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ABSTRACT

 The generation of tsunamis is discussed in connection with the type of faulting associated with the focal mechanism of tsunamigenic earthquakes.The focal mechanism of tsunamigenic earthquakes and of earthquakes not accompanied by tsunamis,occurring in or near Japan during the period 1926 to 1968,were investigated on the basis of radiation pattern of P−waves and for some,of S−waves.Only earthquakes having a magnitude greater than 6.3 were taken into consideration.
 The faulting inferred from the focal mechanism of tsunamigenic earthquakes is mostly the dip−slip type;the generation of large−magnitude tsunamis is especially associated with dipslip type.The earthquakes associated with strike−slip faulting are almost never accompanied by tsunamis,even though the magnitude of the earthquake is nearly seven.Also,most after−shocks are not associated with the generation of tsunamis.
 The generation of tsunamis is closely related to the type and dimension of dislocation in the source,being associated with characteristics of the earthquake,such as magnitude,focal depth,and water depth at the epicenter.

INTRODUCTION

 The generation of tsunamis associated with the focal mechanism of tsunamigenic earthquakes occurring in or near Japan was discussed by the present author(Lida,1967)at the eleventh Pacific Science Congress at Tokyo in 1966.The generation of tsunamis has been found to be greatly dependent on characteristics of an earthquake,such as magnitude,focal depth,focal mechanism,aftershock activity,and water depth at the epicenter.This paper is a progress report.The present investigation is concerned with tsunamigenic earthquakes during the period 1900 to 1968.However,since the more accurate data are those from 1926 to 1968,during which the focal mechanism of all earthquakes having a magnitude greater than 6.3,even those not accompanied by tsunamis,was investigated,this time interval was specially studied.

CATALOGUE OF TSUNAMIGENIC EARTHQUAKES AND TSUNAMI MAGNITUDE

Array A catalogue of tsunamigenic earthquakes,compiled from various reports and papers and from mareographic records of tidal stations distributed along the coasts of Japan,is presented in Table1 along with tsunami magnitudes.Tsunami magnitudes m were determined by the following formula
   m= log_2 h or h = 2.0^m,(1)
in which h is the maximum height,in meters,measured at a coast 10 to 300km.from the tsunami origin.
 The geographic distribution of epicenters of tsunamigenic earthquakes from 1900 to 1968 is shown in Fig.1,classified according to tsunami magnitudes.From Fig.1,it is apparent that most of these tsunami sources are located on the Pacific side in northeastern Japan.Figure2 shows the relationship between the earthquake magnitude and focal depth of an earthquake accompanied by a tsunami.The magnitude of a tsunami is shown in round numbers in Figure2.Earthquakes occurring off the coast accompanied by tsunamis and not accompanied by tsunamis are depicted by filled−in circles and by plain circles,respectively.Data for the magnitudes and focal depths of the earthquakes were taken mainly from the 「Catalogue of Major Earthquakes Which Occurred In and Near Japan」published by the Japan Meteorological Agency(1958,1966).
 Figure2 shows that there is an approximate linear boundary between earthquakes accompanied by tsunamis and those that are not accompanied by tsunamis.This boundary is considered to be a limiting magnitude for earthquakes;below this magnitude tsunamis do not occur,as reported in previous papers(Iida,1958,1963,1965).This limiting magnitude for tsunamigenic earthquakes may be generally expressed by the straight line as
   M=6.3 + 0.005 H,(2)
where the focal depth H is in kilometers.This straight line is shown as the full line in Figure2.The coefficient of H in equation 2 is somewhat different from that in previous papers(Iida,1958,1963).From Fig.2,it can be seen that there are three tsunamigenic earthquakes located on the left side of the straight line expressing the limit of equation 2.Taking those three into consideration,the limit may be expressed by
   M=5.6+0.01 H.(3)
This relation,expressing the smallest magnitude of a tsunamigenic earthquake,is the same as before.
 The limit for disastrous tsunamis,having a magnitude of more than 2,may also be determined by
   M=7.7+0.005 H,(4)
as shown by the broken line in Figure2.Thus the earthquake accompanied by tsunamis having magnitude of less than 2 may generally be located between the full and broken lines in Figure2.It is,however,noticed that four tsunamis(1940 Hokkaido,1953 Boso,1963 Iturup,and 1964 Niigata)having a magnitude of 2 or 3 are located on the left side of the broken line of equation 4.It is noted that the magnitude of the 1963,Iturup tsunami was especially great compared with its earthquake magnitude.
 As seen in Fig.2,even though the magnitude is greater than that located on the right side of the line representing equation 2,there are a number of earthquakes not accompanied by tsunamis.The number of these earthquakes for the period from 1926 to 1968 totaled 79;the epicenters are shown in Figure3.These earthquakes are classified into three groups:(1)the aftershocks,32 in number,(2)the relatively deepfocus shocks,15 in number,and(3)others,32 in number.Groups(1)and(2)of these earthquakes are generally considered not to generate tsunamis.Therefore,the reason group(3)is not accompanied by tsunamis must be explained.For this investigation,the mechanism of earthquakes is considered.

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Table1:Catalogue of Earthquakes Accompanied by Tsunamis In and Near Japan,1900 to 1968.
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Fig.1:Geographic Distribution of the Epicenters of Tsunamigenic Earthquakes Classified According to Tsunami Magnitude.
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Fig.2:Relationship Between Earthquake Magnitude and Focal Depth of Submarine Earthquakes During the Period 1926 to 1968.The numeral outside of the circle is the tsunami magnitude in round numbers.

MECHANISM OF EARTHQUAKES AND TYPES OF FAULTING

 To see the difference between tsunamigenic earthquakes and non−tsunamigenic earthquakes with magnitude greater than 6.3,focal mechanism is investigated.There are a number of papers concerning the focal mechanism of earthquakes.Most of the data concerned were taken from the publications of the Dominion Observatory,Ottawa by Wickens and Hodgson(1965,1967)and publications by Stauder and Bollinger(1964,1965).Further,some of the data were taken from the technical reports of the Japan Meteorological Agency,especially from the report on individual earthquakes(1969)and from the papers by Ichikawa(1966)and others(Hirasawa,1965).
 Some examples of focal−plane solutions of tsunamigenic and non−tsunamigenic earthquakes are shown in Fig.4,which shows the distribution patterns of compression and dilatation of P initial motions.Some radiation patterns of S−waves were also considered,together with those of P−waves,especially in the results by Stauder and Bollinger(1964,1965).From these radiation patterns,not only the nodal planes and their dips can be determined,but also the strike and inclination of the nodal planes.Assuming that the faulting occurs along the nodal plane,the dip and strike components of the unit vector of the movement due to faulting can be calculated.When the strike component thus obtained is greater than the dip component,the faulting is called strike−slip type;when the dip component is greater than the strike component,the faulting is called dip−slip type.In this way the type of faulting was investigated for each of the earthquakes mentioned above.The results are listed in Tables 1 and 2.It can be seen that more than 60 percent of the tsunamigenic earthquakes are from faulting of dip−slip type.
 The geographic distribution of the epicenters of tsunamigenic earthquakes classified according to the type of faulting is shown in Figure5.From this figure,it can be determined that there are few tsunamigenic earthquakes(only 8 percent)produced by faults of strike−slip type.Further,there are a number of earthquakes on the northern Pacific Ocean side for which the types of faulting are not determined.Figure6,which corresponds to Fig.3,shows the geographic distribution of the epicenters of submarine earthquakes not accompanied by tsunamis,classified according to the type of faulting.From Fig.6 it is seen that most earthquakes show the strike−slip type fault,whereas few earthquakes are from dip−slip type faulting.

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Fig.3:Geographic Distribution of the Epicenters of Non−Tsunamigenic Earthquakes,Classified According to Magnitude.
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Fig.4:Some Examples of Focal Plane Solutions of Earthquakes,Redetermined After the Data by Stauder and Bollinger(1964,1965).○ Compression.● Dilitation,P,T,B:Axes.X,Y:Nodal Plane Poles.Left side:Tsunam
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Table2.Examples of Non−Tsunamigenic Earthquakes and Types of Faulting.
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Fig.5:Geographic Distribution of the Epicenters of Tsunamigenic Earthquakes Classified According to the Type of Faulting.S−S:Strike−slip fault type.D−S:Dip−slip fault type.
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Fig.6:Geographic Distribution of the Epicenters of Non−Tsunamigenic Earthquakes Classified According to the Type of Faulting.

TSUNAMI MAGNITUDE AND TYPES OF FAULTING

 The relationship between tsunami magnitude and the magnitude of a tsunamigenic earthquake was investigated in connection with the types of faulting as shown in Figure7.From Fig.7 it can be generally concluded that the greater the magnitude of an earthquake,the larger is the tsunami magnitude.Also,most large tsunamis occur in association with dip−slip type faulting.The tsunamigenic earthquakes marked by N in Fig.7 are from the normal fault type,whereas those without N are from reverse fault type.Therefore,it may be concluded that most earthquakes accompanied by tsunamis in or near Japan are of the reverse fault type and that the relation of the tsunami magnitude to the reverse and the normal fault type is not clearly distinguishable.

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Fig.7:Relationship Between Earthquake Magnitude and Tsunami Magnitude,Classified According to the Type of Faulting.

DISCUSSION

 As is generally known,tsunamis accompany only those earthquakes whose epicenters are at shallow depths,less than 100 km.,beneath the ocean or close to the shore;most tsunamis are generated by abrupt deformation of the sea bottom,either fault displacements or more general deformations caused by earthquakes,including deformations due to submarine volcanic explosions or landslides.Tsunami magnitude can be estimated from the generation mechanism of tsunamis,the dimensions of the area of tsunami origin,the speed and amount of displacement,and the water depth at the tsunami source.The dimensions and degree of crustal deformation of an ocean bottom is closely related to the earthquake magnitude,its focal depth,and the focal mechanism of the earthquake.Further,the area of tsunami origin is closely related to the area deformed by an earthquake and is approximately equal to the area of the aftershock activity.
 Tsunamigenic earthquakes are always followed by a number of aftershocks.The generation of tsunamis accompanying earthquakes is closely correlated with dip−slip type faulting,i.e.,the faulting of most tsunamigenic earthquakes is of the dip−slip type.The faulting of most non−tsunamigenic earthquakes having magnitudes greater than 6.3 is of strike−slip type.The process of tsunami generation is schematically illustrated in Figure8.
 Most aftershocks are generally not,accompanied by tsunamis,but there are some aftershocks which are accompanied by tsunamis(such as 1938 Fukushima,1952 Hokkaido,1960 Sanriku,1961 Ibaraki and Kushiro,1963 Iturup,and the 1968 Aomori),although the tsunami magnitudes are usually comparatively small.
 Abrupt deformation of a sea bottom is really caused by a main shock.By the change in stress or strain due to this deformation,aftershocks are caused.Consequently,we consider an aftershock not to partake in the production of deformation of the sea bottom.Since a tsunami results from sea−bottom deformation,which an aftershock does not produce,aftershocks are not accompanied by tsunamis.Therefore,if an earthquake which is considered to be an aftershock is accompanied by a tsunami,that earthquake is regarded as an independent shock and not as an aftershock of the main shock.
 The crustal deformation due to a large earthquake may be classified into two types:the deformation may completely finish during the vibration of the main shock,or the deformation may continue for several hours or several months after the end of the vibration of the main shock.The former type of deformation is called the first kind of deformation and the latter type,the second kind of deformation.Any tsunami is attributed to the first kind of deformation.True after−shocks are generated by the release of the stress secondarily produced in the area of the crustal deformation caused by the main shock.Therefore,it is concluded that an aftershock does not produce new deformation of a sea bottom in the area deformed by the main shock and,consequently,an aftershock does not generally produce a tsunami.The conditions of tsunami generation are greatly dependent on the focal mechanism of earthquakes.

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Fig.8:Process of the generation of tsunamis related to the occurrence of earthquakes.

BIBLIOGRAPHY

Hirasawa,T.1965.Source mechanism of the Niigata earthquake of June 16,1964,as derived from body waves.J.Phys.Earth.Vol.13,pp.35−66.
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Iida,K.1958.Magnitude and energy of earthquakes accompanied by tsunami,and tsunami energy.J.Earth Sciences.Nagoya Univ.,6,pp.101−112.
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Stauder,W.and Bollinger,D.A.1964.The S−wave project for focal mechanism studies,earthquake of 1962,AFAFOSR−62−458,Dept.Geophysics and Geophysical Eng.,Inst.Technology,Saint Louis Univ.
Stauder,W.and Bollinger,G.A.1965.The S−wave project for focal mechanism studies,earthquake of 1963,AFAFOSR−62−458,Dept.Geophysics and Geophysical Eng,,Inst.Technology,Saint Louis Univ.
Wickens,A.J.and Hodgson,J.H.1967.Computer re−evaluation of earthquake mechanism solutions 1922−1962.Pub■.Dominion Obs.Ottawa.33.