,the time when the microseismic storm reached its maximum would coincide with the time the typhoon was nearest to the land.On the other hand,the intensity of microseismic storms is far greater in T.6014 than in T.6011,despite that the size of disturbance source is nearly the same in the two typhoons.The path of typhoon T.6011 runs from the southern tip of Shikoku,passes through the Kii Channel,and advances to the Japan Sea,whereas that of typhoon T.6014 progresses eastward along the Pacific coast.Judging from these different paths,it may be said that the microseisms observed in Nagoya are more conspicuous when the source of disturbance is located on the Pacific side than when it is found on the Japan Sea side.And yet,it is not known whether this fact is attributable to the difference in the distance from the source of microseisms or to the difference in the coastal topography as suggested by Okano [9] who believes that the terrain of the Pacific coast is in favor of generation of microseisms.In the cases of T.6011 and T.6012 which took nearly identical paths,the latter is of a much larger scale,and this difference is clearly manifested in the amplitude of microseisms.
 The foregoing paragraphs have discussed chiefly on the amplitude of micro-seisms.As to the period,however its changes with approaching typhoons are generally very slight.As seen in Fig.10(b)the maximum period increases from 6 sec to 8-9 sec as the typhoon approaches,but the dominant period is still around 3 sec which agrees quite well with the dominant period in the normal times of microseisms.When the typhoon is of a fairly large scale and is situated near the observation point(as in the case of T.6014),another dominant period is seen at about 5 sec but it does not show any particular increase with approach of the typhoon.
 So far,the relationship between typhoon and microseismic storm has been studied using data of several typhoons.The result suggests that many of the observed variations of microseismic amplitude and period may be explained simultaneously by regarding the shore line near the observation point as the source area of microseisms,taking into account the typhoon’s scope,location and distance from the observation point.
 II.Cyclones and micros isms.Cyclone is another abnormality of meteorological conditions.Now,let us examine microseismic storms that would appear with passing cyclones.Path,central pressure and duration of the cyclones used in our study are given in Fig.11.The cyclones are denoted as LA,LB,LC.....LF,in order of generation.The five cases,LA through LE,are when the meteorological conditions near Japan were relatively simple and the respective cyclones were the only abnormalities,but LF is a case of a rather complex situation where several cyclones existed on both Pacific and Japan Sea sides.By the same method as before,the dominant amplitude and period and the maximum amplitude and period were obtained from their frequency distributions.The results are illustrated in Fig.12.As compared with the data resulting from typhoons,the data in Fig.12 indicate that changes are generally smaller in both amplitude and period,especially in period.This must be due to that cyclones are much smaller in scale of meteorological abnormality than typhoons.
 Figs.11 and 12 tell that the view on the microseisms accompanying typhoons as mentioned in the foregoing section can be applied to LA,LB,LD and LE.In other words,by taking into account the cyclone’s location,scope and its distance from the observation point,we can explain rise and fall of the observed microseismic storms On the other hand,the above view does not seem applicable to LC and LF.For example,in the case of LC in Fig.11,even when the center of the cyclone was nearest to Japan(Nov.2,6 h)it was still at a distance as 600km north of the Japan Sea coast,and yet the microseismic storms attained the culmination within several hours.The central pressure of this cyclone was 998mb,consequently the extension of the disturbance source must have been less than 150km in radius,and the velocity of waves coming from the source less than 25km/h.At this velocity,it would take at least 24hours for the waves to reach the coast of the Japan Sea.Thus,the source of microseisms cannot be situated near the coast.In this case,to regard the cyclone’s central area as the source of microseisms would lit better.As to the case of LF,Fig.12,we cannot take it as a single abnorinality.In fact,there were complex changes in amplitude.
 At any rate,the maximum amplitude of microseisms resulting from cyclones would seldom exceed 30 μ.The maximum period,7 sec,is somewhat smaller than the value of 8-9 sec in the case of typhoons.It is interesting to note that in each of LA through LF the dominant period is always about 3 sec which is similar to that of the normal times of microseisms.
3.Discussion
 Since Longuet-Higgins his theory of stationary waves has been regarded as most relevant for explaining the generation mechanism of microseisms.One of the major reasons for supporting his theory is that the theoretically obtained one to-two relation of the microseismic period to the sea wave period would explain quite well the observed facts.According to this theory,when the dominant period observed in Nagoya is 3 sec the dominant period of sea waves should be 6 sec.The location of the source of microseisms present another problem.In section 2,we tentatively called it a coastal area,but in our case it is still unknown whether the source is situated within Ise Bay or somewhere in the outer sea.
 In May 1962,a pressure type sea-wavemeter was installed in Ise Bay by the Ministry of Construction,which enabled simultaneous observations of microseisms from August to September at a point near Nagoya Harbor in the inner part of the bay and at the Institute of Earth Sciences,Nagoya University.Fig.13 shows an example of distribution of microseismic period as observed at these two points and of period of sea waves.Sea waves inside the bay are very weak all the year round,with amplitude seldom exceeding 50cm even under considerably abnormal conditions.Hence,we are unable to make a comparative study of microseismic waves and sea waves for a long duration.The example given in Fig.13 is the case when typhoon T.6212 passed through the southern coast of the Pacific side and the amplitude of sea waves attained to several tens of centimeters.The period of sea waves is dominantly 12-13 sec,much larger than 6 sec.A similar pattern of sea wave period was observed in another typhoon,T.6214,of the same year.Also,the amplitude of sea waves is generally small inside the bay.These facts suggest that at least the sea waves inside the bay do not contribute much to the microseismic activity.The difference in microseisms between the Institute of Earth Sciences of Nagoya University and Nagoya Harbor will be discussed in Part II.Oceanographic information tells us that the mean period of sea waves around Japan is 8-10 sec,which is supposed to be the value at normal times.The period of sea waves under abnormal meteorological conditions is dominantly 8-15 sec,with maximum period attaining to 15-20 sec,considering from the wind velocity(20-50 knots)in the central area of abnormality.
 In the results of our microseismic observations,however,the dominant period is still around 3 sec,although the maximum period shows a distinct increase with the approach of the abnormality.Thus,as far as our results are concerned,the one to-two ratio relation,i.e.,the 1/2 period theory may not be applicable.
 According to Ewing et al.[2],in discussion of microseisms it is desirable to consider the following factors separately:(a)nature of the source,(b)mode of propagation in oceanic paths,and(c)mode of propagation in continental paths.In our case assuming a source of microseisms near the coast,factor(b)can be excluded.That is,we are to consider the nature of the source and the mode of propagation of microseisms in the inland area.
 Denoting the spectrum of sea waves in the source of microseisms as Y_1(ω),the land characteristics of the path of propagation as Y_2(ω),and the characteristics of the seismograph as Y_3(ω),the spectrum of the microseisms we observe is expressed as Y(ω)= Y_1(2 ω)・Y_2(ω)・Y_3(ω).In Y_1 = Y_1(2ω)the theory of Longuet-Higgins was applied to the relation between the period of sea waves and that of microseisms.Even if Y_3(ω)was ignored,we would get Y(ω)= Y_1(2ω)・Y_2(ω),not Y(ω)= Y_1(2ω).That is to say,unless the land characteristics Y_2(ω)cover a wide range,the relation of the period of observed microseisms to the period of sea waves is not necessarily 1:2.In view of the propagating direction and the result of orbit analysis,microseisms are regarded as the Rayleigh type surface waves.If so,Y_2(ω)should have the band pass characteristics related to geological structure,and could never be flat,Accordingly,there can be cases where the characteristics of Y_2(ω)surpass those of Y_1(2ω),so that the pattern of the observed microseismic period is reflecting Y_2(ω)for the most part.
 If the predominance of 3 sec microseisms in our case was thus interpreted,the above-mentioned results of observations of the period of sea waves and microseisms would indicate merely that the influence of sea waves inside Ise Bay on microseisms is negligible,and would not offer any counter-evidence against the 1/2 period theory.
 In order to separate the influence of the land characteristics on amplitude and period of microseisms from the source characteristics in the disturbance source,the observation results of only one point and of a given duration of IGY are far from sufficient.Therefore,we later carried out comparative observations of microseisms from 1961 to 1962,and achieved our object to a certain extent.The results are reported in PartII.

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FIG.3.Seasonal changes in amplitudes of microseisms observed at Nagoya University.
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FIG.4.Frequency distribution of the amplitude of microseisms in the normal times.(a)Summer season.(b)Winter season.
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FIG.5.Frequency distribution of the period of microseisms in the normal times.(a)Summer season.(b)Winter season.
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FIG.6.Frequency distribution of amplitude and period of microseisms under weak cyclone and cold front(June 18,1958).(a)Amplitude.(b)Period.
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FIG.7.Path of typhoons.
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FIG.8.Frequency distributions of amplitudes and periods of microseisms under typhoon,T 5811 in July,1958.(a)Amplitude.(b)Period.
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FIG.9.Frequency distributions of amplitudes and periods of microseisms under typhoon,T 6014 in August,1960.(a)Amplitude.(b)Period.
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FIG.10.Time variation of amplitudes and period of microseisms under typhoons.(a)Amplitude.(b)Period.
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FIG.11.Location and path of cyclones.
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FIG.12.Variation of periods and amplitudes of microseisms according to various cyclones(LA,LB,LC in 1959;LD,LE,LF in 1960).
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FIG.13.Frequency distributions of periods of microseisms and of sea waves.

PART II.COMPARATIVE OBSERVATIONS OF MICROSEISMS IN NAGOYA AND ITS VICINITY

Array1.Location,duration and method of observation
 From the results of our observations of microseisms during the IGY and later periods,we learned that microseisms of about 3 sec in period and 5μ in amplitude are dominant all the year round,and understood the nature of microseismic storms under abnormal meteorological conditions.Since these are the results of one-point observations at the Institute of Earth Sciences,Nagoya University,during the above-mentioned periods,we cannot assert that they are representing the characteristics of the entire area of the Nobi Plain.On the other hand,the land characteristics of the site of observation may account for the easy occurrence of 3 sec microseisms,as has been briefly mentioned in PartI.However,in this respect also,we should not make a hasty conclusion.Thereupon,we decided to carry out comparative observations of microseisms.Of the two horizontal component seismographs,one was installed at the Institute of Earth Sciences to he used as a key station.and the other was used in mobile observations at various points of the Nobi Plain,centering around Nagoya.Some typical examples of the records of microseisms obtained are shown in Fig.14 together with those used in PartI.Comparative observations were performed from 1961 to 1962 at the stations listed in Table 2.Duration of observation at one station was limited between 2 and 4 weeks,which is supposedly long enough for our purpose.As stated before,one of the two horizontal component seismographs having the same characteristics was used in fixed point observation and the other was used in mobile observation.For the sake of convenience the components were unified in a N-S component,since it is known that the variation of microseismic amplitude with direction is not very great in this district as already mentioned in Part I.Distribution of observation stations and geological outline of the Nobi Plain are shown in Fig.15.
 In analyzing the data the same method as the one stated in Part I was employed.The only difference is that the values at the fixed station of the Institute of Earth Sciences,Nagoya University,were used as the standard for comparison between the values obtained at other stations.
2.Result of observation and discussion
 Fig.16 shows some of the amplitude and period distributions obtained at various stations by the same method as in Part I.The distribution at the key station during the same period is also given in the figure.These records,except that of Minato^1,are generally of calm times of microseisms.
 So far as our observations go,the period of microseisms is dominantly 3 to 4 sec,seldom at higher or lower values.Of the above-mentioned nine observation stations,the dominant period at Nagoya University,Ichinomiya and Gifu was rather small,being 2.7 to 3.3 sec,but all the other stations recorded the dominant period at nearly 4 sec.Putting aside the 4 sec period at Seto,we can say that 4 sec period is dominant generally in the area near the Nagoya Harbor(Minato),whereas 3 sec is the dominant period in the area farther inland from Ise Bay.Nevertheless,variation of microseismic period with geographic distribution of observation stations is very small and cannot furnish information useful for reasonable interpretation of its relation to the ground characteristics.
 On the other hand,variation of amplitude with location is fairly conspicuous,the value occasionally attaining to ten times as much that of the standard value at the key station in Nagoya University.Referring to Fig.16 the dominant periods and amplitudes at the respective observation stations are summarized in Table3.
 As the table indicates,variation of characteristics with location of obser-vation station is more conspicuous in the amplitude than in the period.At stations in the southern part of the district,namely,Tobishima,Kuwana,Shiratori,Ishin and Minato,the values of dominant amplitude are very large,three to eight times as much as that at the key station,whereas the value at Ichinomiya is roughly the same as that of the key station,being about 1.0 to 1.2 times the latter.The value becomes smaller at Seto(0.66-0.8 times),and the smallest at Gifu(0.2-0.4 times).In other words,amplitude is greater in the south and decreases northward.Considering that the main source of microseisms recorded in this district is located near the Pacific coast in the south,the above values may suggest a correlationship between the increasing distance from the source and the decreasing amplitude of microseisms.On the other hand,in Fig.15 showing the location and geology of the observation stations we know that almost all stations in the southern area stand on the Holocene beds,while Gifu and Seto are in the area of Paleozoic strata or igneous rocks.Thus,a correlationship can be assumed also between the hardness of foundation and the amplitude of microseisms.In order to find out which correlationship is more effective,we prepared a diagram,Fig.17,showing the relation between distance and relative amplitude,taking two directions,i.e.,NE-SW line connecting Seto,Nagoya University and Tobishima,and N-S line connecting Gifu,Ichinomiya and Tobishima.In the NE-SW direction,the amplitude attains a peak at Tobishima,southernmost station,and decreases rapidly northward,in the order of Ishin,Minato,Shiratori and Nagoya University.Between Nagoya and Seto,the decrease in amplitude is very small.This is applicable to the N-S line,too,where the amplitude shows a rapid change between Tobishima and Ichinomiya while the change is very small between Ichinomiya and Gifu.
 If these facts should be interpreted as attenuation of surface waves according to distance,as has been generally believed,the source of the microseisms must be situated very close to the observation point(such as the innermost part of Ise Bay),and must be a very local one,too.However,as already pointed out in Part I,the result of our comparative observations of microseisms and sea waves showed that the sea waves in the innermost part of the bay cannot become a source of microseisms.And,if the decrease in the amplitude is explained as surface waves’ attenuation with distance,the period must become longer with increasing distance from the source.Our observations revealed,however,a reverse relation,that is,the period becomes shorter inlandward.According to Okano [8],an area where microseisms can be generated is limited roughly in the near-coast sea of rather dense isobathic contours.
 It is difficult to know an exact location of the source of the microseisms we observed,but the afore-said facts suggest at least that the source is somewhere near the coast at a distance somewhat larger than that between the observation stations.
 Thus,in both Tobishima-Nagoya University-Seto line and Tobishima-Ichinomiya-Gifu line,the correlationship between the amplitude characteristics and the distance from the source of microseisms becomes secondary,so that the effect of the nature of the medium of the path of propagation of microseisms,i.e.,ground characteristics,is considered the primary one.Shida’s study [14] seems to follow this viewpoint.
3.Geologic structure and microseisms
 In an attempt to clarify the characteristics of the microseisms in this district,the observed facts so far described must be fully elucidated.That is,we must find an explanation that would simultaneously satisfy the following facts:3 to 4 sec periods are predominant,and microseisms of 4 sec period are abundant in the southern area;the amplitude varies greatly with location of observation station,and the value becomes larger southward.Now,let us study these problems.
 As has been mentioned in the last section of PartI,the spectrum of the microseisms we are to observe is expressed as Y(ω)= Y_1(2ω)・ Y_2(ω)・Y_3(ω).As correction was made for the seismograph’s characteristics,Y_3(ω),the spectrum can be represented by Y(ω)= Y_1(2ω)・Y_2(ω).Of the values to be used,direct data of the spectrum of sea waves denoted as Y_1(2ω)are very few,but refer-ring to the analytical results of sea waves at various parts of the country during the IGY period we get a comparatively flat distribution of spectrum,although 8-12 sec periods are generally dominant.Thus,Y_1(2ω)is considered to cover a fairly broad area.There are cases where dominant period moves toward a longer period with the approach of meteorological abnormality,and occasionally exceeds 20 sec.However,in the result of our observation of period of microseisms,the microseisms of 3 to 4 sec were dominant with or without existence of abnormal meteorological conditions.This suggests that Y_2(ω)is closely related to the period.
 Pertaining to the underground structure of Nagoya and vicinity,there is a study by Iida and Aoki [4] based on near earthquakes.According to this study,the following two models are equally possible:
 表:Model I・Model II
 Considering from the location of observation stations whose data were used in the analysis,the above models are regarded as representing the under-ground structure of the area around Nagoya City.On these models,the amplitude versus period characteristics of Rayleigh waves can be calculated,but a method of calculation for a three-layered structure has not been established yet.Hence,we carried out calculation by assuming the structure to be two-layered.In calculation we adopted the idea of Tazime [15] i.e.,the dominant period of surface waves can be obtained by the 1/4 wave length law.If the first and the second layers were grouped together as one layer,the resultant period would become about 8 sec which is too large as compared with the known dominant value of 3 to 4 sec.Therefore,we used the first and the second layers only,ignoring the third layer,for the reasons that the velocity contrast between the first and the second layers is considerably large and the boundary between the second and the third layers is at a great depth of 9.3km from the ground surface.Thus,we obtained the period of 2.6 sec in both models.
 From the result of our calculation on the Rayleigh waves for a two-layered structure,the amplitude versus period characteristics,Y_2(ω),and the dispersion curves of the cases closest to the above-mentioned two models were selected and diagramed in Fig.18.As is seen in the diagram of calculated models,both model I and model II are closer to model [A_2] than to model [A_1].In either of [A_1] and [A_2],dominance of the amplitude is found at about 3 sec of period.Especially model [A_2] seems to present the characteristics of a fairly distinct band pass centering around 3 sec.
 Both model I and model II have some defects which prevent us to build a decisive conclusion,so any further discussion on Y_2(ω)would be meaningless.However,the calculated result may present an interpretation of the fact that 3-4 sec microseisms are apt to dominate in the Nagoya district.
 Next,we must think about the difference in microseismic amplitude between observation stations.This problem reminds us of Shida’s study [14].Through the multi-points observations in the Shonai district,Shida pointed out a close relationship between the amplitude of microseisms and the strength of foundation which will be known from observations of ground vibrations.This relationship suggests that the subsurface weak beds would affect microseismic waves more strongly than expected,despite the fact that the wave length of microseisms is much longer than that of ground vibrations.In order to ascertain this,observation of microseisms must be performed in an area of known underground structure.
 Details of underground structure of the whole area of Nobi Plain are not clarified as yet,but fortunately the results of numerous borings down to the depth of several hundred meters are available for a line connecting our observation stations between Nagoya University and Kuwana,via Tobishima.Based on these boring records,Matsuzawa [6] prepared a geologic cross-section.According to the outline(Fig.19)of this cross-section,with an exception of the observation station at Nagoya University which is situated near the base of Diluvium,all other stations such as Shiratori,Minato,Ishin and Tobishima stand on the Alluvial beds,and the Quaternary beds(Alluvium plus Diluvium)become thicker almost linearly in the above-mentioned order.Only Kuwana stands on the Tertiary foundation which is in fault contact with the Quaternary beds.
 By comparing the crosssection with the distribution map of relative amplitude of microseisms in Fig.17(a),a very close relationship is found between the two.That is to say,the increase in the amplitude of microseisms is roughly proportional to the thickness of the Quaternary strata.In other words,assuming the direction of propagation of microseisms at nearly right angle to the cross-section,the difference in the amplitude between Nagoya University and other stations south of Shiratori is largely attributed to the existence of the Quaternary soft beds.
 For quantitative examination of this relationship,the amplitude characteristics of the Rayleigh waves in the three-layered structure must be known,as in the case mentioned before.However,such a task is very difficult at the present stage of our knowledge.Thereupon,considering that the characteristics of Rayleigh waves are most strongly affected by the interlayer velocity of S waves and that our microseismic observations are being made on the horizontal components,we attempted to detect the effect of soft beds on the amplitude.As to the values of S waves the results by Iida and Aoki [4] were referred to,as before.In addition,as the uppermost layer a layer of small velocity was assumed.Thus,we obtained models [B] and [C].The result is given in Fig.20.The figure shows that,if the thickness of the uppermost layer is definite,amplification of amplitude becomes larger with decreasing velocity of S waves in this layer,and if the velocity of S waves,V,in the layer is definite,the amplitude increases with the layer’s thickness.At the same time,the dominant period increases,more so in [B].The result may not be allowed to substitute the amplitude/ period characteristics of surface waves,but it becomes known at least qualitatively that the surface amplitude increases markedly with increasing thickness of soft beds,and the period also increases but not so remarkably as amplitude does.This result is not contradictory to the inclination revealed in the result of our observations of microseisms in the Nagoya University-Tobishima line.For discussion of the same subject on other observation stations,we do not have sufficient information of under-ground structure,so a hasty conclusion should not be reached.Nevertheless,it may be reasonable to apply the same theory to other stations.This theory,however,cannot offer a satisfactory explanation of the amplitude at Kuwana and the period at Seto.Despite the fact that Kuwana is situated on the Tertiary foundation,the amplitude is fairly larger than at Nagoya University.But,considering it from a different angle,we may be able to say that the amplitude,which rapidly increased with increasing thickness of the Quaternary beds in the Nagoya University-Tobishima line showed a sudden decrease in the Tobishima-Kuwana line,is reflecting a large fault supposedly existing between Kuwana and Tobishima.With regard to the period at Seto we cannot say anything decisive until more precise observations are made in the area between Nagoya and Seto.

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FIG.14.Some examples of microseismic records.(a)Nagoya University.Records used for the analysis in PartI.
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FIG.14.Some examples of microseismic records.(b)Records obtained by comparative observations.(c)ditto.
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TABLE2.MICROSEISMIC STATIONS
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FIG.15.Geological map of Nobi Plain and distribution of observation stations.
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FIG.16.Examples of distribution of amplitudes and periods of microseisms at various stations.(a)
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FIG.16.Examples of distribution of amplitudes and periods of microseisms at various stations.(b)
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FIG.16.Examples of distribution of amplitudes and periods of microseisms at various stations.(c)
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FIG.16.Examples of distribution of amplitudes and periods of microseisms at various stations.(d)
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FIG.16.Examples of distribution of amplitudes and periods of microseisms at various stations.(e)
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FIG.16.Examples of distribution of amplitudes and periods of microseisms at various stations.(f)
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FIG.16.Examples of distribution of amplitudes and periods of microseisms at various stations.(g)
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TABLE3.PREDOMINANT PERIODS AND AMPLITUDES OF MICROSEISMS AT VARIOUS STATIONS
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FIG.17.Relationship between distance of station and relative amplitude of microseisms according to the direction.(a)SW-NE direction.(b)S-N direction.
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表:Model I・Model II
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FIG.18.Theoretical dispersion curves and amplitude-period curves for models [A_1] and [A_2] of underground structure λ,μ:Lame elastic constants.V_0,U_0:Vertical and horizontal amplitude.
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FIG.19.Geologic cross-section along the Line connecting the stations between Nagoya University and Kuwana.
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FIG.20.Relationship between the amplitudes and periods for models [B] and [C] of uppermost layers.m:Poisson’s ratio.V_s´´,V_s´,V_s:S wave velocity of each layer.

CONCLUSIONS

 As the result of our researches so far reported,we have learned the general characteristics of microseisms observed in Nagoya and the surrounding areas during the International Geophysical Year 1957-1958 and later periods.The results are summarized as follows.
 The microseisms dominant all the year round at Nagoya University have about 3 sec period and 3 to 8 μ amplitude.The amplitude in the winter season is nearly twice as much as that in the summer normal time.The result of data of several typhoons shows that many of the observed variations of microseismic amplitude and period will be explained simultaneously by regarding the shore line near the observation point as a source area of microseisms.
 From the analysis of comparative observations the predominant period of microseisms in Nagoya and its vicinity is found to be 3 to 4 sec which is understood as the foundation characteristics representing a generalized geologic structure of the area.Amplitude varies considerably with location of observation point,ranging in value from 0.2 to 8 when the value at Nagoya University is taken as unity,value of amplitude is related to the thickness of the soft foundation represented by Quaternary beds.By advancing the conclusion in the paragraph above mentioned,we may be able to know the hardness of foundation or the extent of soft foundation through multi-points observation of microseisms.

ACKNOWLEDGEMENTS

 The writers wish to express their heartiest thanks to the staffs of schools and institutions concerned,who gave them a ready consent to the use of the facilities.To Mr.Kitajima,chief of the Construction Section,Ise Bay Harbor Construction Department,who permitted the writers to utilize the records of sea waves,our thanks are also due.The writers are indebted to Messrs.Yoshio Osada,Toshimi Kitano and Akio Kato,for their cooperation in record reading and other troublesome works.

REFERENCES

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[3] IIDA,K.,and WADA,T.,Microseisms observed at Nagoya University.Preliminary report:Read at the Seismological Society of Japan(1957).
[4] —,and AOKI,H.,Phases in the preliminary tremors of seismograms and crustal structure in Nagoya district(in Japanese):Zisin(J.Siesm.Soc.Japan),2nd Ser.,12,75-81(1959).
[5] LONGUET-HIGGINS,M.S.,A theory of the origin of microseisms:Phil.Trans.Roy.Soc.Am.,A 243,1-35(1950).
[6] MATSUZAWA,I.,The geologic structure of the northern coastal region of Ise Bay(in Japanese):Ministry of Construction,pp.4(1962).
[7] MATUZAWA,T.,ASADA,T.,DEN,N.,and TAKANO,K.,Tripartite observations of microseisms:Japan.J.Geophys.,3,1-74(1962).
[8] National Committee for the International Geophysical Year,Report of microseismic and sea wave observations in Japan during the International Geophysical Year 1957-1958:Science Council of Japan,Tokyo(1959).
[9] OKANO,K.,Microseisms observed at Abuyama Seismological Observatory(in Japanese):Zisin(J.Seism.Soc.Japan),2nd Ser.,2,182-190(1959).
[10] —,Direction of approach of microseisms observed by vector seismographs(in Japanese):Zisin(J.Seism.Soc.Japan),2nd Ser.,13,37-42(1960).
[11] SANTO,T.,Investigations into microseisms using the observational data of many stations in Japan(Part 1).On the origin of microseisms:Bull.Earthquake Research Inst.,Tokyo Univ.,37,307-325(1959).
[12] —,Investigations into microseisms by the observational data of many stations(Part 2).Further considerations on the origin of microseisms:Bull.Earthquake Research Inst.,Tokyo Univ.,37,483-494(1959).
[13] —.The observation of microseisms at a wave gauge station.On the origin of microseisms(Part 3):Bull.Earthquake Research Inst.,Tokyo Univ.,38,241-254(1960).
[14] SHIDA,I.,Relationship between the microseisms observed in Shonai district and its ground condition(Part 2)(in Japanese):Publ.Ground Invest.Group Shonai District,1-10(1960).
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